MG Car Repair: Rough engine...sometimes, twin carbs, float chamber

Question
If I use the manual choke for the first start in the morning on my 74 MGB with twin carbs, or if I don't need the choke for that first start, sometimes the engine will run like it's on TWO cylinders. If I walk away from the car for an hour or two and go back and start it up again, it will run perfectly for the rest of the day. Then sometimes it will start up that FIRST time and run great all day. If it's related to one of the carbs, what is it? I'm baffled. THANKS!

Answer
Hi Bob,

Your symptom does not pin point what is wrong. It only tells you that you must run some tests to narrow the problem into a smaller area. It does sound like it is related to one of the carburetors.

First you need to run the tests while it is in it's failed mode (running on two cylinders).

Since it also does it even when you don't choke it, it now can be either an empty float chamber, a sticking float or float needle or even a ignition problem.

You need to ID which cylinders are not firing first. (This test must only be done if you have a points and condenser ignition system, if you have electronic ignition added you have to run this test differently)

First put a timing light on each plug wire when it is running on two cylinders to see if all four have spark. If they do,
Pull all four plug wires off and just plug them on the plugs part way so that it is easy to pull a plug wire when the engine is running. Use an insulated pliers or such to pull one plug wire at a time when it is started and running on two cylinders. This way you can ID which two cylinders are not firing.

You have now established that all four wires are receiving a spark and you have identified which two cylinders are not running. Naturally if it is #1 & #2 that are not running it is most likely the front carburetor that has a problem. However, if it is 1,3 or 1,4 or 2,3 that is not firing it is most likely not a carburetor problem and you would need to run a compression test as it may be engine related.

However, if you found it was either 1,2 or 3,4 that is not running you must stop right away and turn off the ignition and remove that carburetor float chamber top to see if there is any fuel in it. (It must be looked at right away when it is in it's failed mode not after it is running good or left with the key on so the fuel pump can fill that float chamber before you get the top off.)

Howard