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MG MGB Technical - HELP! It keeps miss-firing

Please help me!!!

My MG B GT 1.8 keeps on misfiring no matter what I do. I purchased this car in june this year after it had been sat still for five years. The car ran fine for about the first fifty miles or so, it did idle quite low but nothing to bad. However after filling it up with petrol it developed a miss-fire, inspection of the spark plugs and dizzy cap everything looked fine. We took apart the su carbs and refubished them, which were full of grime. Once these have been put back on the miss-fire was not sorted. We have also checked the timing and this seems ok.

If anyone has any ideas please let me know.
THANKS
m c wright

What year is your car? Have you attempted to remove the spark plug wires one by one to isolate the missing cylinder? (dont hold onto them with your bare hands - you could get shocked if they are not in perfect condition). Also connect a timing light to each spark plug wire in turn to see if you are getting spark to each plug. Next would be a compression check to see if there is a cylinder low on compression, depending on what you find here, it could be rings, valves, head gasket (with other symptoms in most cases).

Others may have more ideas, but would probably like more info in order to be more helpful.

Erick
Erick Vesterback

Since this happened just after a petro fill I would keep looking at the fuel system. The fact that carbs were full of grime suggest the tank is also and that junk would go through the pump and the fuel filter have you checked them? They could be starving the carbs for fuel. It might just need a tune up. Points and condenser are the heart of the dizzy and you cannot tell if they are good just by looking at them. When does this missfire occur? Idle? acceleration? steady cruising? extra info will help.
Charles O'Brien

It's quite normal to have blackened surfaces to the carb interior, and as long as these aren't built up in thickness they can be ignored.

You did put petrol in it and not Diesel, didn't you?

With a car that has been standing for five years the sudden use may have caused all manner of things to fail now. Really we need more info as Charles asks, as the causes could be legion, and you would end up with half a Haynes manual to cover them all.
Paul Hunt 2

MC. I would suggest that you investigate the things which Erick has mentioned first. A compression check is always in order. Should be done with a warm engine and the throttles completely open. Full instructions on my website at www.custompistols.com/ if you need them. Also consider a "running compression" check if you have a compression gauge with a screw in fitting. A running compression check is simply remove one spark plug, attach the compression gauge, start the engine, release the pressure on the gauge, then, see what it is reading. Should be in the 80-90 psi range. Release the pressure a couple of times and see if the readings vary. Then, reinstall the spark plug in the cylinder just tested, reattach the lead, remove the spark plug from the next cylinder and keep testing until all of the cylinders have been checked.

The advice on points, plugs, condenser and leads is good. You can track down a bad lead that way and knowing the ignition system is in good order eliminates a number of possible faults.

As to the fuel system, Charles may be correct. That was the last change made. Sometimes, the old fuel will evaporate, leaving a form of varnish on the interior of the tank. When new fuel is introduced, the varnish is disolved and the varnish is carried into the carbs and cylinder head. One of the common problems is this varnish gets on an intake valve, keeping it from fully closing. Hence, no compression on that cylinder and fuel/air being pushed out of the cylinder on the compression stroke. A low reading on a running compression check is a good indication of a sticking valve.

Les
Les Bengtson

Thanks for the feed back so far. Its a 1977 1.8, i checked and it deffinately was petrol and not diesel.
m c wright

This thread was discussed between 27/09/2007 and 01/10/2007

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