meinsheizer

Start with draining the float bowl on the carb and putting new fuel in the tank. I'm assuming this is the first time starting in a while? If so you've accumulated water in the tank and it found it's way to the carb float bowl. Water is heavier than fuel and will sink to the bottom. This new ethanol fuel sucks moisture out of the air and that's how the water gets in the fuel.

Clean the carb, dump the fuel and replace with new, and change the fuel filter. It's most likely some water in your float bowl and will idle ok because it's only pulling small amounts in at idle but when you hit it it wants to suck up a lit of fuel but theres to much water so it dies.

Backfiring is usually a sign of the exhaust valve not shutting properly. Time to take the head off and clean the built up carbon and also adjust the valve lash. I'm guessing its past 1000 miles on it and probably never has been done.

It's the needle and seat that are attached to the float actually that is the problem. Take the bowl off, pull the pin that holds the float on, carefully pull it down, check the needle tip for deformation and debris. Take a qtip and place one end in a drill and run the qtip in and out of the seat where the needle sits till its clean. Make sure when it's all reassembled that the float site level when in it's up position. Also with it fully assembled you should be able to blow into the fuel inlet port and air should pass. But if you flip it over and simulate the float being lifted then you should not be able to blow into it. GL

I would still recommend a compression check to eliminate the valve lash issue. And depending on mileage your timing could be off enough that the valves are opening early. The chains can stretch over time and cause run issues.

Make sure to clean the main jet and the emulsion tube and all its ports. Number 1 issue does seem to be water in the float bowl but also change the filter. Number 2 sounds like valve clearance issues. What's the mileage on the second one. If over 1000 then its past due for a valve lash check. I think it's like .004-.006