MG Car Repair: Backfire through front carb on 1965 MGB, float chamber, small spring

Question
Hello, I just recently bought a 1965 MGB that has sat for about 2 years. Put new gas in and changed the spark plugs and she started right up. When sitting as you rev it up suddenly there is a backfire through the front carb. If you slowly increase from Idle it is OK. After letting it run for about 15 minutes I removed the spark plugs. The 2nd and third plug had dark soot that I could easily remove. 1st and 4th plug looked ok. I have not changed the fuel filter or points. Do I need a new carb kit? Thanks  Jim

Answer
Hi Jim,
First thing to do is a compression test. Throttle wide open and at least 4 or 5 revolutions on each cylinder. 135 to 180 PSI on each cylinder with little difference between each. If that is OK check the ignition timing and clean all the spark plugs.

Then check to see that there is oil in the top pot of both carbs then remove the air filters and remove the three screws holding the top of the float chamber on and remove the float and top to see if there is about the same amount of fuel in each float chamber and soak the fuel out of the float chamber with a rag to see if there is any water in the bottom of either float chamber.

Put the tops back on and reach in the intake of each carb and lift the piston and let it drop. You should hear it hit the bottom proving it is free moving. Start engine and warm it up. With it idling, lift the small spring loaded pin under the edge of the front carb pot and note the change. If it speeds up and stays fast the rear carb is too rich and needs the jet adjusted (5/8" nut on the bottom of the carb)(turned up to lean the mixture and down to richen) Only adjust the nut 3 flats at a time and recheck by lifting the pin. Rev the engine between each adjustment to clear the plugs. If when you lift the pin, the engine dies, the rear carb is either too lean or the two carbs are not synchronized and the throttle shafts need to be synchronized. If the engine slows down and sounds like it is running on two cylinders then the rear carb is adjusted correctly, leave it alone and lift the pin on the rear carb and note how it runs. Then adjust the front carb the same way. This procedure is not what is in the books but do it anyway.

If you find anything wrong don't proceed until you correct what you find wrong.
Let me know what you find,
Howard