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MG MGB Technical - Dies when hot, won't restart

1971 MGB with alloy head, roller rockers, H4S, Bosch Hall effect ignition. After a long fast drive on the freeway today I exited, stop at a light and the car dies. Won't restart until about 30 minute cool down. Drove 4 miles and she sputters and dies right after spewing some black smoke out the tailpipe. fuel pump is pumping. Can a sticking carb piston cause this. Any help appreciated

M.A. Barrera

Checked the coil lately?
Steve Simmons

I'm guessing that you are running a bit rich to start with, and when she gets hot, the fuel may be boiling in the float bowls, making the mixture way too rich for the engine to idle. If she does the trick again, it may help to have a misting bottle of water along to cool the carbs quicker. Is the choke sticking on? Might even try misting the body of the coil too, as Steve suspects that.

I don't think it's a carb piston. Or have you had trouble with one sticking before? Wouldn't hurt to check, I guess. A complete check and tune of the carbs may be all you need.

In the olden days we used to try changing the condenser when this happened.
Your Bosch unit doesn't have one though!

The HIF carbs have always suffered from heatsoak and won't idle 10 mins after you stop from a run, the compensation leaning them off too much.
Things on mine improved greatly when I fitted an extra electric fan that runs without the ignition.(It comes on 5 minutes after I stop, and runs for 20

Bit of a meandering answer, but do you have a supplemental electric fan? It would help bring under bonnet temperatures down in the situation that you describe
Martin Layton

Your post says you have HS4's. Check the float bowl vent lines for blockage. If they are blocked the heat under the hood expands the fuel, forcing it out the main jet and into the intake flooding the motor.
John H

If the float chamber vents are blocked it won't need heat to flood, the floats can't rise to shut off the fuel at all and the pump keeps going pumping raw fuel down the intake.
Paul Hunt

Is the heat sheild in place? if so could have possibly lost the insulation pads on the backside of it?
MK Mike

First of all you will have to determine if it is a lack of spark or in fact excess fuel causing your problem.It is a common mistake to go chasing one when it is the other. Simply just check to see if you have spark at the plug end of a plugleed when it is in it's I don't want to go mode. If you have spark this eliminates spark as the problem and then it must be fuel Goodluck Willy
William Revit

Been there - done that. Re-read Steve's post. My money is on the coil too.

Thanks all for the replies. I have rebuilt my charcoal canister, exchanged the coil, wrapped some heat shielding tape around hard fuel lines, and cleaned the pistons and dashpots. I noticed the bridges were at slightly different heights, rectified. Car seems to run well. Have not attempted a long run on a hot day yet. Maybe tomorrow. Wish me luck

M.A. Barrera

Had a similar problem and I was advised to fit a Kenlowe fan, this cured my problem, it now runs like a dream in heavy traffic, no more over heating.
Stuart McCabe

MGBs run in Arizona without aftermarket fans and don't cut-out, let alone England!
Paul Hunt

This thread was discussed between 25/08/2008 and 01/09/2008

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