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MG Car Repair/78 MGB will not stay running

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QUESTION: I'm trying to get my 78 MGB ready to sell. Haven't driven it much in past couple years. Took it out for drive. Would die out, had to keep foot on gas when idling. Found stuck auto choke problem, fixed. Now can start car, runs for 1-2 minutes then dies and won't restart. Seems like it's flooded. Wait long enough and it will start. I had recently replaced spark plugs due to carbon build-up on then that I attributed to not driving enough. Trying to find out what changed. Any suggestions? What should the fuel/air mix be set at?

ANSWER: Hi Craig,

We often had cars come to us in the dealerships with complaints like that and even though symptoms can lead you to a problem, they can often lead you astray too. So the only 100% sure way to arrive at a solution to any running problem is by testing.

All engines only need 3 things to run. Compression, Fire and Fuel with conditions on each.

Compression will usually not come and go like in your case. So it most likely is "Fire" or "Fuel".

First get the car into the "Failed Mode" (will not start) and just spray a little starting fluid into the intake while trying to start it. If it does start or even tries to start and will only run while you spray into the intake, you know for sure it is "Fuel" related. Then you put a fuel pressure gauge in the fuel line to the carburetor and measure fuel pressure. It should be from 1.5 PSI to 3 PSI. (All tests must be run while the car is in the failed mode).
If the fuel pressure is good then it becomes a carburetor problem. So, at that point you need to remove the carburetor and open it up. If that is the case let me know and I will walk you through the procedure of what to look for in the Stromberg carburetor.

If however, the engine never even sputtered when trying to start it when being sprayed, then you need to pull a spark plug and connect the plug wire to the plug and lay the plug on a metal part of the engine and have someone try to start it while you watch the spark plug gap to see if it is sparking. (not in bright sun light) The spark must be blue in color and as thick as a pensile lead. If it sparks but the spark in thin as a hair and colored yellow or orange, it is a weak spark and can be the cause.

Each test is designed to put the cause into a smaller and smaller area until you arrive at the problem. When this procedure is used it is a 100% fix rate. In the dealerships we often had test equipment that made it faster but the same principle is used.

Let me know the test results,

Howard

---------- FOLLOW-UP ----------

QUESTION: Thank you Howard for providing direction on this journey. Following your directions we tried the options. Was able to obtain fuel pressure test kit. After getting to failed mode, hooked up in line and got reading of 2.5psi, so in the range. We also replaced the in-line fuel filter thinking that it may have gotten clogged with stuff when started before due to gas sitting. I had added fresh fuel and an additive earlier in season.
So, sounds like we are on to the carburetor now. I haven't taken apart a carburetor since the early 80's. What will I be looking at?

Answer
You have good fuel pressure so here is the next test. Get a can of starter spray from any auto parts store and spray it into the intake when trying to start it. If it starts but will not keep running unless you keep spraying then you have confirmed that you have a carburetor problem.

First be sure the air filter element is in the air filter. The late model MGBs with the single Stromberg was set so lean that it needed the added vacuum supplied by the restriction of the air filter. Remove the air filter and it just about won't run. Also make sure you have the hose that is on the float chamber vent is off (this is the hose that runs from the carburetor to the charcoal canister. If the charcoal canister or that hose is stopped up the engine will quit after running a short distance.

If the air filter is installed, remove the center cap on the top of the carburetor and lift it up and then push it down. You should feel hydraulic resistance. If not, remove the cap and the long pin with the piston on the end of it and put about a tablespoon of engine oil in the top and put the cap back on and try to start the engine.

If it still don't start without starting fluid then remove the four screws holding the top on the carburetor and lift off the top and the long spring. Then lift out the piston with it's diaphragm and look closely at the diaphragm for any holes. Be careful not to damage the long needle in the piston. If all that looks good you will need to remove the carburetor and turn it over and remove the float chamber to check the float needle and seat and the float level.

let me know if you have to go this far and I will give you the instructions on how to set the float level and test the needle and seat.

Howard

MG Car Repair

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Howard M. Fitzcharles III

Expertise

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

Experience

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.

Organizations
Associate member SAE EAA member

Publications
Import Car magazine

Education/Credentials
ASE Master Auto with L-1 certification up to 2000

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