Xr600 Won't Start!!! Not Carb. Related...

This is actually pretty embarresing, I am a motorcycle technician and ended up purchasing this bike that blew a hole through the center of the piston. I have done the following: carburetor overhaul & dipped in carburetor DIP, replaced timing chain, new piston & rings, new valves, cut new seat to make sure they sealed properly. I am getting spark, fuel and compression. The cam chain is perfectly alligned. I go to start it and it wil fire for 1 revolution of the engine, then backfire. Anyone ever experence this problem and find out what was wrong? Any feed back will be greatly appreciated.

If You have compression, spark, fuel, with air it should run....sounds like cam timing, valve adjustment, or ???....:banghead:

which end does it backfire out of?

+1 on the cam timing or valve adjustment...

Thats what so weird. The cam timing is perfect, valve clearance is perfect, it backfires out the exhaust and sounds like a shotgun going off. I have never been stumped on a bike like this. I have been doing this for 7 years for a living and have never had this problem before. I can't figure it out! I am starting to lead towards the stator since it sits in oil but from bench testing it and kicking it over, it meets Honda's OEM specs. I don't know, what do you guys think???

What about the decompressor leaving the exhaust valve hang open ??

spark and valve timing are out of sync

spark and valve timing are out of sync

I agree.

has to be valvetrain related.

UNLESS, the pilot circuit is clogged or restricted.

If you took it off make sure the cam like thing that makes the timing signal in aligned correctly. Some XR's didn't have a index mark and it could be installed on any spline, not just the correct one.

I'm talking about the thing that is on the clutch side of the engine.

It is unlikely the stator. The ignition winding only supplies the power to the CDI. The timing for the spark comes from the coil on the other side of the engine.

Make sure you check your woodruff key on your crank/stator drive has not sheared...???

My XR650R back fired through the exhaust when new. It quit after uncorking it. When I had the carb apart I turned down the idle adjuster. It ran like yours does, a couple of revolutions then dies, I turned up the idle and it runs great.

Your problem is probably more complicated than mine was but take a step back, and look at the simple stuff.

This is actually pretty embarresing, I am a motorcycle technician and ended up purchasing this bike that blew a hole through the center of the piston. I have done the following: carburetor overhaul & dipped in carburetor DIP, replaced timing chain, new piston & rings, new valves, cut new seat to make sure they sealed properly. I am getting spark, fuel and compression. The cam chain is perfectly alligned. I go to start it and it wil fire for 1 revolution of the engine, then backfire. Anyone ever experence this problem and find out what was wrong? Any feed back will be greatly appreciated.

Is it really cold out? The reason I ask....I just bought a 600 used, and while at the dealer trying to get it started took me, the salesman, and two mechanics kicking it to finally get it to run more than a couple revolutions. Don't know why, but maybe it was just really cold and the mixture was too lean? I'm in the process of putting in a new piston/rings and tear into the carburetor. The previous piston looked nearly new, and the valves were perfectly in spec, so I'm guessing it's in the carb. This bike came from sea level, and I'm at 4800', but it was about 25 degrees, so I'm thinking the jetting was still lean.

Are these bikes typically hard to start when cold? Once it got started and warmed up, it fired up on the first kick...

Check the simple stuff first. Make sure the fuel is fresh and the carby clean. Check the spark - it should jump a distance of 10mm from ground when you kick it over hard (get someone to help you do this check). Also make sure your valve clearances are correct, if you have a tight valve you will lose compression, thus only getting a backfire when you try to start the bike. Failing this check your ignition timing, which requires a timing light that you may or may not have. Good luck, let us know how you go!

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