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i've been having troubles with this sled backfiring. it will only back firing as you are starting the engine, and when it back fires it blows out the mag side seal. i have gone thru replace all the seals, crank, water pump (both mechanical and oil) used arctic cat sealant for the case halves, fresh top end. i thought it may be poor quality seals? i used winderosa so i replaced them with cometic. checked my values at the stator, and they are with in spec. only other thing i havent mentioned is she can be real hard starting if the sled hasn't been ran in a couple of days, has good compression 125-130 psi. reeds seem to be in good shape. it doesnt backfire every time you go start it but when she does 9 time out of 10 the seal will be blown out on the mag side. i am questioning the exhaust sensor and or the timing adjustment dial. any ideas of what i am missing or not seeing. would running ethanol gas cause this? i tried to avoid it but sometimes you are forced to run it.
 

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Excess fuel in the base.
Needle valve is not seating, or more likely, fuel is leaking in from the fuel pump pulse line.
Check the needle valves and do a pump rebuild.
 
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