Ive got a 99 400 scrambler, and just dropped in a new wiseco .030 new rod and new crankcase after the old piston exploded. it never had this problem before, when i start getting it going in higher rpm, not full, it will cut out and then it will just die and roll to a halt. I can fire it right back up and go again, but it just repeats itself. It almost seems like the engine is starving for fuel. Since i put a new piston in it, would that mean the carb needs to be rejetted? I dont know much about that, any help or suggestions would be gladly accepted.

 

Very timely thread, I experienced the same problem on my '95 today with a couple of minor variations. Mine would backfire before it died & would take a few minutes of sitting before it would start back up. After doing this about 3 times to get it back to the trailer, it finally quit starting altogether. I pulled the plug & it was pretty black. Any ideas?

 

You could have your carb vent hose crimped.

 

with a larger displacement you are going to need to bigger main for more fuel maybe even a bigger carb

 

what kind of shape are your motor mounts in? your not shaking to the point of foaming the gas in the carb bowl are you? if not you could be runnin lean on the main jet.

 

i was thinking bigger carb, it just doesnt run like it used to when we first installed it. Today it backfired real loud, i felt the wind of it hit my legs, then it died, i assume this is the same problem, only thing that i can think of is not enough fuel. can you bore the stock carb out?

 

one other thing, what kind of air filter/air box setup do you have? try pulling the snorkel off and running it if your using it and see if it goes away.

 

Boring the cylinder .030 over does not require rejetting unless it was really off before. If it ran good before he worked on it then he did something wrong putting it back togather.

It is fairly clear that it is running very lean from lack of fuel.

He could have crimped the fuel line or the carb vent hose. The carb boots could be loose and leaking.

Or it is possible that the main jet could be clogged. Sometimes when you have a carb off while working on the engine you can turn the carb over and dirt that was lying in the bottom of the bowl can get in the jets.

 

Try exactlly what cornbread said.

 

I had a similar issue while riding one day, my carb vent had flopped over against my pipe and melted shut. check this.

I tried everything else on the trail, plug fuel filter and missed the simple thing. Start easy and work your way up.