loosing power/backfire through carb

How long has the carb been on the engine? Was it running fine for a long period of time, and this only started recently? Has anything been changed that may coincide with the drive-ability problems? Was the carb put on last winter, and drive-ability issues started since the weather has been warming up?

Check the simple stuff first.
Carb:
It could be going suddenly lean, which will cause spitting back through the carb.
1. How old is the carb?
2. How old is the fuel filter?
3. Any recent adjustments like jets, springs or metering rods?

Wires:
1. Look at the engine in a very dark place while it is running. If the wires glow, or you can see sparks anywhere, the wires are likely shot
2. Do you have a good ground from the battery to the engine and the engine to the body, not necessarily in that order?

Timing
1. Timing at idle? (with & without vacuum)
2. timing at 1500 rpm? (with & without vacuum)
3. Timing at 2500 rpm? (with & without vacuum)



Let us know the answer to these questions. It may be something bad like a cracked block or a wiped lobe, but it is more likely something outside of the engine, e.g. fuel and spark.