Hi Dave and welcome to the place! As has been mentioned, backfiring through the carburetor is usually (or I should say often) a sign of improper ignition timing, but it can also be caused by a sticking intake valve. Particularly at low rpm. What happens normally is a spark plug fires while an intake valve is still open...hasn't yet closed. Usually you'll notice the engine isn't running right if a valve is sticking but in any case if a plug fires while any intake valve is still open it allows fire to by pass the valve and ignite the air/fuel mix above that valve (all the way into the carburetor). The wrong mixture can also cause this, Dave but it sounds like you've already taken measures to remedy that. Often it is a very lean mixture that causes backfiring but not always. A vacuum leak can cause an overly lean mixture but with the 235 it won't be the vacuum line going between the carburetor and the distributor because at idle there should not be vacuum present in that line. However you mentioned an auto-choke carburetor. Those will have some vacuum present at idle (to pull back on the auto choke piston as the engine warms up) and a problem there could be working against you. Those are some possible problems.
As has been mentioned a bad condenser can definitely cause backfiring. It will make the spark pattern unpredictable. Please contact me if you need a new one. If you can't use the PM function yet, you can reach me at
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