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MG MGB Technical - Stumped...electrical or fuel?

Gooday all.

This may end up being a bit long but my problem has been ongoing for longer.

MGB 69 australian delivered mk2.

Problem... In warmer whether and on a long uphill climb (constant throttle,2500rpm, approx 3minutes) car begins to backfire through carbs and pings (pre detonates) severely with loss of power. Can do in cooler whether but doesn;t seem as bad. I travel the same route daily and the same hill gets me almost everytime. If i climb hill at a lesser throttle, doesn;t always have problem but if i increasre throttle, bingo.
At all other times and conditions, car runs fine.

Attempted fixes.... new distributor ..ran better but still same problem as plugs (ran better still but same old problem,) set fuel bowl float levels, removed fuel pump and checked for blocked filter, clean. Have adjusted mixtures from rich too lean but no difference on hill. Have adjusted timing from retarded to advanced , no difference on hill.

I,m running out of ideas. next step will be replace coil (possible breaking down under load) and leads (same idea)
Carbs have slight wear in spindle shafts but don;t think thats the problem.(open too further suggestion)

i figure its running short on fuel given the throttle position making a difference but short of replacing the su pump (expensive over here) i have no more ideas.

I like a challenge but this ones begining to drive me batty.

Mechanical knowledge good (30 years playing) but always learning.

Perhaps someone here has had similar problem or just plain knows more..
Any advise will be greatly appreciated.


Cactus - Check my fuel delivery troubleshooting guide at:
Particulrly, look at the rate of fuel delivery, ahould be 2.4 pints per minute (this is US measure, I'll let you do the math to convert that to Imperial) and check for air leaks. Good luck - Dave
David DuBois

Thanks for the prompt reply David.

I have pulled the pump down to check the filter and it was clear. Did replace the outlet valve the wrong way round. After the third strip down , finally found it. All the internals that are visable,ie, the diaphram, valves, and seals all appear to be ok. I have no bubbles at the carb end of the fuel line when pumped into a jar so am guessing no air leaks. I will try the fuel flow rate tomorrow and see what that reads. If it is lower than recomended, what would be the cause? A weak diapphram?.... and if it is low, would that cause pre ignition? If the car simply lost power, i would say fuel starvation but the pinging is what leads me to electrical. Just cant make a clear judgment , hence the replacement of electrical and fuel components

once again, thanks dave


I agree with you that is sounds like fuel starvation. When you check for the fuel pump rate, check it at the end of the line - where the hose meets the carb. This way you get a true reading of the whole system and not just the pump. There may be restrictions along the way. Did you check for kinks in the both the hard and soft fuel lines?
Just a thought.
Good luck.
Mark Rotsky

Firstly a larger throttle opening and/or heavier load on the engine will increase the tendency to pink, which is why we have vacuum advance which reduces the amount of additiona advance under those conditions. Secondly fuel starvation will cause a weak mixture which *is* one of the causes of pinking as it doesn't cool the cylinder as much as a correct mixture charge, and some era's of MGB (like mine) are very prone to pinking, and sensitive to octane. Retarding the ignition should certainly prevent the former, and should tend to reduce pinking from the latter. Are you saying retarding the ignition has no effect at all? Have you tried increasing the throttle even more to see if that stops it pinking? It does on mine, but then I don't have fuel starvation so the greater volume of fuel cooling the cylinder and reducing the chance of pinking is more then compensating for the increased load increasing the chance of pinking. Have you tried driving down hill with your foot on the brake to simulate the load of driving uphill, and hence a similarly wide throttle opening?

You really need a fuel line pressure gauge to monitor what is happening when this problem occurs, or possibly a tee off the pipe with a tap into a container (bit dodgy though) which you can turn on to see if the pump still has capacity to deliver to your container as well as the carbs. If so, it ain't fuel delivery, but could still be carbs. HSs, I presume, have you disconnected the jet pipes from the float chambers to make sure there is no debris in either? The jet pipes will need blowing through in both directions to check, the rubber washers can decompose and bits partially obstruct the flow.
Paul Hunt 2

Paul's comments should lead you to determine if you have too much or too little fuel delivery ~ either amount or pressure or both.

Off hand, it sounds more like too much fuel than too little. That is what I usually associate with backfiring.

You do not say specifically, but I am assuming you are running a genuine SU fuel pump (given the items checked on it). SUs were only meant to handle 2-3 PSI, and if your pump happens, by some quirk, to be providing greater pressure than that, it could conceivably be blowing the float valve off its seat, and to some extent, flooding the carbs, causing the backfiring you are experiencing. Or it could be your float valve(s) are defective and doing the same thing even at 2-3 PSI with a good SU fuel pump.

Most commonly available vacuum gauges sold are setup to also measure fuel pressure and I would use one (or whatever low pressure gauge you can access) to determine fuel pressure first. If pressure checks OK, then I think I would look hard at the float valves as a possible problem.

However, if your vacuum advance is not working properly and advancing your too much or too little, or drawing its vacuum from a wrong port (most HS-4s draw off of a port on the carb body, altho some will draw from manifold vacuum). If your setup is connected to the wrong port, that could be your problem, or if not, then I would check your vacuum advance unit's number against the number it is supposed to have.

One source of general Lucas dizzy info it at Teglerizer's site, , but it does not give exhaustive vacuum advance numbers. Still useful, however, and it may help you. A better source is British Automotive's site, , which appears to have a better handle on what those numbers are for any given dizzy our cars use. Good luck!

Bob Muenchausen

Bob brings up a good point, I assumed that you had a SU fuel pump installed, but in rereading your original post, you don't say what the fuel pump is. If it is a SU fuel pump, the pressure will be correct or possibly a bit low, but not high. The SU fuel pumps are fitted with a volute spring that sets the pressure and it will either be 2.7 psi or 3.8 psi, depending onthe model of pump you have. Either pressure work fine with SU carbs. Mark is correct in saying that the flow rate check should be done at the carburetor end of the line so the entire deliver route is checked for any obstructions.

The filter inside the pump is intended to stop rocks and small birds from getting through, but fine rust particles can pass right through the pump and be caught in any in line filter that has been added to the system and will cause a reduction in flow rate. If yu find that the flow rate is reduced, then the next thing to do is move back to the pump and check the rate there. If it is still low, run a line from the pump inlet to a can of fuel and try again. Any increase in flow rate in either of the last two tests, would indicate a restriction in the line just bypassed. If the low flow comes from the pump itself, then a rebuild in in order, replacing both the diaphragm and points and making the correct adjustments to the pump. If it comes to that point, e-mail me and I'll send you a reassembly and adjustment procedure.

Don't get fixated on fuel flow to the point of ignoring Paul's suggestions of the vacuum advance to the distributor, as that could be causing your problem also. Good luck - Dave
David DuBois

Gentlemen....Thankyou for your ideas.

Firstly...SU fuel pump....secondly...HS4 carbs. (should have mentioned those things)

Response to all that i have tried, and figured out.
One more indication of problem. When i said long hill at high load and throttle positon for approx 3 minutes, the problem does not appear for the first 2 minutes. Its only after that , that the severe pinging appears along with constant backfire through carbs, not exhaust, and loss of power.

Paul. your theory on fuel starvation is my line of thinking as it seems to happen after the fuel bowls have used up their fuel .ie, after a few minutes under load. I wasn;t sure if a weak mixture would induce such a problem but it appears likely after reading these posts. I will check the fuel flow rate today as suggested by David and at the carb end as suggested by Mark. After that, the jet pipes.

The vacumm advance was my first opinion so i replaced the distributor with a rebuilt one. The exact same problem exists so unless i am unlucky and have two defective advance units, i will assume(sometimes a huge mistake) that the dizzy is ok.

Bob, gooday. i don;t believe it to be running too rich. it is the SU type pump and i have no smell of raw fuel at any stage ie, coming from the overflow pipe. The backfire is also through the carbs and not the exhaust. (must remember to explain "exact" circumstances in future) Vacumm advance doesnt seem to be the problem as the pinging would appear as soon as i increaed
load and not after a few minutes. Mind you, i have had all these thoughts and checked all areas even though my logic tell me it may not be this or that.

Once again, thankyou all for your suggestions and i will now go and check my flow rate (cheapest thing to do ) and let you know what results i find.


Are you running standard needles in the SU's? If not, could be that it is running ok on the regular but when under load of the hill, it is running to lean or to rich.
Bruce Cunha

gooday bruce... good point.

I changed needles a couple of years ago because i dropped one of the originals and bent it. The replacement ones i had were not original numbers i recall so will purchase new ones as a matter of course. I had forgotten all about them.

I know for a fact that it is possible to starve SU carbs of fuel in a long hard left hand corner: The floats are forced into the off position and only comming off the corner will alleviate.

Going up the hill may be doing the same thing. How are the pivots on your floats oriented? In my experience the hinge is downhill from the float, so while going uphill, the fuel will pick up the float from the front and force it into the closed position.You said that you re-set the floats. Maybe try setting them a bit higher (later closing)than recomended and see if it helps.


I'll throw one out from left field...

Have you done a compression test?

If so, then what were the results?

A car with low compression will often gasp and
weaken as it climbs hills. Hold the throttle open
while climbing - and it will often spit back at you.
Daniel Wong

Also, You may have a carbon buildup on a piston crown,combustion chamber or an intake valve causing detonation when a certain load is achieved. Ray

One other thing, the vacuum for your advance, if it is like US cars of that year, is ported vacuum from the rear carburetor and there is a flame trap in the pipe between the carburetor and the vacuum advance unit. Often the flame trap or the pipe itself will gt clogged up with carbon and the result is no vacuum to the distributor. Good luck - Dave
David DuBois

Hmm. Backfiring through the carbs. What would you need to ignite a backfire or allow combustion gases to be expelled out the carbs? I might be tempted to guess another type of timing problem.
Bob Muenchausen

Cactus. Quite an interesting problem. Thank you for sharing it with us.

Most of the important points have been covered already. Taking the fuel line and inserting a T fitting into it, then running a line back into the passanger compartment, should allow you to hook up a fuel pressure gauge to see what is happening as you climb the hill. If the fuel pressure drops, you need to look at either the pump or the fuel pick up tube in the gas tank. You do not mention if the effects vary with the amount of fuel in the tank. If they do, look at a damaged fuel pick up tube. If you see fuel pressure variations, David DuBois can be contacted through the tech article he has noted.

Many years ago, I was having performance problems with my 68 GT having a new set of Burlen SU carbs. Roger Parker, who used to frequent here, recommended the AAA needles. These are slightly rich to meet our emissions requirements, but, are the best performance needles I have found. You might check what needles you have installed and compare their profile to the AAA needles which work, uphill and down hill, very well.

Ignition timing may be a factor. If you have a replacement distributor it may not produce the required curve as determined by the factory. The last replacement distributor I examined had far too much advance for the specified application. A problem that others have noted over the years. Thus, if your mechanical advance (at large throttle openings the vacuum advance should be minimal) is excessive, you will get "Pinking/Pinging".

Do let us know what you find and, perhaps, we can offer up some more ideas. The more information you can provide, the more accurately we can focus on what you are experiencing. The website that Dave directed you to, also has some tech articles on setting ignition timing and testing of various ignition systems. A bad spark plug wire, failing under load, could also cause some of the symptoms you describe. All worth checking out. Remember, "It is not a Problem, It is an Adventure!". Does not always help, but does some of the time.

Les Bengtson

My suggestion, in keeping with my rule of trying the cheapest solutions first, is to remove the dome from each carburetor and make sure the inside is clean. Also, clean the vacuum pistons and make sure the needles are centered in the jets. If the pistons are not free, you will have the symptoms you describe.

Also, make sure the air filters are clean, and be sure you have the correct needles for the carburetors. If your engine cylinders have been bored much larger than stock, you may need non-stock needles. -G.
Glenn G

I would suggest that you go back to Les' last comment and do a simple check of the fuel pickup in the tank. I had a similar problem that took me a year to finally diagnose. Try this: Fix a 12" or longer length of clear plastic tubing to the output side of your fuel pump. Add whatever other line you need to make it easy to put its end in a milk jug or other container. Run the fuel pump and look for bubbles in the line. They mean that there are holes rusted through in your pick-up line and it, or the tank needs to be replaced. You can make sure that it is not the pump by running the line from the input side of the pump (again with the plastic tube) and insert the end in the fuel filler pipe on the back of the car. Fuel with no bubbles will confirm that it is indeed the tank pickup and not the pump.

At slow speeds the air has no effect on the engines performance because the fuel demand is low and the air bleeds out of the system. At higher speeds, the air in the lines keeps an adequate supply of fuel from getting to the carbs.

Good luck.

glq Greg

Try some choke. My father used to have do this when he drove his Morris 8 (SU carb.) in the Snowdonia mountains. If it works, it's not a fix, but it might point you in the right direction. The fact that it's not as bad with less throttle suggests a mixture problem to me. Barrie E
Barrie Egerton

Thankyou gentlemen for your suggestions.

I have had a busy week and no time to try any as yet. Not even been able to drive the car, but this week is looking better and i will start with the simplest suggestions through to the more advanced ones. This site is one of the best i have seen for the diversity and knowledge that it provides. I will keep everyone posted as to the results as they come in. With any luck, it will be the first one i try. (murphy;s law sates that it will be the last one)
Again , many thanks.

This thread was discussed between 20/11/2006 and 03/12/2006

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