xr600 misfire at 1/4 throttle

[chopp666]

Hi all...i have a '91 XR 600 , single carb model . it was running fine but has suddenly developed a realy bad misfire at anything over 1/4 throttle, with slight black exhaust smoke. have checked the carb, CDI, coil, plug, fuel and thats all fine....it's like almost valve bounce or riding with the choke half up! i am a very good mechanic and have checked all the obvious things but no luck! any ideas out there ? thanks chopp

[Motosprtman]

cam timing off ? pull the cam cover off and rotate to TDC check the cam sprocket alignment marks flush with the head. Cam chain may be very worn and slipped a tooth - very rare - but possible. They will still run, idle poor, loose throttle response and exhibit a running rich charecteristic (wisps to puffy black smoke) also check your plug I bet it is black....

[chopp666]

....hmmm, the plug is black but dry and sooty, not wet.Pulling the the cam cover is the next thing to do tomorrow night after work! funnily enough the bike was running great yesterday , had a good 4hour hard trail ride in slippery mud and fast road 80/90mph ,bike was fine when i turned if off . In the morning i washed it with a garden hose and brush , and it was a pig to start , and when it did it had the 1/4 throttle problem! ...thanks for the info on checking the cam timing, i'll do that and let you know...cheers chopp

[bigboredbiker]

Sounds like the timing to me m8. My xr600 did exactly the same thing, fine when I put it to bed, running like a pig(pardon the pun) the day after. Pulled it apart to find that the camchain had jumped 11 teeth! Its a wonder it started at all! Now its rebuilt, rebored, new camchain etc, its back to its usual self, trying to kill me at any opportunity. Good luck m8 and let us all know your findings.....

[Motosprtman]

ah yes, The dreaded BSPS (Black sooty plug syndrome) :-) almost always a sign of a few things, crabuaration too rich (needle position, jets etc) weak ignition (spark) too cold a plug (not often if all else is correct) or.... the cam timing is off. I first discovered cam timing off on a 1972 XL250 I restored, I got the engine running, but it would not idle and fouled out plugs easily and (they were black and sooty) also would emit wisps of black smoke from the exhaust when the throttle was blipped. I pulled the cam cover off and found it off only a tooth! reset it.. and wa-la, purrrrfect idle, throttle response etc.

I don't know how far off they can be and still run and not have a valve and piston realtionship (contact) bad news of course when that happens as a valve gets bent. I recently finished an XL350 motor that someone had rebuilt and had the cam timing off 180 out both the exhaust valves met thier demise. Had the head reworked and 2 new valves installed and is now a daily woods runner.

Check out some of the vintage bike pages ofr lots of useful info that applies to these newer motors too!

http://www.oldrice.com/xl_page.htm (check out my bikes there as Dave the Motosportman's bikes)

or

http://www.xlintperformance.com/

Lotsa good links for parts etc and also tons of experience with the XL/XR motors and troubleshooting, rebuilds etc.

[Shawn_Mc]

If it was running fine until you put it away and then washed it, youve washed something into the carb. Pull the carb and douche it with carb cleaner and compressed air.

It could have jumped the timing, and checking it wont hurt, but I dont think thats the issue. The circuits to pay attention to are the AIR circuits in the mouth of the carb. They are super super tiny easy to block which will cause the thing to go rich suddenly.

[Motosprtman]

carb is simpler to check and I would not discount it as a possibility, but unless you are running without a filter or a filter that is almost completely non restrictive and also have a combination of a loose boot connection to the carb. Then it is unlikely that anything found it's way into the carb. But! I concur I would check it anyway - quick and easy, if problem still exists, then timing.

[rmhrc630]

If it was running fine until you put it away and then washed it, youve washed something into the carb. Pull the carb and douche it with carb cleaner and compressed air.

It could have jumped the timing, and checking it wont hurt, but I dont think thats the issue. The circuits to pay attention to are the AIR circuits in the mouth of the carb. They are super super tiny easy to block which will cause the thing to go rich suddenly.

I agree Shawn - I think it might be water or something in the carb somewhere?

Does anyone think he's referring to the infamous "cough stall" of the big XR range? I remember once on my old 600 almost being thrown up a hill I was chugging up and needed some gas, whacked the throttle and then lights out bike stalled!!! worst timing!!

[Shawn_Mc]

Water will wash silt right through a properly oil foam filter. And its amazing how little it takes to foul an air bleed circuit in a lil thumper carb causing the thing to act like you just bumped the pilot jet 10 sizes.

[Motosprtman]

Yea Shawn, think you have a good point there. Besides IF the cam timing would of jumped - he should of known while riding it - with diminished performance and possibly even some backfiring.