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MG MGB Technical - B fuel issues

Greeting all,
I have a 75, mgb roadster with a stock Z-S carb. Last week I took it out and had it was running well.
I stopped for 20 minutes, and when I came out it would barely run, and then quit completely. I had to have it towed back.

When I get it to start, it runs very rough, and need to keep it floored to run. It pumps out white smoke, and as soon as I let off the gas, it dies. fuel pump is working fine and I dis-assembled the carb and all looks OK. The plugs are fuel fouled.

Any ideas, gang?

Steve in Santa Fe.

White smoke suggests head gasket (steam). How's the radiator level and is there water in the oil?
Steve Postins

I think Steve in Oxfordshire is right. White smoke usually = antifreeze in the combustion chambers. There are other explanations, but if this is the case, you need to attend to it ASAP!!
In addition to Steve's check of the radiator, check the oil for signs of antifreeze contamination. Pull all the spark plugs and spin the engine on the starter, looking out for antifreeze spurting from the plug holes.

If there is antifreeze in your combustion chambers, and if you allow the engine to sit this way too long, one or more of your pistons will sieze, and that is not a "Good Thing"! If you can't get the head off right away, drain the cooling system, spin the engine (spark plugs out) enough to clear as much of the antifreeze as possible, and temporarily fill the cylinders with ATF, or Marvel Mystery Oil - until you can get the head off.



If you have just bought gas, check what you bought. Diesel mixed with gas will produce very light gray exhaust and terrible running. Saw this happen to a Lexus.
John H

Thanks for the input, guys.

I see no evidence of antifreeze in the oil, and the rad is full up. The plugs are wet with fuel (strong gas smell). I recently installed electronic ignition...Is it possible that my spark is suddenly very weak? Coil failing??

Could your timing have shifted horribly? I had something similar out of the blue when an advance spring flew of and jammed the working of the dizzy.
Steve Postins

Your best bet is to do a careful troubleshooting squence. Start by laying a plug against the head with the HT lead connected and have an assistant crank over the engine. Look for a strong spark at the plug. If you have a spark your problem is fuel or timing related. Weak (or no) spark, then work you way back through the coil, dist cap, rotor, and LT wires.
Bill Boorse

I'd go along with the 'eliminate ignition first' camp - as per the adage '90% of SU carb problems are due to Lucas ignition'. If you haven't got a spark tester - which consists of a scissors-type device that goes in the HT lead and varies the gap - then progressively lifting the plug off the block to see how big you can get the spark may have to suffice. However bear in mind that doing it that way the HT will try to ground through the poor sould holding the plug when it can no longer jump the gap to the block, even if you are holding an insulated plug cap or lead. Not the case if you remove the plug leads from the distributor cap, or the coil lead from the coil instead. However the white smoke does indicate something is in the combustion chambers that shouldn't be.
Paul Hunt 2

It could be that the diaphragm in the Zenith has a hole in it. I know Steve said he checked the carb, but it doesn't take much of a hole...

John English

I had a diaphragm perforate once. The resulting problem was gradual in onset, and the engine still started easily and idled smoothly. It just didn't have any power - at all. Doesn't sound like Steve's problem.

I agree with all the diagnostic stuff that's being mentioned here, but first, I'd presume the worst and make sure the combustion chambers are free of any antifreeze.


Steve It could be the needle and seat in the carby sticking open, flooding the engine. We did not have that carb setup in OZ so its just a possibility(guess

This thread was discussed between 24/05/2006 and 27/05/2006

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