Update (11/13/21). The intake valves were completely closed up. Readjusted them with a feeler gauge and the damn thing fired up instantly. Throttle cable is garbage so it's idling high so I'm waiting on that to come in. Only thing I can think is valves were damn near closed up when we test drove and when we took it home and she got cold everything shut all the way. Just a huge coincidence I suppose.


So I recently purchased a 2002 grizzly 660. I test drove it at the sellers house. Shut it off and back on multiple times. Everything worked. Ran like a Champ. We loaded it on the trailer and brought it home. We put it in the garage for the night and in the morning, it would not start. All it does is crank and occasionally backfires. I should note that the seller already had the atv running when we showed up as he drove it to the end of his dirt road to meet us.

I have tried everything I could think of with basic engine knowledge (never worked on an atv).

I have:

Rebuilt the mikuni carb with a mikuni rebuild kit (it was a bit dirty as they didn't have an air filter in. No I was not happy at all to find that out.)
Replaced the spark plug
Confirmed fuel is flowing to carb
Confirmed sparkplug is firing
Tried using starter fluid with no change
Tried jumping battery with a charging/jumper box in case battery wasn't able to give enough power.
I've tried adjusting the mixture screw as well to no effect.

If it was a bad kill switch it wouldn't turn over. And I've made sure kill switch is in "running" position.


My only ideas at this point is a bad cdi or magneto/stator. I just don't see what has changed from sitting over night that could have happened. Like it literally will not even start for a moment. Reminds me of when you run out of gas. Everything turns but nothing happens. The only thing I haven't confirmed is that fuel is getting from the carb to the engine. Which would mean some sort of vacuum issue with the carb. I'll be putting it all back on tomorrow as I had pulled the carb back off and Confirmed everything was put back correctly.

 

I assume you had it on choke when you first tried to start it? Check compression, much under 90psi and bikes get bad to start, if it is, set the valve clearances, Yams are prone to loosing clearance. Fit a new plug too, just in case it is that. Check the inlet manifold, again inlet cracks are a known Yam fault and an engine which is sucking in air through a crack, isn't going to start. Never, ever, fiddle with carb settings when an engine that was running OK suddenly doesn't, if it was dirt in the carb that caused the fault, it will run fine on those settings once the dirt is out. Altering settings just puts another fault on.