I bought a 2002 Scrambler 400. When cold it fires up fine but a couple laps around the yard it will start to die out almost like it is running out of fuel and then won't restart. If you let it sit overnight it will start up fine again and then do the same thing. If you try to restart it it will backfire bad out the exhaust.

Things I have done:

- New top end rebuild. Cyl bored to next size and new Wiseco piston. 125psi compression
- Checked reeds and they appear to be in good shape. I even tried swapping to a different set and it made no difference.
- Completely went through the fuel system. Has new lines and filter from tank to carb. Checked the outlet fittings from tank and they are clear. The carb has been completely rebuilt and cleaned. I even tried another known good carb on it and it does the same thing.
- Did an Ohms test to stator and everything is within spec. Changed CDI box to a known good one. No difference. The motor has good spark when it is acting up.
- Checked for water pump leak. No coolant in crank case. Doesn't seem to be losing coolant.

I'm pretty handy at turning wrenches but I'm not too familiar with these engines. Does this sound like a crank case air leak or something? I visually checked the case for cracks but don't see any. I'm stumped.

 

loosen the gas cap see if it runs maybe tank vent plugged

 

If tank vent is not the cause then immediately after it dies check resistance on stator pick up and charge coil and compare with cold readings. Could be stator itself or pickup beginning to fail when it gets hot.Also plug cap should be around 5k ohms resistance. It could be shorting out also. If electrical can be eliminated as the cause,then make sure the three way fuel valve and fuel line and filter to carb are free and not restricted. I've had fuel valves partially stop up also. Also if this started after top end rebuild,was the coolant system bled at the head to purge any air pockets? Does it seem like its getting hot? OPT

 

It didn't start after the top end rebuild. I rebuilt the top end hoping to solve the problem.

The tank vent is open and the vent system on the tank and the atmospheric vent on the carb are both good.

It's not electrical because I still have a good hot spark when it is acting up and the timing is correct.

It is not getting hot and I properly bled the coolant system when I did the top end.

I didn't replace the three way valve but when I cleaned up the tank outlet elbows I blew the valve out with compressed air and everything seems to be flowing like it should. I can see fuel coming from the carb when trying to start which is why I think it backfires so bad.

I did eliminate the reverse speed limiter because there was a short in that that caused the 4x4 hubs to not engage so I just removed it and grounded the brown hub negative wire which fixed that issue. The running issue still happened before and after I did this so I don't think removing the speed limiter did anything.

 

I'm down to about a half stick of dynamite solution at present time Maybe when I'm fully awake and the old brain can think of what else to check later? It just sounds like something that would happen on a stator going south,fuel starvation or when a top end looses compression when warm,but you said the problem already existed? Keep looking,maybe others have some first hand knowledge on what you're going through while I shake some cobwebs loose! Try also to unplug the black kill wire from the cdi and see if it helps?One other thing is the crank seal behind the drive clutch could be distorted and starting to loose bottom end compression? OPT

 

I was thinking of the crank seal issue. I'll try unplugging the cdi.

 

The CDI wire makes no difference.

This morning it won't even start. It just backfires. I'm wondering if I sheared a flywheel key that catches when cold and lets go as it warmes up.

 

does sound like maybe a timing issue if it's back firing. wouldn't hurt to check the key.

 

I gave up and took it to the dealer. I don't have the flywheel puller anyway.

I'll keep you guys posted after they diagnose it.

 

I have an older version with same problem. Ended up being worn cranks bearings and the seals were leaking.