MG Car Repair: MGB flooding issue

Question
Hi.
My 1972 MGB is for the USA. I just installed a Weber conversion with aan electric choke, and it started right up on the first crank and runs great. However, after running long enough for the motor to get sufficiently hot, the engines sputters and stalls as if it were flooded. Then it won't start until I leave the motor to cool down for a half hour or so.

I thought maybe the problem resided in some form a vapor lock resulting form the heat from the motor, or in the charcoal canister which likely needs taking apart and the charcoal being recharged or replaced, but I honestly am at the limit of my weekend warrior mechanical capacity.

I'd appreciate any expert advice you could help me out with.

Doug Goodwin

Answer
Hi Doug,

To diagnose a problem with running you need to first ID the system that is failing (there are only 3) Compression, Fire or Fuel. And they MUST be tested in that order. Working in dealerships for years I learned to avoid using symptoms too much and depend on a set procedure of testing.

Even though your problem does not point at Compression as a fault you must not leave stones unturned when testing an engine. Just from what you said you are not at the limit of your mechanical ability. You just need a few facts and a procedure to find and correct the problem.

Here are a few facts that will help. Excess fuel to any engine will produce black smaoke out the tail pipe and put a fine black powder on the spark plugs.
If your Weber carburetor has a float chamber vent pipe and that connected to the Charcoal Canister and if that canister is stopped up with fuel, that can and will cause excess pressure on the float chamber fuel and thus excess fuel through the main jets and fload the engine.
Overheating can cause an engine to stop for several reasons. An over heating condition should not be diagnosed just from a gauge reading, you must get a conformation like coolant blowing out of the over flow.

Here are my recomended tests and procedure.

First get a compression gauge if you don't have one. (available at most auto parts stores) run the engine until it dies and quickly Remove all four spark plugs and examine them for appearenace and color. Flat black powder on the center porcilin and the whole end is an excess of fuel. Glosy black carbon is either excess oil burning or a extreamly flooded condition. A very clean white porcilin and a discolored negative electrod is a sign of a lean condition.
Set the plugs aside and run a compression test on each cylinder (Throttle open) and at least 5 or 6 complete revolutions of the crank on each cylinder test. You must see about 120 PSI to 170 PSI on all four cylinders and only a small difference between cylinders. It is important to run tests while the engine is in it's failed mode.

If the compression test is OK and ONLY if it is OK do you procede to "Fire".

If you were able to do the compression test while the engine was still hot just connect all the plugs to their plug wires and lay the plugs on a metal part of the enging and have someone spin the starter as if they were trying to start the engine. Watch the sparks (not in bright sun light) the sparks at the plugs should be colored blue and as thick as a pensil lead. If the sparks are thin like a hair and colored orange or yellow then you have a very weak spark and it most likely will not spark under compression. Even though you do not want to skip the compression test, you can run this spark aperance test first before the compression test.
Incorrect timing can cause an engine to over heat but you didn't say anything about poor perforamnce before the overheating. Incorrect timing produces poor performance then overheating.

Last thing to test is always fuel. First when the engine dies and will not start quickly take a spray can of starting fluid and spray some into the air filter and try to start it. If that makes it start but will not continue to run unless you keep spraying into the air filter then it for sure is a loss of fuel. This can be due to either the carburetor, carburetor vent system or the fuel supply to the carburetor.

These test will establish which of the three systems has failed and then further tests should be run on that failed system.

Here is a mechanic's short cut up front. First put the car in the failed mode (quit and won't start) Just spray the starting fluid into the intake and if it don't try to start then it is either "Fire" or "Mechanical" that is the fault. If it tries to start then it was "Fuel".
"I know, why didn't I give you the short cut first? It was because I didn't want you to get lazy and not run the tests necessary".

Let me know.

Howard