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That's a lot of stuff that has been replaced. What is the history on this engine? Why was all of this done?
When does it backfire idle or all speeds?
When you did the carbs did you change the intake gaskets and carb gaskets?
Have you checked the timing belt that it hasn't jumped a tooth? What you are describing fits the timing belt being off.
I am experiencing internet problems so please hang in there with me.
Before changing the head gasket we need to do a compression test. This will give us an indication of the condition of the head. Preferably we would like to do a leak down test. This will put air into the system and help pinpoint where there are any leaks.
Yu could very well have a problem in the valve train. A boroscope inspection of the valves and cylinder are warranted. You may be able to borrow these tools from a local auto parts store like Auto Zone. You could have a valve that is hanging open slightly.
What we want to do is to look at the valves while rotating the engine by hand, spark plugs out, ignition off. See if one is not closing all the way.
Post back your results of the compression, leak down test.
The carbs are set up differently so you can't swap them as you want. Have you checked the primer/starter. It could be leaking which would screw up lower speed operation and not affect high speed operation as much. Do you have a service manual or do you need help checking this? It is the round white electrical part on the top carb.
There being a problem in that carb would make sense. Swapping the carbs depends on the serial number of the engine, sorry about that.
There is an idle mixture screw, it is behind a plug on the to, side of the carb. At this point I would drill and remove the plug and see if you can richen the mixture a little bit. Also check the plate over the idle mixture area on top of the carb. That gasket must seal.
No hurry, let me know Jon, thanks.