Experience Dealership line mechanic on MG, Triumph, Jaguar for 15 years, Instructor in commercial mechanics school 2 yr. Product information manager for piston and valve manufacture, Instructor & hotline answer man for import car parts importer 15 yrs.
Organizations Associate member SAE
EAA member
Publications Import Car magazine
Education/Credentials ASE Master Auto with L-1 certification up to 2000
Question hello i recently purchased a MG Midget, and it seems to misfire and try to sputter out, it only does this after its been running for a few minutes and warms up, do you know what is causing the problem
Answer Jeffrey, your symptoms narrow the problem to about a thousand possible things so you need to do some testing to narrow the problem to a smaller area.
The first tests you need to do is to monitor fuel pressure at the time it is misfiring and trying to quit. To do this you need a fuel pressure gauge with a "T" fitting. Put the "T" in the fuel line close to the carburetor and connect the fuel gauge and note fuel pressure when it is cold and after you warm it up and start getting the misfire. It should maintain from 1.5 to 3 PSI at all times.
If that is ok put a timing light on the coil wire and either hold the trigger down or tape it down so you can watch the flash when cold and when it starts to misfire. If the misfire is in the ignition you will be able to see that the flash is not steady like when it is running good.
Even though compression can cause a hot misfire it is less likely and you should just check valve clearance to be sure a valve is not getting tight when hot.
