1961 MGA sputtering/backfiring once it gets hot

I have a great running 1961 MGA but this summer it has done something odd. It runs great at first start-up, and then after 5 or 6 miles it starts sputtering and losing power even though the engine keeps revving when i give it gas, and after that it starts backfiring like a gatling gun. Same happens if i leave it idling for about 15 minutes. My friends and i are debating if this is an ignition or carburetor problem.

You might adjust the valves and look for a weak valve spring while the cover is off.

I seem to recall the coil was susceptible to heat.

With Lucas Ignition, sputtering and backfiring are considered normal…

I would remove the distributor from the engine, carefully inspect it for worn parts and wiring, install new points and condenser, a new quality coil, Inspect any voltage dropping resistor or wire in the primary circuit, and see if that cures it.

Sounds like the choke is not operating properly to me. An easy test of that is to pull the air filter and see if the choke is opening fully when warm. Another common problem is the timing slips, so check that also.

Yes, anytime the name Lucas appears it becomes an area of suspect.

However, this car has the twin SU carbs that weren’t much more reliable than the Lucas equipment, so I’d say it’s a toss up between fuel and spark.

Check for good fuel. Cars like this sometimes don’t get driven much and it may have varnish in the carbs. Check the fuel filters and then check for the proper synchronization of the two persnickety SU carburetors. A factory MG tech once told me back in the 60’s that if these cars climbed a 100’ hill the carbs would have to be adjusted for elevation…LOL.