My arctic cat 4X4 with a Mikuni BST 31 carb suddenly lost power and fouled the plug.New plug makes it start instantly and it runs great for 10 sec then fouls plug.Diaphram is good. Bowl not leaking

fuel supply problem
blocked fuel lines , filters , filter in tank at tap
blocked main jet , blocked needle and seat
incorrect fuel float level
the problem is carby and fuel flow

Try running it with the cap off the tank. Sometimes the cap will fail and wont allow air back into the tank when you draw fuel from it. This starves the engine of fuel.

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check for a dirty plug may be sooty.if so clean it.

Fill it up with premium gas, change the plugs, and run some sea foam through it. It will blow smoke after you add the sea foam and then clear up. That helps keep the varnish out of the carb (and unstick the choke etc.) and the high test gas will make it a lot easier to start and keep the plugs from fouling out as often. Also change/clean the air filter. This will give you optimal conditions for fuel air mixture and combustion. If you still have problems, go from there.

well sounds like you need to clean air filter if that does not help i would go with your choke is sticking on if its a cable pull one make sure its pushing all the way back on carb check or if it is electronic one i would replace auto choke good luck

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Try checking valve clearance. I had a Honda that did same thing, Set the valves and would start right up when cold.

check the bowl of the carb unscrew bolt on bottom clean as nessary