QuestionHello Howard. I have a 78 MGB that runs fine for about an hour. I then shut it off for about 15 minutes and when I start it up to go she just loses power in all gears. Also when shutting down its dieseling. Any adjustments you suggest are appreciated.
AnswerHi David,
When an engine diesels after turning off the ignition it is a sign that the combustion chamber is over heated to a point that the fuel explodes rather then burns. This is destructive to the engine and it causes a loss of power.
Unfortunately part of the problem was BLM who attempted to meet US emission standards by overheating the combustion chamber.
There are many things that can cause the dieseling and you need to look at each and every one of them to find which one or more are giving you this problem. I will list them in the order that I would test a car for this problem.
First and most important is the grade of fuel. Use a high test gas.
Second, check and set the ignition timing to specs.
Third, Test and correct (if not correct) the factory anti-dieseling valve system which applies a vacuum to the carburetor float chamber when you turn off the ignition.
Forth, check and adjust the float level and the fuel mixture and BE SURE the idle is 800 or below in RPM at idle.
Fifth, check the number heat range on the spark plugs. If you are not up on heat ranges of plugs call a local auto parts store with the brand and number and ask if that number heat range is a cold plug or a hot plug, most have a chart for each brand of plug. There is no standard in the industry so a chart on each brand is necessary to determine the heat range. You need a middle range or a little on the cold side not on the hot side which helps cause dieseling.
Check the cooling system temperature as a over heated cooling system will cause dieseling too.
Dieseling can also be caused by excess carbon build up in the combustion chambers.
Dieseling is very destructive to the engine and you must attend to it right away. Sorry that there is no easy fix but if you follow the sequence I gave you and DON'T SKIP ANY of the items, you will find the cause.
Howard
