xr600 misfiring????

Hi guys i'm having some weird problems on my '94 xr600r.

It seems that misfires once in a while or at least is what i think it's happening.

I'm a bit pissed off because if it happens while iddling at a red light on the city, the bike deads. You can imagine how frustrating can that be.

I think it's a misire because it also does it once in a while when cruising at very little throttle, you can feel a power loss (a shake) during a small instant and the power comes again.

When it does while iddling it makes like a little explosion and dies.

Oh yeah, if i'm not wrong it only does it when hot, but not shure 100% as it's an intermitent problem.

The bike always starts at first or second kick when cold and when hot maybe at 3rd as much.

What can it be?

I read somewhere that the spark plug boot can go bad over time, can that be the problem?

check auto decompression -could be catching the exhaust valve at low rpm

How do i check that?

You do mean the camshaft one controlled by rpms, not the one you use it with the lever manually, right?

Soon i will replace the cam chain as it's a bit streeched. Maybe i can check it then or disable the autodescompressor forever?

After replacing the cam chain it didn't do it anymore. At least not 'till now.

Your idle may be a bit low. You may need to adjust your idle screw a bit higher.

about that spark plug cap..the end that goes on the plug un-screws with a wide flathead . A resistor will drop out with a small spring.Sometimes that will burn up rattle apart etc.

that's kinda rare though..sounds like you would "flame out" while at idle. That can be several things, like you said cam chain is one. A little lean on the fuel screw\pilot is one more. slighty leaky valves or out of adjustment.A pulse coil going bad,but that would be mainly when it would get hot then not start for a while. ..that's all i can think of right now.

oh ya.. use A good premium 92 octane gas. my 650r will flame out when i use regular.

Thanks scalejockey.

I took out the spark plug cap resistor and it seems there's no problem with it. No rust or anything "weird"

I'm using 98 european gas (that's like 93 in USA) and the iddle isn't the problem, it happened even iddlin at 1700rpms.

The iddle mixture it's ok i think. After starting the bike when cold i can take out the chocke completelly in 3 secconds and it iddles fine. Also the spark plug gets a little black a tiddle.

Leaky valves isn't the problem for shure, i rebuild the head 600miles ago and i installed a NX650 cylinder and a 101mm 10:1 wiseco piston. So now it's a 640cc.

It only remains the cam chain and maybe pulse coil starting to go bad. But as i said it hasn't happened again in 60 miles.

A mechanic told me that here was few XR that came with a problem on the head that used to ocour that same sintoms. The only solution was make honda replace the head under waranty. I don't know if that's my case. Hope is not as the warranty is gone.

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