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MG MGA - Bad Misfire Under acceleration

Hi

I have an MGA with a 3 bearing MGB 1800 18G engine which seemed to run fine until I had to replace the clutch which I did no problems. Now however when I try and accelerate hard the engine misfires and spits through the exhaust like it is lean, when I ease off the problem stops. I can accelerate under light acceleration but any touch on the throttle and the problem returns.
So far I have done the following:

Replaced the distributer with new.
Set the timing with a strobe.
New plugs
New ignition leads
New rotor
Set rockers
Checked valve timing
Renewed all inlet /exhaust gaskets
Checked drop of needle assembly
Changed oil in the dash pot (20W)
New floats and valves
New fuel pump
Checked fuel flow no blockages back to tank
Run with the filler cap open
Removed the breather control valve
Checked compression on all 4 cylinder cold 150psi hot 146 to 150psi
Checked engine earth
The car ticks over perfectly and I can adjust mixture ok so lifting the piston with the plunger checks out ok.
I can rev the car staionary no problem with the occasional misfire at high revs.

Before I go and buy new carbs and waste more money has anyone any ideas? I have posted this on the MGB tech forum but wondered if anyone here had an idea.

thanks

Barry
B Bridgens

Coil?
Art Pearse

Double check your timing and your vacuum advance (make sure there's no leak).

http://mgaguru.com/mgtech/ignition/ig116a.htm
Gene Gillam

Barry - Check that the pistons in the carburetors move us smoothly, without any drag at any point in their travel. I had a similar in our TD that turned out being a spot of dirt on the chamber that one piston traveled in. Cleaned both bells and have not had any problem since. Cheers - Dave
D W DuBois

Hi

Art it is a new coil.

Gene I sealed off the vacuum advance no change. I'll try the timing again in the morning at higher rev range as per mgaguru.

Dave that was what I first thought but I've cleaned them well twice now, no difference.

It had better not be another nice sunny day tomorrow or I might just go crazy at not being out driving. Today was blue sky 27 degrees C and I was stuck all day with my head in the bonnet.

thanks

Barry
B Bridgens

Steve Gyles favourite...check that the rear air cleaner is not upside down such that the air balancing port on the carb is blocked.
Andy Bounsall

You beat me to it Andy!
Steve Gyles

Very frustrating for you - while checking - make sure the carbs are tight on the inlet manifold and the manifold itself has no leaks.
R
Roger W

Hi
Agree with Andy/Steve
Also if it is a new coil check the polarity
Popping out the exhaust is usually electrical missfire or rich mixture not lean does it take the throttle ok before it misses
cheers willy
William Revit

Fuel starvation? Easy to check fuel delivery - just disconnect flexible pipe to the carbs, turn on the ignition and catch the fuel in a container.

I had a problem with "misfires and spits through the exhaust" and found an air leak into the inlet manifold. Worth checking. Good luck, Barry
BM Gannon

Barry
I've been thinking about your problem
Could it be that your plug gaps are way too wide
Did you set them before fitting-- .028"
All the new parts listed ,Were they ALL fitted trying to fix this problem or were some fitted while the engine was out---If so which ones , when

willy
William Revit

Hi

Willy all the above was done to try and fix the problem. The engine was not touched when out of the car. Plug gaps are correct even tried new plugs.

Today I went through the whole lot again, tappets, timing and carb tune. Went out for a run seemed fine then after about 1 mile the problem returned. I hobbled back and got into the carbs which had the air filters off and what do you know the back carb damper was stuck down and took quite a bit to lift it. I took out the needle and reassembled and the piston was as free as a bird replaced the needle same sticking. Could this cause the problem? Checked the plugs 1 and 2 OK but 3 and 4 were lean. I was going to re-centre the jet but thought I might as well replace the assembly in case of a bent needle or problem with the jet. I dont know how easy it would have been to knock the needle assembly during removal and replacement of the carbs before and after the engine lift. Anyway B & G will take a couple of days to ship the parts so will have to wait till then to find if that's the problem. Any thoughts if an intermittent stuck damper this could cause these sypmtoms?

thanks

Barry
B Bridgens

Barry

Not sure what you meant about the "rear carb damper was stuck fully down" . Did you mean the piston was stuck fully down?

Many years ago I found that if I tightened one of the three screws on the cylinder/damper pot before I had the other two moderately tight it was sufficient to cause pot to be ever so slightly out of alignment and the piston would stick. I am always very careful now when I tighten those 3 screws, a bit at a time on each, checking the piston movement as I go.

Steve
Steve Gyles

Hi Steve

Yes I had to use a small amount of force to lift the damper, the carb was hot as normal after running. When cold I progressively tighten the bell screws checking continuously that the damper falls with metallic clunk.
With the carb still hot I removed the needle and reassembled tightening the screws progressively. The damper rose and fell no bother no sticking. When the needle was put back in no matter how I tightened the three screws the damper stuck after I lifted it to the top of its travel and let it clunk onto the jet holder, it fell quite freely. Hope this is clear.

Barry
B Bridgens

You need to center the jet so it does not drag on the side of the needle.
Barney Gaylord

as Barney says - simple fix to centre it up and yes a sticking piston will cause all sorts of mixture problems
William Revit

Just to explain-
If the piston were to be jammed at say 1/4 open position
Idle and low speed running would be lean from too much air- normal cruise speed would probably be somewhere near normal mixture but open throttle driving would be rich from the choking effect in the carb.caused by the piston stuck too low for full throttle
This would give the popping through the exhaust under load
cheers willy
William Revit

Hi Guys

I am loosing the will to live with this problem.

Recentred the jets, no change but damper piston now move freely.

Put in 20/50 damper oil, no change

Blew out with compressed air the fuel lines, no change.

The engine ticks over beautifully and when I raise the damper pistons with the plunger, slight rise in engine revs then settle down. Reving the engine stationary there is no problem but as soon as I try and accelerate!*!?*. It seems to go bad at about 2500 revs.
Should the outer casing of the coil be earthed?

Time for lunch

Barry
B Bridgens

Barry

Did you set the the timing dynamically with the the strobe? i.e. 33 degrees at 3500rpm with the vacuum disconnected?

Could it be something with the vacuum advance system - leaking?

Steve
Steve Gyles

Hi Steve

I'll check again with the strobe, it is a new distributor but I disconnected and plugged the vacuum pipe and went for a drive, no change.

Barry
B Bridgens

Barry

That would suggest your vacuum pipe is leaking/blocked.

You are not getting any vacuum advance when you hit the throttle.

Steve
Steve Gyles

Barry
Did you check the polarity of your ign coil
Also if you happen to be using an electronic dizzy , have you got the correct coil for it as some electronic coils won't operate properly on std. coils
Are you running positive or negative earth-----
William Revit

Barry
Did you check the polarity of your ign coil
Also if you happen to be using an electronic dizzy , have you got the correct coil for it as some electronic coils won't operate properly on std. coils
Are you running positive or negative earth-----
Also check that the throttle is pulling Both carbs open
willy
William Revit

Barry

Just to clarify my last post. I got the impression from your previous response that you observed no difference to the backfiring/spitting etc with and without the vacuum line attached to the distributor. Thus my comment that this is the source of the problem. Had your vacuum been working correctly you should have felt an even greater degradation when going for a drive with it blanked off. During your engine out/in process you will have removed and refitted the pipe from the carbs. It's fiddly down there behind the carb and my guess is that you have cross threaded or not tightened the union.

Steve
Steve Gyles

Barry

I fitted a new conventional distributor in the Spring and I have had 2 problems with it hence don't assume new means it works OK. My problems were duff condenser, almost instantly, and last week I discovered the screw that holds the points was locking the plate below it hence no vacuum advance or fine tune.

Paul
Paul Dean

What about being a bit drastic? Try leaning carbs off 3 or 4 flats (check), richen 3 or 4 flats (check), retard distributor a few degrees (check), advance a couple of degrees (check), if these don't highlight anything then get someone else to check the car using the Mgaguru's Casual Tune-up method while you go to the pub (so not to influence the person doing the checking).
Mike
Mike Ellsmore

Paul

I concur your comments that a new distributor does not always mean it is okay. I had an original 1500 distributor rebuilt by a well known UK supplier. I had horrendous issues, not unlike Barry's. The engine went totally flat above about 2500rpm. It turned out that they had fitted the incorrect advance plate inside the unit. It had only a 7 degree cam instead of 13 degrees. This resulted in a mechanical advance capability of only 22 degrees instead of about 34 (8 static plus 14 = 22 instead of 8 static plus 26 = 34).

Steve
Steve Gyles

Hi Guys

Not been playing with car today, depressed!!!

Yesterday I took of the vacuum pipe and sucked to check the distributor advanced OK. I richened the rear carb slight improvement. Taking out the plugs 3 and 4 were lean coloured 1 & 2 normal to rich. Like Steve going above 2500 to 3000 revs feels like a rev limiter is cutting in. The distributor and coil were bought as pair from Simonbbc the distributor is a 25D which is correct for the engine. I like Mike's method of tuning but I have no-one to call on. Despite the fact that I renewed all the gaskets on the inlet side I am going to strip it down again and put in a new set. Interestingly on another site, MG Experience, a guy fitted a new stainless exhaust and had real problems under acceleration, it was a blocked exhaust. Don't think this is my problem as the exhaust although stainless worked OK before. I might also rebuild the original distributor and put that back in. This is what I thought was the original problem way back, I found the vac advance spring broken which is why I bought a new electronic. As Steve said I might have swopped bad for bad.
Back to the TD for some light relaxing restoration.

Barry

B Bridgens

Barry

You never mentioned the orientation of the rear filter box - upside down? Or even if it is up the right way do you have the correct gasket between the box and the carb that has the holes for the rear piston balancing. See here: http://www.mgaroadster.co.uk/odds_and_sods.htm

Steve
Steve Gyles

Hi Steve

For the purpose of testing I am running without the air filters.

Barry
B Bridgens

Barry

That rules that one out. You had mentioned that above but I missed it! I have been trying to restrict my thinking to the stuff that gets disturbed during an engine out job. One thing you have not mentioned is the connection between the carbs. Has that come loose?

Also you can do a quick check to see if the carbs are more or less in balance by unscrewing and removing the dampers; put two equal length pieces of rod in the tubes, then run the engine and observe that the rods rise and fall together. The SU balancing rods in the following link show the principle: http://www.mgaroadster.co.uk/su_carburettor_tool.htm

Steve
Steve Gyles

Barry, I had this trouble some years ago (see archive - "plug colour" read all of the thread). Long story short; the mis-fire was caused by the front two plugs fouling due to the front air filter over-oiling and causing richness at anything much above idle. The air filters were a heavy felt type, probably made in China, but supposed to be replicas of the original Volkes units. I replaced mine with K&Ns and the problem was solved having wasted weeks of checking just about everything else!

Picture shows plugs from cylinders 1 and 2. Although served by the same carb, number 2 seemed to suffer from fouling more than number 1. This symptom lead me right up the garden path and into the realms of everything from strange ignition faults to valve and head-gasket leaks. Try running your car without the air cleaners to see if my diagnosis is correct.

Lindsay Sampford

Hi Lindsay

Thanks for taking the time to read the posts and comment. My plugs are a bit like that but unfortunately I have not got air cleaners on just now during the diagnosis of the problem, so that is probably not the cause. I have kind of given up just now as it was becoming very frustrating as I was getting nowhere. The car is sitting in the garage very sad that it can't enjoy the good weather. I am working on my other cars just now and will probably get back to the MGA after the show at the NEC.

thanks

Barry
B Bridgens

Got to be the fuel. I had bad misfire and popping through the carb, checked everything as you have about five times. Drained the tank cleared out the carbs and filled with new fuel retuned carbs and problem gone. Worth a try before you go completely bald.

Alan
AR Terry

I'm voting for ignition. Sparking is always harder when the going is faster or harder.
Art Pearse

Barry, sorry, I missed the post where you said you were running without air filters.
Usually a misfire when under load is caused by a weak spark at the plug. This can be caused by the plug being fouled so that the spark tracks through the carbon build-up instead of jumping the gap or by a weak HT supply reaching the plug. Paul Dean mentioned a "duff condenser". There are plenty of these about these days (along with duff rotors). Make absolutely certain that you have a good condenser, most of the ones on eBay are rubbish so get one from The Distributor Doctor or similar. Also make sure that the coil is getting a good supply. With the ignition on and the points closed you should see at least 11 volts across the coil. This fault is going to be a weak spark at the plugs for one reason or another. Most fuel problems turn out to be electrical!
Lindsay Sampford

Hi Guys

Alan, I ran a pipe to a can of new fuel in the car same problem. I had already replaced both floats and needle valves.

Art, I have replaced everthing with new, distributor, leads and plugs. The only thing I can think is I replaced bad with bad.

Lindsay, I got a complete new electronic distributor from Simonbbc with red rotor and new leads from The Green Spark Plug Co. This was so I did not have condensor and points problems. I thought the initial fault was a broken vac. advance spring in the original distributor way back.

The problem still seems to be, starts and idles perfectly, accelerates under light throttle, will not accelerate when throttle is floored combined with f**ting out of the exhaust (no backfire through the carbs), lift off problem clears under light acceleration. Condition is worse above 2500 revs. When stationary revving the engine, it starts to misfire above 3500 revs with no load. Funnily enough I cannot make the problem happen in reverse gear, I guess it's a load problem.

thanks for thinking about guys

Barry
B Bridgens

Barry, do check that you have a good supply to the coil. If your car has the original ignition switch, remember, it is more than 50 years old and contacts can pick up a lot of resistance in that time. A temporary 'hot wire' to your coil will prove whether or not that is the source of your trouble.
Lindsay Sampford

Barry

Did you check the balance between the carbs that I mentioned above?

Steve
Steve Gyles

Barry, take it to a shop with a diagnostic set up - scope etc. They can monitor the spark voltage etc with it running.
Art Pearse

Hi

Steve
Yes I did that both the same.

Lindsay
I'll do that, it is a new wiring loom.



Barry
B Bridgens

Barry
Just to get that clear - Are you saying that you can't get it to happen in reverse at all even uphill---
William Revit

Just another thought Barry, does your electronic ignition earth through the distributor body as a normal points type does or does it require a separate earth feed? If the latter, is that earth good?
Lindsay Sampford

Hi

William I think I can't go fast enough in reverse to get it to appear there are no hills where I live.

Lindsay It earths through the body. I checked coil voltage 12.6v.

I have ordered a vacuum gauge to see if I can track dow anything that way, should be here next week.

Barry
B Bridgens

Barry
I'm tending to agree with Art, that the best move now taking into account that we can't come around to your house for a look would be to take it somewhere that has a scope and get your distributor output etc checked out to eliminate that from being the problem.
With it hooked up to the scope the whole ignition system can be checked in a couple of minutes at minimal cost compared to buying heaps of not needed parts
Any reasonable tuneup joint should be able to help you out with that. Give them as much info as you can before they start to save them guessing what to look for.
cheers willy

I'm still thinking it could be the coil for some reason ,but the scope would pick that up
Tha carbon post in the dist. cap isn't missing I suppose
William Revit

Barry

Looking through your list of replacement parts you do not mention the spark plug caps. Did these get replaced with the new leads or transferred across?

Steve
Steve Gyles

Hi

Willy I wish you guys could come round not only to help me with the car but you could also help me with a few bottles of wine and some fine malts. I changed the coil for a know good one, the carbon connection is still there.

Steve I do not have caps, new connectors and covers were bought to make up the new leads. I replaced the old leads into the new distributor to check that it was not the leads.

Barry
B Bridgens

Barry, that voltage looks too high! Connect your Meter across the coil then, if it is possible, arrange for the electronic 'points' to be closed or failing that, connect a good earth to the side of the coil that goes to the distributor (it would be best to disconnect the wire that goes from the coil to the electronic unit so that you don't subject it to any possible damaging current). 12.6 v looks like the open circuit voltage you would expect to find on the switch side of the coil. To check the integrity of the coil supply through the ignition switch etc, it needs to be tested under load, that is, with a full earth on the 'contact breaker side of the coil.
Lindsay Sampford

Where is the timing set?
dominic clancy

Lindsay, surely with the "points closed" you get full battery volts across the coil? There is no ballast resistor or any other significant resistance in the circuit.
Art Pearse

Art, the coil is only about 3 ohms so with the resistance of the wiring and a slight drop across the ignition switch the voltage across the coil should be about 11.5 with a fully charged battery.
This fault has got all the hallmarks of a weak spark. As you indicated in your earlier post, "sparking is always harder when the going is faster or harder", you can add to that, hotter. Practically everything has been changed, so we are not left with much (only the switch and wiring?) unless a faulty component has been replaced with another faulty component; these things do happen, but usually to me! If it were mine, I would be tempted to temporarily re-fit the old points distributor to simplify testing and eliminate any possible weird goings on in the electronic unit.
Lindsay Sampford

Barry
what are your plug colours like now that the carb piston isn't sticking and both pistons are moving together - have they all evened up or is there still a problem there-----------
Also I am still thinking about your coil
There are three types of coil with different primary resistances 3ohm -1.5ohm- .75ohm (measured across the primary posts of the coil with wiring disconnected)
It might just pay to measure what you have and check with your distributor supplier whether they supplied the correct one
Also did you get around to checking the polarity of your coil ------- If your car is neg. earth the 12v supply to the coil goes to the positive terminal and
If pos. earth it goes to the neg terminal
but I guess there would have been instructions on this with your dizzy

willy
William Revit

Barry
I have just read through all the postings on here and your responses to them and really it gets back to the fact that the problem was there at the start and all the work you have done has been to try and fix this problem ----------
Now, thinking about it, the rear carby must have got a knock either while removing/reinstalling the engine or while it was out, and it was enough to move the rear jet tube enough to move it off centre. You have now recentred this and that should be good but--
I'm thinking - could the little hose from the bottom of the float bowl to the jet be damaged ,kinked or maybee blocked up a bit.
Have a good look at the hose and make sure it isn't kinked or flat (common fault)----If it looks ok you will then need to remove the dashpot piston assy. and the top off the float bowl and blow the jet tube out from where the needle goes in ,back into the float bowl and see if there has been some rubbish blocking the tube.
Probably pay to do both carbs and then you can compare by blowing through the jet with your mouth and a piece of hose that they are both nice and clear and equal----

My thinking is that if one of the tubes happens to be damaged or blocked a bit, there could be enough flow for idle and slight throttle but when the throttle is opened the blockage restricts the flow causing it to run lean and misfire on these cylinders--

Also check for fuel supply with the fuel lines still connected to the float bowls but with the tops off the bowls,--like actual fuel supply to the bowls
If there was a blockage here, like a piece of rubbish that has found it's way into the float bowl lid tube or onto the top of the needle and seat this could also have the same symptoms on the road

Over, my head's sore now--cheers willy
William Revit

The tubes only apply if the car has B Carbs. But if you have A carbs, it is probably worth checking that the thimble filters at the carb banjos are not blocked, and that you have the correct carb needles, as the 1500 needles are going to be lean and cause a misfire under load for a 1800 engine.


dominic clancy

1500 needles on an 1800 engine will run rich---------
William Revit

Barry

The last few posts have highlighted once again the one true fault you have identified to date: the sticking rear piston. You say that you now have freedom in the rise and fall of that rear piston. Is that just using the lifting pin or checking the rise and fall over the entire range of that piston?

Thinking out of the box (desperation, really I suppose) do you have an air tight seal between the exhaust manifold and the down pipe?

Steve
Steve Gyles

No, a 1500 needle will deliver the correctly metres fuel for a 1500 engine when the carb is set correctly. It will always be too lean for the extra 20 percent capacity of the 1800 engine.
dominic clancy

Hi Guys

The prize for getting closest to the cause has to go to Willy.

I decided to take off both jets tube assemblies to check thoroughly if they were clear. When I took off the rear carb tube I found that through deterioration the jet tube rubber seal had formed a neat flap which would seal the tube under heavy fuel demand but open under light fuel demand. Needless to say I checked the front also, not as bad but clearly deteriorating (damn modern fuels). I had been blowing through the jet tube from the jet side so I was not seeing this problem as the flap would open in the reverse direction. I managed to get two new rubber seals and washers and refitted the carbs. I have just been for a 10k drive with no hint of the problem. I think it might have been coincidence that it happened after I put a new clutch in, which really sent me off on the wrong tack.
Anyway thanks for all your help and suggestions it is great to get the car back to normal running and I hope that something has been learnt from this experience.I tried to take a picture of the offending seal but it was not very clear so of little use.
Willy your prize is a bottle of wine which you can collect anytime from le Mans.

Thanks again guys you can all go and solve another problem now.

I'll be at NEC on Friday if anyone else is around.

Barry
B Bridgens

Well done Barry. How many Euros did that cost you to find that out? You never said you were using MGB carbs, otherwise you may have got Willy's suggestion a week or so ago! I think most of us who went the 1800 route simply transferred the MGA carbs and retuned.

Enjoy your motoring. Clutch okay?

Steve
Steve Gyles

Hi Steve

Yes it cost a lot of money and time for a 2 part. Fuel pump, distributor,coil, plug leads, plugs, carb. floats, needle valves, gaskets etc. At least my clutch kit was a reasonable price from Leacy Classics who still offer free shipping. The clutch is fine no more slipping when I am tearing round the country lanes scaring everybody including myself.

Barry
B Bridgens

Barry,

Pleased to see its finally sorted.

It will be interesting to see what other components are effected by the changes in fuel composition.

Where in France are you located?

Regards...John
John Bray

Hi John

I live near le Mans in Sarthe, a small village to the east. I have already changed all the rubber fuel lines to latest spec. for ethanol based fuels. Although I don't use E10 I suspect that most fuels contain some ethanol up to 5%. I sealed the fuel tank during restoration to stop rusting with an ethanol resistant sealer. When I replaced the needle valves during the trials of seeking the fault I had convinced myself that the small rubber section at the valve had expanded and I am sure that the seal felt sticky, anyway they are all new now. It is a beautiful sunny day here so I am now obliged to go for a drive to make up for lost time.

Barry
B Bridgens

Barry,

Like many others, where possible my classics are run on super unleaded which I think is still 5% or less ethanol in most of the uk.

We visit Bagnoles-de-l'Orne on a regular basis about an hour from you, will have to arrange a meet up at one of the shows at some time.

Enjoy this mornings drive in the autumn sunshine

Regards...John
John Bray


Barry Bridgens

Well done mate,
I've been out of action for a few days, and when I checked here you have fixed it - I hope the weather is kind to you to get some reward for your effort.

Unfortunatley with problems like this there are always well intentioned inputs from posters that can at times be a little off the mark, but I suppose that in itself makes you think a bit more so, all good really
Most inputs in this case were really good and really before all the new parts had been fitted any one of them had the chance of being the cause of the problem
I'll have to have a chat to Dominic about needle choice though,
Anyway my theory is --
Drive em hard and drive em often
I'm back to the midget section now, don't quite know how I got here--

Cheers from the bottom of the world -Willy

Barry -Just checked Sarthe out on google maps, nice looking area you live in there-close to LeMans. That Prost circuit looks crazy, have you ever had a lap on there. There appears to be an oval track just to the East of the Prost circuit, Would that be a speedway or a bicycle track ?? or


William Revit

Hi Willy

When you sit on your own in the garage it is nice to know that the rest of the MG community on the forum are with you. When members throw ideas they might not fix the problem but they certainly make you go and check, which is why after your suggestion I checked the carb. tubes. The oval you noticed is a hippodrome for " Les Courses Hippique" in essence a trotting circuit where races are held. It is a very popular sport here in France.

John Let me know when you are next in France and fancy a coffee and chat at a halfway meeting point.

Once again guys thanks for all the input, really enjoying the bracing autumn drives just now. Off to NEC in the morning for a Friday visit.

Barry
B Bridgens

This thread was discussed between 18/10/2014 and 11/11/2014

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This thread is from the archive. The Live MG MGA BBS is active now.