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Hello, I just found this forum and could really use some help! I bought a 2002 Yamaha Kodiak 400 a few months ago thinking it was a simple carb issue keeping it from running but I rebuilt the original carb and then ended up replacing it entirely but still does the same thing. To best describe the issue, When I start the ATV it only starts fully choked and the engine has to be cold then it slowly sputters out over a short period of time. When you give it gas it revs up but backfires and crackles then dies out anyway. It may restart once or twice but then will only crank and crank and will not start. I noticed as it warms up a little smoke builds up coming out the crankcase vent to the airbox. I did adjust the valves though the access ports but that did not seem to help.

Want I tried: Carburetor rebuild and replaced, Adjusted valves.

I will be doing a compression test and testing spark to see if its constant tomorrow and provides more information. Maybe even take a video to show what im saying better. Any useful tips would help out.
 

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When I had my 2002 Grizzly after a few years I needed to leave the choke open to keep running and was hard to start when winter hit. I had the valves adjusted and solved that problem so you are on the correct tract. The carb replacement was a good idea to rule out.
I wonder since this is a recent purchase if the machine had been sitting for a long time and some buildup crud in the fuel system that is starving the fuel?
If you got good spark with new plugs might need to revisit fuel delivery. J
 

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Welcome from Colorado.
Proper valve lash lets the valve(s) close fully for highest compression, as these are low compression engines at best, proper valve lash is critical.
Then a plug can show a spark yet be slightly fouled for a cooler spark, a new plug is highly recommended.
I would go back to the stock carb, remove and clean the tank and fuel line. While adjusting the valve lash you will confirm the timing is dead on. With a properly timed engine with hot spark all that's needed is fuel in the proper ratio, the carb may need slight adjusting at the end.
 
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