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MG Car Repair/once at temp engine cuts out .... help


my mate has mk i 1967 mg tried new ign switch cleaned carbs ,fuel pump. cuts out when at normal temp help ,,, summers coming please

Hi Chris,

I am gussing you have a MG Midget not a MGB or any of the other kinds of MGs? "Cuts Out" meaning it missfires when up to normal temp? Or does that mean it stops running?

Any time a car is brought to me in the dealership, I had to do the exact same thing to find such a problem. The test procedure must be done in the correct order and nothing skipped.

There are only three things that need to be looked at and they must be looked at in this order.
"Compression", "Fire" and "Fuel".

"Compression" needs to be tested even if it was ok a day before or even a hour before.
The Compression test must be done with the throttle open. It should be 145 PSI to 170 PSI on all cylinders and with very little difference between cylinders. If Compression is not tested, it is no use proceeding to "Fire" nor "Fuel". A compression gauge is not expensive and will work on any gasoline engine. The compression test should be done with the engine hot. If the compression is off just a little check the valve adjustment first. The run what is called a "Wet" test. You run a second compression test but squirt about a teaspoon of engine oil in the plug hole just before the second compression test. The readings will be higher but shouls NOT be more then 10% to 15% higher.

"Fire" (Ignition) must be second. In the dealerships I worked in we had a Scope that would check the whole Ignition system in just a few seconds but most people don't have access to a Scope but there are other ways to test the ignition system.
First is a preliminary test. Remove all four spark plugs and place each wire on it's plug and lay each plug on a metal part of the engine. Now place your thum over #1 plug hole and have someone spin the engine or you can do it with the starter solenoid which is handy. As the engine spins the compression will blow your thum off of the hole with a "Pop" sound and at the same time you will see the spark at the plug and it will make a "Click" sound. If the timing is close enough to run, it will seem that the "Click" is caused by the "Pop". (This does not mean that the timing is correct, only that it is close enough to run) If you get a "Click-Pop" or a "Pop-click" then the timing is off. You need to do each cylinder the same way. This confirms that each plug it firing and at about the correct time and in the correct order. Don't skip this even though it most likely is not your problem. Each time a plug sparks the spark should be Blue in color and as thick as a lead of a pencile. (not in bright sun light).
If the plugs are carboned up you need to either clean them or replace them.

Now you need to check point gap and look for any carbon tracking in the cap. Then check the ignition timing accuractly.

Now and only now can you look at fuel. First turn on the ignition switch (not start) and listen for the fuel pump to start clicking and then slow to a stop and only click very few time or none at all after that. If the pump continuse to click remove the float chamber vent hoses from the top of each float chamber to confirm that no fuel comes out. Now warm the engine up and let it idle (about 750 to 900 RPM) Under the edge of the top pot on each carburetor is a spring loaded pin sticking straight down. With the enigne warmed up and at idle lift that pin all the way up on the front carb and note what the engine runs like. If it speeds up and stays fast then the rear carb is set too rich. If the engine slows and sounds like an old two cylinder tractor, then the rear carb is set right. If the engine dies instantly, then the rear carb is either set too lean or the throttle butterflies are not set evenly. Then rev the engine a little and let it return to idle and lift the pin on the rear carb which tests the front carb the same way. This method is not the same as what you will read in books but is served me well for many years in several dealerships and and in my own shop since I started in 1960.

Let me know the results.


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Howard M. Fitzcharles III


MG from 1956 (USA versions only) up and Engine theory.


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.

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