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2002 Yamaha Grizzly 660- aftermarket exhaust, 2500 miles, 900 hours
Please help this thing is pissing me off. I bought this quad with the engine torn apart knowing it needed a new crank. Was told it ran great but who knows.
Anyways, threw it together with the new crank and the engine seems to run fine in idle, but dies when I give it throttle, even in park. It backfires a bit too.

I've checked:
timing
valve lash
cleaned carb/put new one on
adjusted idle and fuel/air mixtures
carb boots
was told CDI box was replaced, connections are good
cleaned spark plug and I have spark.

Side note: i noticed the cam had a small burp in it. I could see the exhaust valve open very slightly when it wasn't supposed to. Didnt see any noticable bumps on the cam.

What else could it be?
 

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Welcome from Colorado.
I just found your thread.....
I does start and run so its not to far out of wack, are you doing this with the air filter box installed and the lid on?
If so there might be an air leak at the intake boot.
 

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It has an aftermarket exhaust. Was the carburetor jetting adjusted to compensate?
I would inspect the jets too see if they are stock or aftermarket, or simply see what size they are.
 
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