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MG Car Repair/Engine ajustment


Hallo Howard!
When I accelerate in my 1965 mgb the engine is not running right. I I go why gently on the accelerator then engine runs smooth but when I step on it the engine starts "coughing".
Can you direct me? What should I do?

Best regards

Hi Miller,

When a car came to me in the dealership, with a complaint like that, I had to go through a test procedure as that could be many different things.

A miss or caugh under load does not pin point the problem so you need to follow my procedure to find the problem. The procedure must be done in order.

First is to test the compression. It must show form 135 PSI to 170 PSI on all cylinders with little difference between cylinders and you must run the test with the throttle wide open.
First remove the white wires off of the coil and remove all spark plugs to runb the test and be sure to spin the engine over at least 5 or 6 revolutions on each cylinder test.

If the compression test is not correct it is a waste of time proceeding until it is correct.

Second is fire (ignition). Examine the plugs and clean or replace them. While the plugs are out connect the plug wires to each plug and lay each plug on a metal part of the engine and connect the white wires back on the coil. Now spin the engine over and watch the plugs spark. The spark must be blue in color (not in bright sun light) and the spark must be thick as a pensil lead. A hair thin spark that is orange or yellow in color is a weak spark. If the color is good check and set the ignition timing per specs. (10 degrees BTDC in the US version)

Third, and only if the Compressiona and Fire are correct, you can look at Fuel. Since you did not say the engine seemed to cut off under load, you don't need to test Fuel pressure.

Warm the engine up to operating temperature and let it idle (750 to 900 RPM) and locate the small spring loaded pin sticking straight down just under the lip of the top pot on each carburetor (SU Carburetors). When it is idling lift the pin of the front carburetor and the engine should drop in RPM to a very slow idle and sound like an old two cylinder tractor engine. If the engine dies as soon as you lift the pin, your REAR carburetor is too lean or the two throttles are not in sync. Do the rear carburetor the same way. If it too makes the engine die the FRONT carburetor is too lean or the throttles are not in sync.
If both the front and the rear cause it to die you need to lower the jet adjustment (the 5/8 inch hex nut on the bottom of the carburetor) but only move the nut no more then 3 flats (a half turn) then run the lift pin test. Do this to both carburetors. If the engine speeded up and stayed fast when the pin was lifted then the other carburetor was too rich. (unlikely on your car from what you say)

When finished unscrew the top cap on the top of the carburetor and lift the cap up about an inch and push it back down and you should feel hydraulic resistance. If you don't remove the cap and the long pin with it's piston and put in a few squirts of engine oil.

If all of this has not corrected your problem you will need to check the ohms reading of each plug wire and the coil wire and examine the distributor cap for carbon tracks and test the rotor by removing the coil wire from the cap and try to jump a spark to the center of the rotor while spinning the engine to see if it will jump a spark to the rotor. If it does the rotor is bad. You will also need to remove the tops off of the carburetors and set the float levels and look for traces of water or trash in the float chambers. Also check the float chamber air vents to see that they are open.

If you have a difficult time getting the idle down to it's correct RPM you need to run a simple test for a vacuum leak. With it idling you just slowly start restricting the air to the carburetors and as you restrict the air, if the idle at any time speeds up or gets smooth there is a good chance you have a vacuum leak somewhere.

Let me know the results. don't skip anything and do it in the order I outlined.
The tools to do all of this are not expensive.


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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.

Associate member SAE EAA member

Import Car magazine

ASE Master Auto with L-1 certification up to 2000

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