MG Car Repair: 73 midget, shock absorber, air velocity

Question
73 midget  700 miles on complete rebuilde started to backfire when first started sometimes ok when warms up carbs and engine prof rebuilt  could it becontact breaker points car i only used for shows

Answer
Hi Brian,

When you say "backfire" do you mean backfire out the exhaust or a cough back out the carburetors?

Points are the least likely in either case.

If you mean it coughs back into the air filters, it is most likely that the chokes are not set correctly. The SU carburetors don't have a "choke" like some carburetors have. When you pull the "Choke" cable you actually just drop both jets lower in the carburetors which gives you a richer mixture which a cold engine needs to run. When an engine is cold and does not have sufficient fuel per air it will cough back into the carburetors. Have someone pull the choke and watch BOTH jets to see that they drop down about a half inch.

Another thing you need to look at if it is a cough into the carburetors is the oil in the top pot. This oil is a shock absorber to prevent the piston from raising too quickly when the throttle is opened too quickly and to dampen the pulses at idle. By restricting the upward movement of the piston on a throttle opening you would think that holding the metering needle deep in the jet would make it run lean but the opposite is true because the increase in air velocity across the jets draws an excess of fuel thus making the fuel mixture rich which is needed on a throttle opening. This takes the place of an accelerator pump in a conventional carburetor.

If it is backfiring out the tail pipe, (only when it is cold) You need to check first for a leak in the exhaust pipe to exhaust manifold and if that is ok and no leak there. You need to go through a complete tune up procedure.

Howard