At the end of last summer I was out riding the scrambler and on the way home it lost alot of power when full throttle then when I came to a stop it shut off and hasn't started since then. I have been messing with it on and off over the last couple of months trying to figure it out and am at a loss.

When I tried to start it before it would just crank but would never turn over. It has spark, its getting fuel, I have changed the reeds, new spark plug, new spark plug boot and wire. I just replaced the stator but am not sure if it has to be lined up a certain way. Now when I try to start it, it just cranks and backfires.

Does anyone have any suggestions?

 

What is the compression reading? OPT

 

I for some reason forgot to even check compression so I just went out and checked, 30 PSI.

 

I think you found your problem! OPT

 

Yes I definitely did, Thanks

 

Well I am back, with pretty much the same problem of the scrambler not starting. Since my last post I have had the piston replaced with new gaskets, another new spark plug. The compression is now 130 but it still won't start. It will try, run for a few seconds then shut off. I think the oil to gas ratio is a little off, would that keep it from starting? I have tried adding more gas to the mix but still nothing. Any suggestions would be great, this is driving me insane!

 

If you're sure the carb isn't the problem,another thing is crank seals. If you haven't replaced them,could be part of the problem? If you're premixing, 40-1 is the rate I'd use. If you're still using the oil pump it's preset from the factory at 50-1. You can just pull the front plate and see if the mark on the oil pump wheel lines up with the case mark.

 

I don't think the carb is the problem but I am not positive on how to check it for sure. I have put on the old carb to see if there is a difference but no luck. I was able to get it to start twice for about 45 secs then it shuts off, it mostly backfires alot! I am not using the oil pump. I replaced the throttle cable hoping that was the problem because it was an issue before. I am not familiar with the crank seals as I have not had to mess with them before. Now there is oil leaking somewhere around the exhaust as the jug is covered.

 

Just swapping carbs may not help if both need cleaning/kits,etc. Here's a couple vids on your carb. Backfiring can be a simple carb problem, compression related,130 psi is a little low on a new bore job and piston on the 400,could be leaking crank seals (lean condition) or even an electrical problem. Start at one thing at a time and eliminate it as a problem! Recheck compression,clean the carb/replace the carb kit,pull the front clutch and replace the crank seal(most people neglect these) As far as oily exhaust,it's common with wear/vibration on the pipes over time,Malcolm Smith Racing products did have some good pipe sealant tape,but I can't find it anymore. You can use some hi temp silicone sealant at the pipe connections and see if it helps on the leaks.