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MG MGA - MGA stalling - need ideas!

My '56 MGA 1500 roadster, which I have had for more than 20 years, is giving me hell right now. It runs fine until it warms up (190-200 F), and then will stall out as I try to take off from lights or a 4-way stop. It then takes anything from 5 - 15 minutes before it will start again, and then runs very rough, like it is on 2 or 3 cylinders only.

This has been getting progressively worse over the past 2 -3 years, starting with a little hesitation at take off, and now full blown stalling and refusing to restart.

I need some ideas what to look at, but I can tell you what I have done: Three weeks ago I fitted new throttle shafts as mine were worn, and it was suggested it was leaning out while idling as it was sucking in air past the shafts. When I installed new shafts, I also fitted new jets, new needles, new jet glands etc (carb repair kits from SU)and checked the float levels. It seemed to be better, especially the idle, but the stalling when hot has continued.

Last night I removed the distributor which had pertronix ignition fitted (over 5 years ago, and no problems), and installed a distributor with points and condensor. I also fitted a new distributor cap and new plug leads.I took it for a run, and within half an hour it died on me at lights. It eventually restarted, but died again before I got it home.

I have fitted a new coil, but this made no difference.

When it stalls, the fuel pump (a small Mazda replacement)is pumping, there is fuel in the float bowls, and there is spark. I have checked compression and each cylinder is 120 - 130 lbs. Tappet clearances are correct, as is the timing at 7 degrees BTDC.

It might be a vapour lock, but the plate between the carbs and the exhaust manifold is intact, and the correct isolators and gaskets are fitted between the carbs and the inlet manifold. Nothing has been changed in this area, and I have checked the manifold nuts for tightness.

Any suggestions or similar experience?, would be most welcome.

Peter.
P. Tilbury

Peter

How old are the flexi fuel pipes leading to the carbs? Could be possible that they are cavitating as they warm up and only allowing a dribble of fuel through.

Steve
Steve Gyles

Peter,

I experienced something very similar in my A a few years ago. In my case replacement of the fuel pump (with one of new electronic SU's) cured the problem. Whilst my old pump was still pumping (and hence appeared to be functioning adequately), it obviously wasn't as effective as it should be, and when under load would fail to deliver sufficient fuel to the carbs (hence rough running and even stalling).

Steve
Steve

Another one that has been mentioned before is a blocked fuel tank cap. i.e. you slowly create a vacuum in the tank, resulting in the pump being unable to pump petrol out of out.

Steve
Steve Gyles

I would look to electrics, something shorting out in the ignition circuit probably. are all the cables to the ignition circuit in good condition. Check the ignition switch is not getting past it by bridging it. Check also that the connections at the fuse block are secure.

In spite of the fact that you have just replaced the condensor, I would replace this again as the first step, as your symptoms are a classic sign of condensor failure. Make sure that the nylon insulating washer in the points/ condensor assembly has not failed from incorrect assembly.

dominic clancy

In my Meyers Manx it was the fuel filter $.79 and 3 hours of teching car ran fine , I would check all fuel / hoses .
karl

Possibly a dist. rotor on its way to the grave. But you indicated that you have spark. I assume it is good and strong.

Does the normal operation of the Mazda pump cause it to run all the time? I like the fuel filler cap as a possibility. Related to that might be a clogged pick-up in the tank.

Problem is.... if you have fuel and spark, That is all you need. Except the spark has to be at the right time. I can't think of anything that would shift the timing based on temperature and then refix itself as it cooled down. Something is amiss in the analysis me thinks.

Have you tried shooting ether in the carbs when this happens? Or removed the plugs to see if they are fuel soaked? These tactics might help in trouble shooting the root cause.

Chuck

Chuck Schaefer

Thanks to all for the input.

Could not include all measures taken so far in my original message but can add: drove car with fuel filler cap open, checked all lines from tank to carbs, carb to carb pipe is new as are rubber pipes in/out of the pump and filter, original distributor with Pertronix ignition had no condensor fitted, rotor is new, all plugs are a greyish colour (no sign of fuel wetting), fuel filter is clear, have tried another new fuel pump.

It's as if fuel is in the carbs, but not getting to the cylinders (or the opposite - too much fuel) !

Last night I fitted replacement plugs, another coil, and leaned the carbs two flats. Drove the car for half an hour and it did not die on me. However, it was acooler night so am not convinced the problem is solved. Saturday morning into the city will be a good test. I'll let you know how I fare.
P. Tilbury

Peter, I had a similar problem but not as pronounced as yours. Rather a stuttering when running my Coupe (Marina 1800 engine), and difficult to start, whether engine was hot or cold. I changed from Champion NY7C to NY9C, enriched the carbs by two flats and problem was solved. Am told that I might need to revert to the NY7C plugs in summer. Joe
Joe Gates

Peter. The one thing no one has mentioned is an air leak around the manifold or the carbs. It almost sounds as is a leak were developing as the engine warms up, then, when power is required, the engine is not capable of producing it. Can this problem be exhibited in the drive way? If so, that would make the testing much easier.

Les
Les Bengtson

Hi Peter, If it is vapour lock associated with unleaded fuel - I had the same problem on my GT (before I got the A)cutting out when it got hot especially on hot summer days or after standing in traffic - I used some of the insulation fibre from my wife's kiln behind the heat shield (not visible when fitted) and it cured the problem - cheers Cam
Cam Cunningham

Thanks for the last three comments. I've had the same (halo) plugs in for 2 years, but just changed them for Nippondenso BP6ES which I used to run before that. No improvement.
Re air leak - yes I checked manifolds when I replaced the throttle shafts - no leaks discernible.
Heat shield has not been modified, and I might do this next, although it does not seem to be excessively hotter around the carbs.
Drive into town on Saturday resulted in coughing and spluttering before I reached the show site, so I leaned the carbs one more flat for the drive home. Result was higher running temp 200 - 212, some hesitation on pick up, but it did not stall on me.
Maybe I'm getting closer?
P. Tilbury

"I leaned the carbs one more flat for the drive home. Result was higher running temp 200 - 212, some hesitation on pick up, but it did not stall on me."

How far down from the carb bridge are the jets? Just adjusting the flats doesn't mean you have the correct setting. the jet bearing assembly could be wrong. There are 2 cupped washers that can be installed wrong and 2 cork gaskets that need to be soaked in oil overnight before assembly or they are too thick. The new parts going around these days can also be suspect. Your best bet is to measure the jet position down from the carb body bridge with a verneir caliper and observe the movement when you use the choke to make sure they return.
Fred H

This thread was discussed between 17/08/2006 and 21/08/2006

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