Backfire through carb on acceleration

[nkaminar]

Backfire through the carburetor is an indication of a lean mixture or a sticky intake valve.

The lean mixture can be caused by a manifold leak, a leak at the carburetor gasket where it mounts to the intake manifold, or a leak at the throttle rod. Or it could be caused by a partially clogged jet in the carburetor.

Sticky valves can be caused by sludge or carbon buildup on the valve stem or a carbon or other particle under the valve seat causing it to be partially open. It could also be caused by a bent valve stem. Unlikely, but if there is not valve lifter to valve stem clearance, the valve will stay partly open. When the spark plug fires it will send hot gasses out into the intake manifold to ignite the air/fuel mixture there.

The Model A coil is used in the inverted orientation. If the coil was designed to be used in the upright orientation it can be filled with a fluid that will leak out when inverted. The coils that are designed to be used inverted are filled with a solid material.

Do a compression test or leak down test to see if you have a stuck valve or one that is leaking. Use the trick Crosscut talked about to find any vacuum leaks. Clean the carburetor using carburetor cleaner and compressed air. You have to take it apart but it is usually not necessary to take all the jets out.