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MG MGA - Acceleration cut

Hi. I am a happy owner of MGA 1600 from 1961 for a short time. I am buying spare parts for SU H4 carburetors but I can not solve the problem. I need help. My problem is that the MGA with motor 1622 and camshaft fast road of Brown and Gammons cuts the acceleration to about 4500 rev. and 70 mll / h when I keep it like this one minute. Before this happened at 55 m / h and 3000 rev. but changing the carburetor springs to yellow and the needles to the AZ has improved but the problem has not been solved. The green springs do not solve the problem either. Climb the float either. All the carburation has been checked and there are no obstructions. The fuel pressure is adequate. Now I have many useless spare parts that I have bought. If I keep the accelerator pressed when the acceleration is cut, valve misalignments occur and who knows if there is any internal damage. Please, I do not have a carburization specialist who can help me. Can you help me?. I keep trying new needles ?. Thank you.
Gabriel Martínez

¿Puedes escribir en español, tal vez tu problema sea más fácil de describir?
Dominic Clancy

I wonder if it is distributor related rather than fuel? Vacuum advance prob ok on acceleration but the bob weights possibly seized or their springs out of limits.

Steve
Steve Gyles

Hi Gabriel,
Have you checked the fuel pump?
I
I had a similar problem with my last MGA many years ago after I replaced the fuel pump.
I thought I had bought a HP SU fuel pump but it turned out that I had bought a fuel pump for a Morris 1000 by mistake.
That pump just did not supply enough fuel for the MGA engine and this caused the engine to run fine up to about 55 mph but above that, the engine would begin to misfire.

Cheers
Colyn
Colyn Firth

Question,
Are you saying the engine "cuts out" at 4500 rpm or merely that it won't rev past 4500?
Allan Reeling

What do you mean "valve misalignments occur"? How does it sound when it is resticted? Is it misfiring?
The 1 minute delay in seeing this problem does point to an inadequate fuel delivery.
Art Pearse

Hi. I appreciate your answers. First, if the majority prefers that I write in Spanish, I'll do it, obviously it's easier for me. It seems that the problem is of fuel because the response of the engine has improved with changes in carburetion (yellow springs and AZ needles) and distributor tests are correct with and without vacuum. With red springs and standard needles the engine misfiring at 55 m / h and 3000 rev. approx. However, the green springs do not help, they seem to overfeed the engine. The sound is of engine stop but without stopping. If at that moment I stop accelerating in about 20 seconds at idle the sound and power return to normal, but if I keep accelerating it sounds like a can and loses power. If in addition to this I continue to accelerate I have found valves blocked and misadjusted, terrifying. The fuel pump is new, it's a electrónic of Moss for MGA. The fuel pressure is ok with vaccum test.
Gabriel Martínez

One more clarification. The engine can go up more than 4500 rev. No problem. But you can not keep them for 1 minute on a flat road or half a minute on a road with climb.
Gabriel Martínez

Sounds like a distributor problem - do you have access to a dial back timing light or a good mechanic?
Mike
Mike Ellsmore

I'd be refiting the standard red springs and needles - they won't be the cause

blocked or restricted fuel supply is most likely

Fuel filter-?
Does your car have the little strainers in the banjo fittings on the float bowls- if so are they clean-?
Blockage in fuel line from foat bowl to the jet tube-?
Check the fuel supply at the carbs-take a fuel line off and measure the pressure first, then the ammout-you need probably 250-300ml in 10 seconds

willy
William Revit

Pull the fuel feed hose off the rear carb, and hold it in a catch vessel. Switch on and observe fuel flow. You should get a gusher, at lest one pint per minute. If not, look upstream for the feed problem. If you do get good fuel flow, look for some obstruction in the carburetor, like crud in the bottom of the float chamber, or perished rubber washers on the banjo bolt between float chamber and throttle body.
barneymg

Debris in the bottom of the float bowl is unlikely to be the issue but debris in the float valve is.

Remove the fuel chamber caps and take out the needle valve bodies clean both the valve bodies and the caps thoroughly using compressed air if possible.
Chris at Octarine Services

It does sound like fuel starvation. A stationary flow test can often be misleading, if there is a developing tank vacuum or crud being drawn into the tank outlet while underway. "Valves blocked and misadjusted" is slightly confusing. Are you saying the valve clearances are altering?
Allan Reeling

Tested fuel flow: 450ml. in one minute at the entrance to the first carburetor. The carburetors were checked and there are no obstructions in the bridge, neither in the entrance to the float, nor from the float to the jets, the filters are clean. I have a strobe lamp and the distributor values ​​are appropriate according to the workshop manual.
Gabriel Martínez

I agree with everybody it does sound like fuel starvation. I am sure the dash pots are clean but would ask if they are clean and dry and the pistons slide up and down freely. Over-oiling the dash pots can cause an oil film which attracts dust particles which can hold up the pistons giving a horrid lean mixture when you try to accelerate causing the engine to misfire and cut out at speed and can then be OK again at low throttle. Worth asking as I had this once. I cleaned the dash pots and that cured it.
Pete
PeteT

Rear air filter on upside down. http://www.mgaroadster.co.uk/odds_and_sods.htm

Steve
Steve Gyles

Again I appreciate the ideas, I'm sure there must be some solution, however, the dashpots are clean and dry and the air filters are correctly placed, in fact the problem improved partly by placing the filters, I think the air restriction It benefits the use of fuel.
Gabriel Martínez

I think fuel starvation also.

I had a case where dust, debris and detritus from the fuel tank had gathered together in the copper pipe going from the fuel tank to the fuel pump; right at the curvature of the top pipe, restricting full fuel flow.

Symptoms were similar to Gabriels. I had checked the full flow at the carbs which appeared slower than I expected. Once the crud was cleared (and I checked the entire pipe line with a compressed air blast), the flow rate was vastly improved.

Regards
Colin
Colin Manley

What ignition advance do you have at 4000 rpm?
Mike
Mike Ellsmore

One easy check.........when this misfire occurs, stop the car and immediately remove the petrol cap. A "gasp" from the tank would indicate a blocked tank vent. Probably the vent is in the cap itself.
Allan Reeling


Here we have bad weather now. It's raining, it's cold and it's snowing. When I can get the car out I'm going to try two ideas: look if there is gasoline in the floats and enrich the mixture with the choke at the first symptom.

The ignition advance at 4000 is 32 without vacuum, 43 with vacuum. I do not think it's a problem of ignition time because the problem persists delaying and advancing time. However, the problem improves by changing the springs of the pistons and the needles.
Gabriel Martínez

Looks to me like you are throwing all sorts of possibilities at your engine. My advice would be to get back to original. i.e., No 6 needles and Red type A damper springs. If you want to go richer go to RO needles. AZ are considerably richer than standard! Going to yellow or green springs will make the piston and therefore the needle ride lower. Yellow are 8oz and green 12 oz whereas the standard reds are only 4.5oz. Riding lower will mean less fuel and air at any given throttle opening, which might explain why your engine is gasping at higher rpm when demand is greater and also explain why the very rich AZ needles make a bit of difference. Lighter damper springs (light blue) will "balance" out the piston higher and make use of the richer section of the needles.
Allan Reeling

Hello again. Finally I was able to drive it and I checked the following: Float chamber full; operating the choke does not solve the problem or to prevent it; the four spark plugs white, the temperature rises to 200ºF (usually 180) and the liquid is expelled from the radiator; revolutions at idle rise from 600 to 1400 (a slow use of the vehicle lowers the temperature and the revolutions in idle in about 15-20 minutes.

I have cleaned everything from the floats to the bridge, but I do not know if there is something wrong. Apparently everything is fine. The engine only has 700 km. since the restoration (new pistons, rings, mechanized to .04, bronze guides, new and uprated valves and springs, ...). Will you need more driving?
Gabriel Martínez

Thanks Allan for your help, however I repeat that I started with red springs and needles nº6 and the problem appeared before (at 50 mll/h and 3000 rev), then I put the needles RO and improved slightly but almost imperceptibly. Then I put the yellow springs and improved noticeably. Needles CT, MME, ... until the AZ. With the latter I got the best result but almost the same as with the MME and much better than the RO.

I think the effect of the springs is the opposite, and coincides with the experts, more spring pressure more fuel in the mix.
Gabriel Martínez

So as I understand you, the problem only occurs at high engine revs under load. High revs in the workshop without load is no problem.

Does it have mechanical points in the distributor or do you have an electronic ignition module? If the mechanical points, try changing the capacitor. How hot is the ignition coil when these symptoms occur?

Can you take a picture of the spark plugs and post here.
Dominic Clancy

Gabriel,
The spring is carefully chosen for the engine characteristics to balance the piston against the vacuum pulling the piston up and in doing this controls the "range" of the piston travel. The vacuum is constant, hence the spring should remain constant. If you think you have a carb tuning problem stick to playing with needles. Even my overboard highly tuned engine ran with standard springs, albeit new ones.
You still haven't explained what you meant by "blocked and misadjusted valves".
Another check for when the weather improves, turn the ignition on till it stops the rapid ticking then check the fuel level in the float chambers.............take the car out and when it gets to it's stuttering misfiring point, turn the ignition off, and coast to a halt. Then look at the fuel levels in the float chambers again.
Allan Reeling

Good question Dominic, Now I remember that it also happened in a prolonged climb of about 20 minutes at less rev. and much less speed. In that climb I did not exceed 3500 rev. nor of 50 mll / h. I changed the coil to a sports col, the leads and the cap of the best quality according to Moss Motors.

I have not changed the capacitor but I can do it although I think it is not an electrical problem.

Gabriel Martínez

I think what is happening is that the engine is poor fuel at times of load even if there is fuel in the float chamber. The problems derived I think are those of lack of fuel: overheating, rev up. in idle, misalignment of valves with engine sound canned. There will be some cause in the head ?. If I put H6 the problem will be solved ?. Or weber ?.
Gabriel Martínez

Refit the red springs and no6 needles
They are not the problem

450ml in a minuite is not enough
and your white sparkplugs are telling you that
with the springs and needles you have in there at the moment it would be jet black plugs but because the fuel supply isn't up to the job they are lean/white be carefull, you will burn a valve or melt a piston or two if you keep going like this

Either the fuel pump isn't up to the job or you have a blockage down the line there somewhere
Have you got a fuel filter on it-if yes try fuel flow testing before and after the filter

William Revit

Another way fuel delivery can be restricted is by having a slight leak between tank and pump. This leads to air being sucked rather than petrol. This is difficult to spot as there won’t be any petrol coming out just air getting in. My incident was caused by short from metal pipe to battery clamp, scary!

Paul
Paul Dean

Gabriel. Please do the checks I suggested i.e., the Tank vacuum/blocked breather check and the fuel level in the float chamber check. Throwing bigger carbs or a Weber will only help to reduce your bank balance!!
The engine is running lean, hence the white plugs and the overheating. Normally an SU pump will deliver in excess of 0.5 litres per minute. Later ones double that. But I say again a stationary test won't necessarily indicate the problem, if, 1) the tank vent or cap breather is blocked or there is debis being pulled to the tank outlet.
Allan Reeling

Could the valves be sticking in the guides? Is this the valve noise referred to ?
Art Pearse

450ml is about a US pint a minute, so is about right as a crude measurement.

It sounds like a combination of lean running and overheating

You need to turn the big brass nut underneath the carb downward by a whole turn on each and see if it improves.

The strange noises are probably rampant pre-detonation caused by the lean running and consequent overheating - if you don't richen the mixture you will destroy your new engine very quickly. It may also be wise to check the ignition timing - the standard factory figures don't apply to modern fuels 1:1

But as most carb problems turn out to be electrical, please tell us what you have inside the distributor - mechanical points or electronic module, and how hot is the coil when you have the problem.
Dominic Clancy

I note you say you have bronze valve guides. These need extra clearance for the valve stems as they expand at a different rate to cast iron, and can bind in certain conditions if the extra clearance is not there. Your overheating problem may have an effect on the guides.
AR Terry

Ok. According to your contributions I should check: hot coil, obstructions or air inlets in the fuel line from the tank, ignition time for a fast road camshaft. And I clarify: the spark plugs have a correct color except when this problem occurs, the float chamber is full when this problem occurs (stop the engine when it occurs); the lid of the tank is ventilated and not blocked; the problem does not improve by lowering the nut of the injectors or enriching the mixture with the choke; the distributor is a 25D with points; the problem occurs under sustained power load, for example, an climb of 8% causes the problem in about 5 minutes at 50 mph.
Gabriel Martínez

Reading back through this, I think AR.T is onto it
Valve guides too tight and holding the valves open a tiddle when it gets hot
That would explain your valve noises and lack of power and why pulling the choke etc makes no difference
For me, I still think the fuel supply is a bit on the low side but at 50mph it shouldn't be causing the issue
You have checked fuel level in the float bowl when it happens and your ign. timing is ok so it has to be mechanical
Have you used stainless valves by any chance,
Stainless exhausts do need a little extra clearance as do bronze guides and if you happen to have both they need even more clearance

Also- Check that the TDC timing mark is correct with the piston at TDC--I've had pullies slip on the damper rubber and put the timing mark out causing overheating

willy
William Revit

I will see if the temperature goes up before the problem occurs or is the other way around. In the first case there would be a mechanical origin, in the second it would be poor performance of fuel.
Gabriel Martínez

Gabriel
When I mentioned the valves grabbing in the guides when it gets hot, I didn't mean the actual engine getting hot
I meant the valves getting hot from the load
When the car is heading uphill under load the valves will be hotter than when running along on the flat
Running under load the heads of the exhaust valves run red hot and after running like this for a while the heat gets into the stems and if there isn't enough clearance in the guides they will stick
This can happen without the engine coolant temp. rising above normal
Your coolant overheating problem is a different issue and needs to be fixed

Blocked radiator---faulty thermostat
Are you running the belt driven cooling fan
William Revit

Great breakdown on the bronze valve guides issue William. Makes perfect sense to me, all seems to add up.

It stuck in my mind when I had the head converted to unleaded that the engine builder didn't like the bronze guides as its difficult to judge what the correct clearance should be to allow for different expansion rates of different materials.
AR Terry

that all points to an issue with too much advance in the distributor under high vacuum conditions (under load) - try retarding the timing by a few degrees and try again
Dominic Clancy

Hello. Today I checked the car again. The temperature rise only with de problem, and after of the problem if I don´t stop the engine, not before. If the overheating came from the valves, temperature rise would be observed and then the problem would come.
I have asked "Burlen uk" and they indicate possible mismatch between the camshaft and the carburetors. My camshaft is a fast road of Brown and Gammons. They also indicate possible failure in ignition or low fuel (7 gallons per hour: 440ml minute).
This morning the coil was hot but same all the metals around the engine.

I have removed the air filters and the problem has worsened. It is clear that the small air restriction of the filters is beneficial for my engine. It's the same effect as the harder springs.
Gabriel Martínez

by removing the filter you are making the mixture leaner as more air will flow. you need to richen the mixture - are you sure that the choke is working correctly when you say it has no effect when you have the problem?
Dominic Clancy

Dom.
If the thing is short of fuel as is thought the choke won;t make any difference as there is't any fuel there for it to do anything
I feel the car either has this valve guide clearance problem or is running short of fuel causing overlean mixture (supported by the white spark plugs)
Maybe a combination of the two

"
Gabriel Martínez
Spain

Posted 18 March 2018 at 21:42:04 UK time

One more clarification. The engine can go up more than 4500 rev. No problem. But you can not keep them for 1 minute on a flat road or half a minute on a road with climb. "

Maybe fit a fuel pressure gauge to it and see if the pressure drops off when the fault is happening
William Revit

Two things Gabriel, one contributing to the other. A leaning out mixture will cause overheating, the higher temperatures expand the metals, including the valves and guides. Worth bearing in mind is what some don't realise, the fuel entering the combustion chamber does have a cooling effect, particularly on the inlet valve, cylinder head and the piston crown.
Allan Reeling

Try taking a fuel pump that you know works maybe out of a running MGA and a 5 gal can of gas pump the gas straight from that can to your carburetors. See if that solves your lean running.
S E Bryan

Everyone has focused on the fuel flow issue - have you checked that you haven't got an air leak somewhere?
Do you have a brake servo fitted? or a manifold vacuum valve?
Chris at Octarine Services

Hello. I think the problem has not been understood. The engine always works at 180ºF (82ºC). If I raise the demand does not rise in temperature. The spark plugs have a correct color, the mixture is correct. On secondary roads, where speed is accelerated and reduced, I can raise revolutions without problem, but on the highway, where these speed changes do not occur, it does not maintain 4000 rev. 70 mph more than a minute. At that time it gives symptoms of drowned and low power. If in those conditions I insist and try to continue then temperature increases. The spark plugs are now white and show a weak mixture. This is the order of the process.
Gabriel Martínez

I think most of us do understand. If there is a fuel delivery issue it will be less evident if fuel demand varies, i.e. "where speed is accelerated and reduced", fuel flow can catch up with demand, but a constant demand will show up a delivery problem, fuel will drop in the float chambers and the mixture will go lean as a result.
Allan Reeling

Lost the will to live one this one...... great response from everyone as usual trying to help...
AR Terry

At constant speed in damp / humid conditions the carbs CAN suffer from icing up - evaporating fuel will drop the temperature by 20 or more degrees.

My V8 suffers from this occasionally.
Chris at Octarine Services

I think that, although the pump carries enough fuel (in theory) and there is fuel in the float chamber when the problem occurs, but the problem may be insufficient pressure for a reconditioned engine that is no longer like the original. I bought a electrónico FACET COMP pump and I'm going to try it. It will arrive in about ten days. I will tell you the result.
Chris. The problem happens in weather dry and wet, with cold or hot. 45ºC in summer and the problem existed.
Gabriel Martínez

Feed pump rate - on my Jetta, avg consumption 7.5 L/100 km, I see it go as high as 50 L/100 km climbing hill or just accelerating. This is 500 ml/km. At a speed of 70 km/h or 1.17 km/min, this would require 585 ml/min if pushing hard.
However, I only see that figure of 50 at relatively low speeds, taking off uphill, never when at speed.
So, I would think that 450 ml/min would be OK.
I'm still voting for sticking exhaust valve (s)
Art Pearse

Gabriel please could you tell us what the engine water temperature is when the problem occurs
John.
J Hughes

Gabriel - Fuel quality is a constant which we seldom question - my garage report no end to the problems caused by stale fuel. They suggest that one should discount all test results which have been conducted with fuel that is older than six weeks.

This may not be the cause of your problem but I hear that your fuel may contain up to 10% alcohol. One possible reason for its short life.

Working on the principle that the cause of many problem stem from the last change one has made - I would be concerned about the operation of the valves. Questions such as tappet clearance and camshaft profile/quality come to mind.
Roger Walker

Fuel quality is a good point and I was wondering about the quality of the oil and whether the engine is starting to lock-up – water temps may be an indicator.
J Hughes

Pressure is not the same thing as flow rate. If you are going to install a Facet fuel pump, which will flow more than enough fuel, you will also need to install a fuel pressure regulator set to no more than 3.5 psi. The float chamber needle valves can't deal with much more than that. A Facet without a regulator, will flood the carbs straight away.
Allan Reeling

Method: Same speed, same rev., Same weather, same road, same gasoline, same driver, but I put a stopcock before the carburetors that restricts the passage of fuel in half (250 ml per minute). Result, the problem appears in twenty seconds instead of one minute. I love science. However, there may still be more factors involved. We will see them.
Gabriel Martínez

Water temperature please.
J Hughes

On Facet pumps, they are like anything else
There are several to choose from, I think they have about 4 different pressure ratings in that little solid state pump
If the correct low pressure pump is selected a pressure regulator isn't required and I think their lowest pressure one still pumps out about 2 litres/min.
You have semi proven what is happening by reducing the fuel amount and getting it to happen earlier, at this stage it sounds more and more like a fuel supply problem and there may not be an issue with sticky valves at all
or maybe they were getting heated from the lean mixture as well and playing a part- but we'll see
Good luck Gabriel ,I think you're on the right track with the fuel pump
William Revit

There are too many variables in this diagnosis. Personally I would put it all back to standard and change one thing at a time. (Well all except camshaft as that is too difficult) This would follow standard diagnostic principles.

By the way I am sure Facet won’t do any harm but there is no doubt a correctly working SU pump and good piping will not cause this problem. I would visually check every mm of the pipe route particularly from tank to pump as any issues there wouldn’t show leaking petrol but invisible air entering system.

Paul
Paul Dean

Gabriel
if the problem occurs even sooner when you limit the fuel flow then I would think that it kind of proves that the issue has to be fuel flow related to some extent.

I once suffered similar problems when some clear silicon sealer was floating around inside a fuel filter and, after a couple of miles of driving, it would be pushed over the exit of the fuel filter and block the fuel flow.
Then after the engine began to cut out and stop, the silicone would float down out of the way and the fuel would flow again allowing the engine to start again.

The silicone was semi transparent inside the fuel filter and it was virtually invisible.

Cheers
Colyn
Colyn Firth

I don't think the stop cock test proves anything. Isn't the result to be expected on a well performing car?
Art Pearse

Hello. Temp. In post day 20 and 23.
The pipes is clean and free of obstructions. Not inlet air. Clean with compress air in the two direction. After, test of flow: 450ml mi.
Gabriel Martínez

If you want to prove this fault, a simple way is to run the car out of a can on the dash and gravity feed the fuel via a pipe onto the carb inlet, this will eliminate everything up to the carbs, I have used this method quite a few times over the years, with good success, from minis to vehicles with 3.5 ltr V8s in them, a lot quicker and you can prove the fault.
Andy Tilney

Bit difficult Andy barrelling down the road at 60 m/h!
Art Pearse

not if you get someone to hold it in place,
Andy Tilney

'Hello. Temp. In post day 20 and 23'
No No it should be ca 170F / 76C. Read it again!
J Hughes

As Mr Revit pointed out Gabriel, whether a regulator is absolutely necessary depends on the model of Facet chosen. Personally I always fit a filter king, as it says it's also a filter. But do check the output pressure of the one you have bought.
Allan Reeling

Gabriel

I was talking to a very experienced restorer, particularly on SU based MGAs and E-Types, who is currently doing some work for me so I thought I would relate your issue in case he had any new ideas.

His main response was slightly low fuel in float chamber. That would be OK when doing variable driving but would struggle with constant higher revs. I know you say chamber is full but he is only talking about it being a bit low so suggest you check setting of valve as per workshop manual. He also mentioned blocked air filter but this sounds fine from what you say.

Also on fuel pump flow he said you really need to check flow flow against pressure as sometimes it is fine against no pressure but doesn't cope with back pressure. His suggestion is you restrict the outlet pipe so it is a 'strong squirt' rather than more of fast dribble. What he does is reduce the pipe diameter by inserting a bit of brake pipe into outlet but there are other means. He isn't a fan of Facets by the way.

I know Allan Reeling has mentioned these points but fuel chamber level may need a more precise measurement.

Paul
Paul Dean

Not convinced. The higher the needle/ piston runs, as it would under load, the less susceptible it is to minor level changes.
Art Pearse

There was indeed fuel in the float chamber but I can not assure that it was full. When the pump arrives I can leave doubts checking all these variables. To reduce the pressure or reduce the flow rate I can use the stopcock of which I spoke or an additional filter. We will see it. If a pressure regulator is necessary, I can also put it, but everything in its time.
Gabriel Martínez

Good idea to check the float levels while you are waiting for that new pump--
Your stopcock may well reduce the flow (don't know why you would want to do that when flow is probably your problem)but it won't reduce the pressure at idle where excess pressure could be a problem if the pump is too strong-
If you have got enough pressure to push the needles off their seats a regulator will be the only way of fixing that-OR the correct pump
--Which one have you ordered-?
William Revit

I think you are going around in a circle Gabriel, so lets go back and start at the beginning again
You say you have had the car for a short time.
1. So did you have this problem from day one?
2. If yes was it a standard tune 1600cc lump?
3. If not did you fit the fast road Brown and Gammon camshaft?
I’ll wait for your reply before carrying on.
Regards
John.
PS. If you now know what the engine water temp was it would help.
J Hughes

Unfortunately the new pump has not solved the problem. I do not know what to do. In some way it has to do with the fuel, change springs, needles and put air filter, delays the appearance of the acceleration cut; restricting the passage of fuel advances the appearance of the problem. While someone comes up with something I answer questions and give new information in case it is useful.

With the new pump the engine works well at idle. I have had the problem since the first day. The engine is a 1622 with new pistons, valves, valve springs, camshaft, bearings, ...;

The new information is that I tested the car in the same place, at the same speed, ... but I have taken a second pass and the problem has appeared later. It is as if the engine needed to run for a time to respond better, or as if it were cold.
Gabriel Martínez

Gabriel
Have you checked the setting of valve in float chamber as per workshop manual yet as this would explain your problem. Just saying there is fuel in chamber isn’t enough it is all about level. Have you got details of how to do this, if not I can send you them.

Paul
Paul Dean

I have just shown my local village garage man this thread and his immediate diagnosis is very much along the lines that Alan Reeling described in the 13th post.

He told me that he had an MGA in a few years back with similar symptoms. After a few basic under the bonnet checks he went straight to the fuel tank. The bottom of the tank was badly dented from either bottoming the car on speed bumps etc or, as he thought more likely, a previous owner had used the tank as a jacking point. He then diagnosed that as a result of the dents the fuel pick-up point was closer to the bottom of the tank, picking up the larger pieces of crud (debris) rather than just soft sediment. i.e. not necessarily small bits that get sucked up into the pipe, but larger bits that under high fuel demand get sucked towards the pipe inlet. This caused fuel flow restrictions at high demand and could even result in cavitation.

Having decided on the cause he replaced the tank and the problem was solved. Out of curiosity he then cut open the old tank to have a look. His suspicions were more than confirmed. The denting had not only lowered the fuel pipe pick-up point but had actually made contact with it, bent it and slightly kinked it.

Steve
Steve Gyles

Been a while since I posted my prior note on measuring fuel flow. I can't believe no one has picked up on this yet. Gabriel has measured the fuel flow at 450-ml/min. Problem is the same with new fuel pump as it was with the original fuel pump. The flow rate he measured is only 60% of the flow rate specified in the Workshop Manual. It should be more like 750 ml/min, or 1.6 US pints per minute.

The Workshop Manual section D.6 describes a method (and tool) for more accurately measuring fuel flow with the commonly expected pressure head. Fuel delivery is specified to be 10 Gal/Hr Imperial, 45.4 Liter/Hr, which is 12 Gal/Hr US, 1.6-US pint/min, with 48-inches output lift (1.83-psi back pressure)

No cheating on the fuel flow. If you don't have that much fuel flow, go back and look again for an upstream flow restriction. Disconnect the output pipe from the pump and measure the flow rate there. The restriction may be in the pipe between pump and carburetor. If the flow is still sub-standard at the pump output, then the restriction will be before the pump.
barneymg

Well, I have removed the caps from the floats and I have measured the amount of fuel that enters the two float chambers and the result is 1.6 lit. per minute, 96 lit. per hour. If there were any restrictions this amount would not be possible, and if there were an occasional restriction on the deposit, the problem would not occur at the same speed, same rev. and same place on the road, it would be a miracle that this happened, the normal would be a power drop under any circumstance.
Gabriel Martínez

Gabriel
Sounds like you have an adequate fuel supply now--I usually aim for 250-300 ml in 10 secs as quoted earlier so you have that now with the new pump
Have you measured / checked the float levels, and checked to make sure the timing mark is on tdc when the piston is at the top of it's stroke
Also to check that fuel flow isn't dropping of a temporary fuel pressure gauge "T"ed into the line up near the carbs will show if there is a drop when this problem happens
willy
William Revit

With apologies to all involved, can’t help thinking this thread is going nowhere now. So many theories from a distance likely to run full circle and start back at the beginning again. Probably would be a good idea to get an expert involved and do some rigorous and systematic diagnosis. I’m sure it would then get sorted or at least diagnosed quite quickly that way.
Personal view only of course!
Bruce.
B Mayo

Here here. Interesting to hear the professional outcome.

Paul
Paul Dean

The ignition time was the first thing I looked at months ago. All right, also the problem happens both delaying and advancing it.
I have seen in some thread about the use of other carburetors that the SU work weak at high rev. It could be an extreme case of this phenomenon.
However, I am seriously considering the intervention of a professional. I have already talked to someone and in the first place they recommend me to put weber 45 dcoe. But I would like an accurate diagnosis before doing the expense. And I always prefer originality in my cars.
Gabriel Martínez

Gabriel - Dominic asked earlier if you changed the capacitor and I didn't see a positive reply to that.

The reason I bring this up is I had a very similar problem that was caused by a poorly grounded capacitor.

Best,

- Ken
KR Doris

Gabriel
As far as I can see you still haven’t checked the float valve setting. Nothing to do with flow. Certainly it was the first thing my very experienced expert thought of when I described your symptoms.
Paul
Paul Dean

As always, I appreciate the proposals. I have not changed the capacitor but I'm going to do it. I changed the wires for cobalt recommended by MOSS, the distributor cap, the points, the spark plugs, the coil. However, I believe that the capacitor should fail regardless of the fuel and the same revolutions, but here the problem arises under increased demand. About the float level, I got it to 7mm, but nothing changed.
Gabriel Martínez

By the way, if someone is interested I have some videos on YouTube with the car. We have good roads here for an MGA.

https://www.youtube.com/watch?v=0QWSDPWF_yo

https://www.youtube.com/watch?v=eTmh0QWA9VU

All the videos in "classicdriver mg td, mga, xjs"
Gabriel Martínez

Sorry Gabriel I’ve been away since my posting of 30 March.
I’ve read all the suggestions since then and note 1. You’ve had the problem since day one. 2. The new fuel pump did not solve the problem.
3. You’ve checked float levels. 4. You are going to try the capacitor.
Ok then I’ll go back to my list of the 30th and carry on.
4. When folk fit camshafts they omit to replace the cam followers or use the special lube that came with the cam. Failure to do both of those things wear the cam lobs at a frightening rate and likely to cause just the problem you have.
If you are unable to speak to the person who fitted the cam and the other parts you mention, I agree with Bruce Mayo it would be best to find an engineer who can:
A. Measure the valve lift. B. Valve timing. C. Compression.
Just for info I’m running a 3 bearing 1800cc with a factory AEH 714 competition cam on original carbs, ignition, fuel pump etc with no problem. I suggest you put everything back to standard and do one more test before seeking out an engineer.
J Hughes

This will create a bombing but here we go--

Gariel- Have you refitted the red springs and 6's
Just out of interest what are your plug gaps set at
What ign coil are you using

Did you get around to checking your TDC mark
William Revit

A bombing!! I think I know what you mean Willy. This post has gone round and round. My "much" earlier post advised getting back to original.
I'm afraid this has got to the point where someone with experience of B series engines needs to get their hands on this car and get back to basics.
Allan Reeling

A bombing!! I think I know what you mean Bill. This post has gone round and round. My "much" earlier post advised getting back to original.
I'm afraid this has got to the point where someone with experience of B series engines needs to get their hands on this car and get back to basics.
Allan Reeling

Ok, the engine have new cam followers, metal lube special lubricant for install. The installation was made by me. Of course without hurry and following the manual step by step.
The problem occurred with the old spark plugs and with the new ones, also with the old coil and with the new sport coil.
The compression is between 145-150 per cylinder. Of course the TDC point is well placed and the ignition time is correct. What can the valve lift measure serve?
Gabriel Martínez

Measuring the valve lift would be to determine if the cam lobs were worn.
Do I understand that you've had the problem on the day the day you bought the car and before fitting the cam or any other work you have done on the engine.
J Hughes

The camshaft is new and ok., the problem is from first day.
Gabriel Martínez

Time for some lateral thought perhaps; an air leak anywhere between the cylinder head or distributor and the carbs or a wornout vacuum chamber ? Alternatively the main distributor lead breaking down under load ? or a flap inside a rubber fuel hose operating to block the line at higher fuel rates. perhaps a malfunctioning timing chain tensioner ?
Roger Walker

Roger, that was what was behind my plug gap question, I was thinking they might have been huge and overworking the coil
Gabriel
At this stage I feel you need to take it somewhere and get it on the rollers to run it hooked up and monitored while it faults
There's only so much that can be done over the net without actually touching the car-
One last thing to try would be to get a decent sized container, like 10 litres or so and run your fuel line into it for a while to see if the fuel supply actually stands up or drops off over time--cheap test and you can tip it back in the tank after
I only suggest this because of your plugs going white at the time of the fault, but having said that, plug colours will normally lighten off at cruise/light load anyway
And
I'm still interested in more of an explanation of
" valve misalignments occur"
Is this a noise or just a choice of words or--?
Cheers
willy
William Revit

Ok Gabriel I think it’s time to have someone on site with you to sort this. I don’t know where in Spain you are so might I suggest the people below will know who is close to you and help you.
Best of luck
John.

MG Contacts in Spain.

Hakan Sigemark
Boulevard de Musicos 23
03581 Albir
Alicante
Spain
Home 0034966868642
Mobile 0046705947576
mail: hakan.sigemark@mailbox.swipnet.se

MG Club Catalunya
President: Ferran Hernandez de la Cruz
Apartado de Correos 90032
E-08080 Barcelona, Spain
Tel/Fax +34 902 36 38 37
europeancontact@mgcatalunya.com
secretaria@mgcatalunya.com
www.mgcatalunya.com
J Hughes

Thanks for your help. I live in Granada, 350 km. from Alicante and 900 km from Barcelona, ​​as you can see in YouTube videos. However, references can be useful once you have tried to fix the problem here at a local workshop. This will take me several days. I will inform when I have an answer.
Gabriel Martínez

Hi Gabriel. Just been viewing your excellent videos with one eye on your rev counter and one ear on the exhaust note. Are you sure you haven’t got a rev limiting rotor arm fitted?
J Hughes

Can't stop thinking about this-
Like others on here probably,I wish I could get my hands on it
Anyway, thinking about it, symptoms are--

Starts to loose power
Gets hot
Won't take throttle
Won't respond to changes to mixture, needles,springs choke etc
Fuel supply now ok and with that new pump and AZ needles it should be rich if anything
Ign. timing is supposedly correct
And yet the plugs go white-lean
--

It could be a blocked or partially blocked exhaust
William Revit

I think Gabriel said it takes a little while at speed for the problem to occur. I think with a blocked exhaust you would not even get up to the speed where it starts?
I'm still betting on tight exhaust valves
Art Pearse

I agree Art, but if it were partially blocked causing a heat buildup over time it 'could' be a factor
I'm just trying to think of possibles from the other end of the planet
It's interesting that the plugs go white,-with a good fuel supply now and with AZ needles it should be running fairly rich, so something apart from the carbs is doing it
It can't be the valves/guides, .Originally I thought that as well and also I thought cam timing might be way out but Gabriel tells us this was happening earlier before the engine build, if I read it right, so it must be something on the car itself---------
Cheers
willy
William Revit

Maybe another parameter to look at would be the fuel consumption ? Might point to something if it is way too good.
Art Pearse

News: Yesterday I was going to visit a workshop and I filled the tank with about 20 l. It was in half more or less. A hundred meters later the car stopped. Fuel pump running but gasoline did not reach the carburetors. The filter dirty and empty. After returning home with assistance, I have decided to clean the entire system and I am doing it. However, with the inspection micro camera I have seen only some small rust spots. I will also look at the fuel suction tube.
Gabriel Martínez

Gabriel just a thought do you have a vented fuel filler cap because it seems funny that it cut out just after you had refilled the petrol tank.
Regards
David
D M SPEAK

I suppose David, the process of filling stirred things up a bit. But if we go back in the history of this posting, s**t in tank was mentioned many times!!
Allan Reeling

I have seen what alcohol did to the fuel hose in our 1800. As we head for 10% in our fuel - could this be a warning for us all.
Roger
Roger Walker

Why not run a temporary suction line from fuel pump to a clean fuel drum sitting in the passengers seat to eliminate the dirty fuel tank possibility.
This is the most frustrating thread on this forum for sometime. Wouldn’t it be good to have Barney on site to bring some order to the fault finding process!
Mike
Mike Ellsmore

Tip that car upside down and give it a severe shaking!
Barry
Barry Gannon

Most on here agree that the basics need to be covered first, which is where advice given on here has been based
Now Gabriel has been whacked with some grotty fuel just to confuse things
Hopefully he can find something out of order while he's cleaning the system out
William Revit

Cleaned the fuel system, tank and pipes. Today I go back to try and it seems that the last engine stop was due to fuel blockage: it did not reach the carburetors. But the main problem continues and it has happened again, 4000 rev (70 mll per hour) and in one minute the acceleration drops and the engine gives stop symptoms. I forced the acceleration and stopped with metallic sounds and explosions. For those who wanted to know what the valve maladjustment like, I attach photos (see valve 4).

Gabriel Martínez

Another photo

Gabriel Martínez

Wow. So #2 cyl exhaust is not functioning! Push rod is floating around. Has it been like this all the time? If so, how could it sustain normal driving?
What is the actual tappet clearance on this valve (and all the others for that matter!
Art Pearse

30 sec. functioning so, not normal driving, stopped and adjusting the valves to 0,015 in hot. Alls valves is in correct clearance, but not the number 4.
Gabriel Martínez

#4 valve is exhaust valve for #2 cylinder. If it is backed all the way off, and it still does not have specified clearance, then you have a severe case of valve recession. That calls for a valve job, and hardened steel valve seats to be installed in the process.
barneymg

Gabriel
With the pics-
Is this how it was when you pulled the cover off, with the adjuster right up and the pushrod out
It's a bit confusing as one pic shows the locknut right up as well -- while the other has the nut down against the rocker and the pushrod out

To me now the plan would be to slide the rocker along and get the pushrod out--giving it a good wriggle as you lift it to make sure you don't lift the cam follower out of it's bore, then check that the pushrod is'nt bent
Refit or replace it as required then adjust the valve clearances to spec. and see if that adjuster sits the same height as the others-

It's looking more and more like a sticking vales problem
Were they stainless steel valves that were used in the rebuild

Has this happened before, or was this what you meant by 'valve misalignments occur'
William Revit

The valves are Moss " stellite faced".And yes, this is what I want to say with misaligned valves. The photos are to show it, because this only happens if I force the acceleration when the engine comes to a stop because of this problem. If I do not force the acceleration this does not happen. The pushrod is straight.
Gabriel Martínez

Gabriel
When your new bronze valve guides were fitted, can you or your machine shop remember how much clearance they gave the valve stems
With the bronze guides you need-
.0015"-.002" for inlets and
around .003" for exhausts
William Revit

Actually, rethinking this, -because of the thickness of the material in the bronze MG guides(they have a fairly large od compared to modern guides)--I'd probably aim for-
0025"-.003" for inlets and
.003" for the exhausts

Not much choice here ,the head is going to have to come off and get the guides machined out-
With the bronze guides you need this extra clearance because the guides grow inwards as they get hot.
- but the extra clearance isn't an issue as it will be at normal clearance value at running temp and being bronze isn't prone to wear like a standard guide so will stay in spec for longer

willy
William Revit

Okay, but what I do not understand is why the problem improved with yellow springs, weight on the piston and needle AZ on the carburetors. If I remove all this and leave red springs, without weight on the piston and standard needles, the problem occurs at 3000 rev. and 60 ml. per hour with the same valves and guides.
Gabriel Martínez

Clearly when driven hard, and the temperature increase, the valves and the valve guides expand. The insufficient valve guide clearance causes the guide to pinch on the valve stem, holding it open and letting the push rod jump out of place when the rocker comes off the camshaft.

I think it is just coincidental that the other changes (springs & needles) appear to cause it to happen; they may be affecting things, such as fuel burn temperature in turn causing hotter valves & seizing up. The yellow springs are likely stronger, so managed to pull the valve closed for longer when the tolerances were lost due to heat expansion, until they also, couldn’t close the valves.

As Willy says, cylinder head off and bigger guide clearances.

Regards
Colin
Colin Manley

Yep
I reckon the heavier springs and richer needles are enough to richen the mixture enough to keep things cooler a bit longer compared to the std. needles

willy
William Revit

As others say clearly head needs to be taken off and properly sorted. Nothing else is worth discussing for now.

Paul
Paul Dean

Agreed Paul! Head off, fix it, put all the other bits and pieces and experimentations back to bog standard configurations, then this discussion closed!
Bruce.
B Mayo

However, before opening the engine I need to be safer. I ask if anyone knows: The white spark plugs indicate poor mix in those revolutions. How do I fix this ?, because poor mixing means overheating and therefore a stuck valve. You have to consider that if I put the mixture richer, at idle I have black spark plugs.
Gabriel Martínez

Maybe your mixture is weaker at higher revs/loads.

You need a different needle, rather than adjusting the idle mixture with standard needles.
Dave O'Neill 2

I would solve the cause first rather than addressing the symptoms. Get the head correctly machined so there is no chance of seizing the valve. Then tune the carbs using as close to standard as possible. Once the problem is then proven to be cleared consider improvements to suit your own performance requirements.

Steve
Steve Gyles

Gabriel
Plug colours will naturally lighten up at cruising speeds
Don't forget you had a low fuel supply problem which replacing the pump fixed
Perfect plug colours are achieved at perfect mixture under full load, full throttle with a mixture around 12.5:1- At cruising speed the mixture could be back as far as 15:1 and the plugs will naturally be a lighter colour

I understand your concern, thinking that the white plugs means lean mixture and heat causing the valves to stick but the valves shouldn't stick under any conditions

head off first
William Revit

If most of your mixture is being pushed out of a valve which is stuck open, it will be running extremely lean.
Allan Reeling

Hi Allan
Yes I agree but I'm not sure if it would be happening long enough to effect plug colour unless maybe the valves are being held a bit before they actually stick enough to be noticed
The head needs sorting first and then 'if' there is a remaining issue it can be sorted out after, but I feel all will be well
Cheers
willy
William Revit

In fact, the valve was sticking. i have rectified the guide and the valve is slides correctly. also i have taken the occasion for verify the remainder of valves and were well. Not there is recession of valve and the seat was well. all the seats are well. But I'm sorry, the problem continues. Can that of some way the problem has fact block the valve of a form easier, but is clear that this not is the cause. but a effect. Not i want generate frustration but after so much debate I think it would be very useful for this forum apart from me to find the cause of this problem. I appreciate all the contributions, more when I see a sincere effort to help.
Gabriel Martínez

OK Gabriel. Sorry to hear it is happening still. But do you still get the bad mechanical sounds you reported previously?
Art Pearse

Wow. Been a month since I poked my nose in here, and this problem is still not solved. I just spent and hour re-reading all of the prior posts and considering the logic. Gabriel has been following up on most of the discussion and checking everything that was suggested. From the entirety of the discussion, I would say the answer is, "None of the above". It appears to be quite certain that you do not have a fuel feed, carburetion or ignition problem. I will conjecture that you can take the car out, drive it casually around town for a bit to stabilize running temperature, then do a full throttle high revs (up to 6000 rpm) drag race acceleration run from zero to 60-mph (100-kph) with no problem.

The problem occurs when you are running at highway speed for a while, or hill climbing at road speed. This means heavier throttle setting for more power output for an extended period of time, like 4000 rpm for a minute, or 30 seconds up hill at full throttle. This is the condition that will generate the most heat in the combustion chamber and in the cylinder head. You also have a picture of a displaced pushrod where the rocker adjuster screw ball end has hopped out of the pushrod ball socket. If the lock nut was loose and the adjuster screw backed itself all the off, you would have a notable loss of power and some valve noise, but the ball should still not jump out of the pushrod socket.

There are a couple of mentions of the Fast Road Cam and also bronze valve guides. These can present a few different problems. First the bronze guides have higher rate of thermal expansion than steel or cast iron (one or two people have mentioned this issue). The guide is tight press fit in the iron cylinder head, so when the guide expands at higher temperature it will constrict inward to seize on the valve stem, causing the valve to hang up and not close as quickly as it should (or maybe not at all). When this happens it immediately reverts to running on three cylinders, which seems like you just lost half of the power output. That is a very dramatic drop of torque, but it is still running on three cylinders, possibly backfiring or having explosions in the exhaust system. Get off the throttle, and by the time you pull over to the side of the road it may clear up and be running normally. This will only occur under conditions of high throttle for an extended period of time. The heavier the throttle setting and the high the engine speed, the quicker it will happen. But ease off the throttle and let it cool down for a very short time, and it will go back to normal running. When the valve hangs completely open the pushrod will see 1/4-inch end clearance (full cam lift), and it can get disengaged from the rocker arm.

You will not see much of a change in coolant temperature when this happens. The point we are concerned with here is the exhaust valve and the valve guide, and the cylinder head material immediately around the valve guide and in the exhaust port. These parts run at significantly higher temperature than the coolant near the thermostat (where the temperature sensor resides). With full throttle operation you can easily generate at least 50dF higher temperature in the metal around the exhaust valves, but turbulent coolant fluid stabilizes the coolant temperature by the time it gets to the temperature sensor location (so you don't see the sudden spike in temperature of the exhaust valve guide and port in the head).

Been there, done that, the first time that I had bronze guides installed in my engine. The solution is easy enough, but does require disassembly of the cylinder head and valve gear (so you buy another gead gasket). Remove the valve springs, and tip the head right side up, and all of the valves should fall out of the guides by gravity alone. You should not have to push the valve to get it into or out of the guide. Iron guides should have 0.0015 to 0.0020 inch (minimum) diametrical clearance. This is at least half the thickness of a sheet of paper, so if you hold the valve head or end of the stem and wiggle it, you should definitely feel and see the side to side motion of the valve stem. If it doesn't wiggle, it's too tight.

Bronze guides need a little more clearance, like 0.0025-.0040 inch diametrical clearance. This is going to seem like a lot more wiggle, to the point where you may be concerned it may be too loose. But in this case, looser is better. Do not be concerned about the looseness of bronze valve guides. They are very durable and will have a very long life without wearing our far enough to cause any problem. A tight iron guide will wear out to be sloppy long before a loose bronze guide will wear that much.

You can fix this tight valve guide problem at home in short time. Get a used valve, and cut the head off of it. Chuck the valve stem in an electric drill. Smear a dab of valve lapping compound on the stem, and run it gradually into the valve guide at full speed of the power drill. Do not jam it in hard to stall the drill. Go at it easy and let the abrasive do the work. Three to five minutes in each valve guide should be enough to clean them out to good running clearance. When you can smear lapping compound on the spinning stem tool and run it thorough the guide with little effort, it's done. Wash the guides clean with solvent, and try the valves in there again. A nice little wobble at the valve head would be good news. Clean up, reassemble, and take it for a test drive. Drive it as hard as you like, full throttle racing, or a good fast hill climb up the mountain, and it should run like a champ and never hang up.

The next issue is the Fast Road Cam. This is likely to have either higher valve lift or significantly more duration (or valve timing overlap). The high performance cam often calls for higher compression ratio for best performance. There will be a bunch of questions about what you had to do to get higher compression ratio, like flat top pistons or shaving the bottom of the head to reduce chamber volume. Even if you do not shave the head, higher valve cam lift will push the valves closer to the top of the engine block, and you do not want the valve heads to ever touch the block.

You should maintain at least 0.060" (inch) clearance for intake valve at full lift, and 0.080" clearance for exhaust valve head at full lift, and 0.100" clearance between any valve head and the moving piston at point of closest approach. The piston is no problem here, because the vales are not at full lift when the piston is at top of stroke. For the 1500cc engine, the intake valve could hit the block, but for larger displacement (stock) engines the intake head valve falls entirely within the cylinder bore. For all of the Austin B-series engines, the exhaust valve head always lies over the top of the block outside of the cylinder bore. If there is not enough clearance for the vale head at full lift, you need to cut "eyebrows" in the top of the block to remove the material that would interfere with maximum valve motion.

The point of all this is that if you do anything non-standard with one of these engines, you need to know exactly what the valve head clearance is at full valve lift. If you don't have this information, ask the shop that built your engine. If they can't tell you exactly what the valve head clearance is at full lift, then you need to measure it to be sure. If the valve head runs too close to the engine block you can end up with bent valves, which is not only annoying but also expensive. If you don't know how to measure valve head clearance at full lift, just ask. It ain't rocket science.

The other issue with a performance cam is the combination of long duration, larger valve timing overlap, and intended higher engine speeds can run you into valve float. This happens when the valve springs cannot push all of the valve gear home as fast as the cam lobe drops, so the valves will not close quick enough. The most common solution for this is heavy duty valve springs. Less common perhaps is reducing mass of the valve train parts, like using the later model short bucket tappets and longer pushrods from the 18V engines, or maybe installing alloy spring caps. If you get all of this right, and it runs well, but you get valve float at maybe 5500-6000 rpm, then you need stronger valve springs.

Let us know when you can report the exact numbers for your valve stem to guide clearance, and valve head to block clearance at full lift. If you don't know these numbers, I would advise you not to drive it again until you do know. Best of luck at turning it into a real MG again.
barneymg

Barney,
" I would say the answer is, "None of the above"" ----That's a pretty big statement--
What we need to know is, did Gabriel actually measure the stem clearance while the head was off--He mentions that it 'was' sticking but now slides ok and checked the rest, but as we know it needs extra clearance with the bronze guides---------Did that happen-????
Also ,the shop that fitted his guides should have a Sunnen guide hone or similar to do the job, without resorting to the valve grinding paste to do it, -sounds a bit dodgey to me
William Revit

I had my bronze guides broached with sized ball bearings pushed down them.
Art Pearse

Yeah, it may be a bit dodgy. The head I did that to only lasted a quarter million miles on three different engines before it sucked a valve last year. The valve that was causing me grief had to be tapped out with a hammer. A couple others needed a good push to move, and the rest had very little clearance, hardly enough to see any wiggle (even though they would fall out by gravity).
barneymg

Dodgy probably wasn't the best choice of words on my part-
Your valve grinding paste method obviously works, but the point I was making was that we should probably advise correct machining practice first and alternative methods second
No offence intended --
Cheers
willy
William Revit

Most engine builders will press in the guides then ream them to the required clearance.
Allan Reeling

The mechanic who rectified the engine believes that the cause is poor valve lubrication. I have explained your proposals and the changes I have made in the fuel supply. He proposes to me to review the lubrication system. In fact I could see how the valves slid with their own weight in the guides when this work was done. Poor lubrication increases the temperature and dilation of the materials occurs. I'm going to check the lubrication system. He also proposes that I remove the valve seals.
Gabriel Martínez

Gabriel
Your engine man needs to listen to advice -
Having the valves slide under their own weight isn't enough clearance when using the bronze guides--You "HAVE TO" have the extra clearance as the clearance reduces as the engine warms up-
With the old cast iron guides the expansion rate is the same as the head and you can get away with small clearances, BUT
With the bronze guides the expansion rate is greater and because they can't expand outwards into the solid cast iron head, they expand inwards and reduce the clearance with the valve stem
I advised 2.5-3 thou for inlet and 3 for exh
Barney went a bit more at 2.5 - 4
I'd be leaning towards Barney's advice, it sounds a lot but at operating temp it will be back to normal clearance
As far as the poor lubrication theory goes, Your man is wrong--The whole reason for running bronze guides is because they don't need lubricating--If you remove the seals you will have smoke.

I know it seem wrong to open new guides up to these clearances but it is what you have to do if you want to fix it

Nothing to do with MG but I had the head off a late model variable timing BMW the other day to replace cracked valve stem seals,A common fault with these- It was low mileage and with the springs off the valves wobbled around in the guides like they must have had 6-7 thou clearance
Phoned the BMW dealer to ask if they had an issue with them and he said they are like that new, and "don't put tighter guides in it, the valves will grab"
willy
William Revit

Gabriel

You must get your mechanic to listen to what Barney and Walter are saying otherwise we will be going to message 200!

I had a couple of valve oil seals fail once and it quadrupled my oil consumption

Paul
Paul Dean

Up to a point there might be something in the lubrication issue, particularly regarding the exhaust valves. Obviously these get hotter as do the guides, as a consequence I leave the oil seals off the exhausts.
BUT three questions you need to ask your "mechanic";
1. Did he ream the guides AFTER pressing them in. The process of installing them reduces their bore slightly and the press upsets the top end, depending on how tight the interference fit is
2. Did he use chromed valves. According to Peter Burgess non chromed valves tend to gall in Bronze guides.
3. Did he consult the suppliers/manufacturers of the guides. Bronze is an alloy and as such is a variable commodity with different expansion rates and lubrication needs. They would have told him the clearances to run and whether they recommended chromed valves.
Allan Reeling

I keep all the comments in mind and I'll tell the mechanic. But we must consider that I have enlarged the hole of the guides up to 8.7mm taking into account that the stem of the valves measures 8.63 mm. I think it's the right thing to do, but in order to dismantle the cylinder head again I have to make sure, given the time and the work it needs.
Gabriel Martínez

That's .0028 inches - too tight by the experts comments here
Art Pearse

I did some spreadsheet calcs, assuming the head runs at 200F and the exh valve stem and guide runs at 710 F Not sure where these came from, maybe someone knows a good number.
The head expands normally from 60 F and the guide thickness grows inward
If you start off with .002 clearance cold, its all gone.
For a steel guide you would have 0.4 thou running clearance
The valve stem grows 1.5 mils and the head bore grows 0.5 and the guide thickness increases 1.0
It seems that you should allow at least an extra .5 mils clearance for bronze guides.
Art Pearse

I hope I haven't missed it but nobody appears to have suggested using cast iron valve guides with bronze liners. That's what a cylinder head specialist recommended and fitted to my engine as the solid bronze guides need too much clearance due to their expansion and are then half worn out before you start. These have been perfect. He also diamond machined the valve seats with three angles for a better seal, better valve cooling and a better flow. So no horrid grinding and the seal was simply checked with a vacuum test reading once the valve was dropped in by gravity. I assume these guides are still available.
Pete
PeteT

Pete these are the K-line inserts, too late now he has bronze guides.

http://klineind.com/k-line-service-tools-3/valve-guide-liner-system

Mike
Mike Ellsmore

I have not said anything because I'm waiting for a new Moss head gasket, but I've machined the exhaust guides to 8.8 mm. The others do not need machining, they looked greased and sliding freely, however, in the assembly of the engine all had the same measure. For some reason the exhaust guides have been tightened to the valve, probably due to overheating. When I can use the car I will inform about the results.
Gabriel Martínez

Gabriel
Rather than advise guide diameter can you advise what the actual clearances are between valve stem and guides are?
Mike
Mike Ellsmore

He said 8.63 mm valve stem so 8.8mm bore gives .0067" gap
Art Pearse

8.8 mm is about 0.1mm clearance which is double the factory for cast iron guides. But don't grease the valves when you assemble, use engine oil, or my favourite, a mixture of STP and engine oil.
Allan Reeling

Pete / Mike
The K-Liners are great but you do have to be carefull on some heads , If the guides are tapered off fairly thin in the port the original part of the guide can crack and fail
On the exhaust it doesn't cause damage as the broken off piece of guide gets blown down the exhaust without an issue but if it's the inlet there can be a disaster waiting to happen
If it is a pointy guide it's best to cut the thin part of the guide back till it's a bit more robust
Fantastic on T type heads where the guides are nice and chunky
William Revit

Found a picky

William Revit

I find it incredible and I still have not overcome the feeling that at any moment I could fail again, but I think it's true, the problem is solved. I have put back red springs, standard needle, and the car is driving wonderfully, it goes softer and faster at the same revolutions, maintains speed and accelerates when I ask it. My calculation has been correct, now we have good weather here and I am going to enjoy the car. Congratulations to those who were successful in the diagnosis, but I am very grateful to everyone for the simple fact of worrying about helping me. Thank you.
Gabriel Martínez

Really!!!!
Bruce.
B Mayo

Gabriel, we are here to help!
Art Pearse

Am I reading this correctly - the problem was down to the valve guides?
J Hughes

Correct, the problem was the guides of the exhaust valves. When the engine was rectified all the guides had the same measures, but for some reason, those of exhaust, they have had to expand and narrow up to imprison the stem of the valves. On the way, increase the fuel in admission I think it had a refreshing effect as discussed here, and delayed the problem from 3000 rev. at 4000 in sustained march.
Gabriel Martínez

Many thanks Gabriel.
It seems despite all the years working on these engines every now again a new issue crops up.
Enjoy all the extra rev’s and power.
John.
J Hughes

This thread was discussed between 17/03/2018 and 28/05/2018

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