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Discussion Starter · #1 ·
I am new ATV's but have been working and playing with sleds and pwc's for years. I bought this 03 600 sportsman at an auction and it didn't run. I just rebuilt the carb with all new jets, new needle & seat and set the float, per the manual at .51". Fuel pump was shot so I replaced it to.

Here is what is happening. Its starts/runs/idles but not smoothly. It will idle properly for a few seconds the start to die and the run properly again for a few seconds. When you rev it up it starts to rev but then backfires and sputters and it seems to me that compression is blowing back out the carb. In fact the carb is not leaking but when you keep it rev'd up and sputtering gas starts dripping out of the airhorn.

I have good spark, 150# compression and to make sure I didn't have a plugged exhaust I pulled the muffler off. Currently the carb is open to outside air and not going through the airbox and it will idle and do as I have described above. When I connect the airbox up it will not even idle, with or without the airfilter.

I am puzzled but thinking valve timing? I can't figure it out so any help anyone can give would be appreciated.:confused::confused::confused::confused:
 

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Discussion Starter · #2 ·
Update

Today I messed with it some more. I pulled the carb and hit the starter, had strong vacuum, no compression leaking by as I had earlier suspected. The carb is a CV unit, a Mikuni BST34. A friend of mine had spare carb that came off of a running machine so I pulled it apart and switched the Main Jet, Main Jet tube, pilot jet, air jet and jet needle. The needle and seat are the same and it held 6# of pressure closed for the 10 minutes it took me to disassemble the original carb.

I cleaned the spare, blew everything out. Drilled out the air mixture screw seal so I could clean it and adjust for my machine. O-ring looked good so I seated it and opened to 2.5 turns, I could adjust it later if needed.

I put everything back together, and noticed that spring on the slides were different so I used the one that came with my carb. Before screwing on the top of the carb I adjusted the enrichener then buttoned everything up. I also put all of the boots back on and the air cleaner back together but left the muffler off.

The unit still runs terrible, barely idles, and seems rich. I remain confused and hope someone on here can give me some ideas.:confused:
 
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