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MG MGB Technical - fouling plugs/ stalling

1974 chrome bumper.
the car ran great this summer, but the other day ( after sitting for a few weeks) the car would stall out when at idle. It would appear to run ok when the choke is pulled out. The front 2 spark plugs are sooty, the back 2 look OK. I had to replace the anti run on valve because the front (small) hose broke off (car is still stalling at idle) Would this have anything to do with my problem. Any suggestions on what the problem and fix would be. Appreciate any help

Rick










rick baranyi

The anti-runon valve is connected to the inlet manifold as well as elsewhere, and any problem in the plumbing for this or the vapour recovery system can easily cause a vacuum leak, which could be the cause of your problem. Depending on exactly where each pipe connects to the inlet manifold can affect some plugs more than others, but as a 74 it could be twin carbs so simple imbalance possibly caused by a problem on one of the carbs could also be the cause.
Paul Hunt

The Ethanol being added to our fuels today can eat a hole in one of your floats and cause the symptoms that you describe. Also, adding to Paul's advice, check your EGR valve. When these units fail, they allow a large amount of air into the intake manifold and the engine simply won't idle. I've replaced quite a few of these, on many makes of cars, since their introduction in '68. RAY
rjm RAY

Ray,
Are you refering to the gulp valve?

Thanks
Rick
rick baranyi

Rick, no. There's a valve, plumbed into the intake manifold, that directs exhaust gasses back into the intake to reduce cylinder temperatures and reduce emissions at the same time. At least that's the theory behind it. It's operated by vacuum and only operates at higher engine speeds. If the diaphragm sticks in the open position, your engine won't run below 2,000 rpms due to the extremely lean mixture that it creates. I can't count the number of these that I've bypassed over the years. Usually, I simply cut the vacuum hose, leading to the valve, and insert a fuse in the two halves of the line, eliminating any vacuum from reaching and operating the valve. This way, the emission system retains its stock appearance. RAY
rjm RAY

1977 MGB GT. No four plug keeps fouling. Changed plugs leads diz cap rotor 3x about 8 sets of plugs a grade hotter, carb service kit twice and needle. its not blowing blue smoke. changed air filler checked breather was fine. What else could this be. Also compression test between 170 174.
Sean Bicknell

Valve guides? A symptom of this is usually blue smoke when accelerating after leaving it idling for a while, or after a long downhill run on a trailing throttle. Have you done that specific test for blue smoke?

Rings/bores? What are the wet and dry compression figures for each cylinder?

Any oil running down past No.4 inlet port on the head that might be getting sucked in?

How many miles before a new plug fouls? Any misfire in exhaust or intake?

What's the dynamic timing like using No.4 compared to No.1?

Could possibly be a bad lobe on the distributor for No.4, have you got another distributor you could try? Alternatively you could shift the drive gear round by 180 degrees so lobe 4 would be firing No.1 and vice-versa, or dismantle the distributor and reassemble the two halves of the shaft 180 degrees out.
Paul Hunt

If you are just idling the car in the garage without running it on the road the plugs will soon foul. Take it out on the road and they will soon clear. This is a common phenomenon of engines of this age.
Iain MacKintosh

Gain a fouled appearance maybe, but in my experience it shouldn't be enough to cause running problems unless there is something else wrong. I've had both my roadster and V8 virtually idling for up to an hour or more when inching forward in very heavy traffic on more than one occasion without a problem.

This is different to the known 'feature' of HS carbs where the idle gradually reduces over time, which is where fuel pools in the throat causing a gradually richer mixture, and is why one has to run the engine at a couple of thousand rpm for several seconds every couple of minute while setting these carbs up. HIF carbs don't seem to suffer s they have an additional bypass port immediately downstream of the jet to just under the butterfly.
Paul Hunt

This thread was discussed between 22/11/2014 and 18/12/2014

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