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93 XR600R with a problem

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Hey folks, new to the forum, hopefully I'm not being redundant here but I've got a problem with a 93 XR600R where it's starting to drive me a little batshit.  :)

 

I bought used a few years ago, have been tinkering with it getting it ready to (likely jump on the group buy here) install a dual sport kit and get some riding in.  The issue is this.

 

Acceleration off the line goes something like...  Gang busters up to about 2K RPM then falls on it's face, sputters, pops, shoots flames out the exhaust, then gets up and goes, albeit roughly, the power pours on...

 

It's a bit of a bitch to start, you have to get it just right on the compression stroke, have the throttle a 1/4 turn and a good bit of luck.

 

Brand new (tested) XR's Only Mikuni 41mm flat slide, jetted and needle set for my altitude, carburetion shouldn't be the issue.  I suspected an issue with the carb, to the point where I sent it back to Mike (the carb guy at XR's Only) and he went through it, confirmed the build and installed it on an XR600 this past weekend and said it performed perfectly.  The carb it on it's way home now so I'm still very likely going to have this problem.  Mike suggested the jumped tooth possibility on the cam / chain.  So just this-evening I've adjusted the valves and the one intake was only a tad loose.  Having the crank at TDC on the compression stroke, everything looked as it should, I was hoping it was the jumped tooth, that would account for how it runs and starts but this proved to be a dead end.

 

Other do-dads, it's got an FMF silencer and header.

 

I believe a head job by XR's only a long time ago.

 

The decompression has been disconnected from the kicker.

 

Was raced in California (Baja) so it's likely had other work done, it's clean, powder coated, tight, engine internals look meticulous / clean.

 

Over-all the bike was well maintained, well cared for, just need to get past this issue where it's hard starting and falls on it's face off the line and I'll execute my plan to get it dual sporterized.

 

What, I ask you, could it be??

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Have you checked for any bent valves? try forcing air into the intake and see if anything comes through the exhaust when its at TDC.

Also, have you check your cam chain tensioner? I'd recommend a manual tensioner if one isn't installed already. I've heard its fairly common for the auto tensioner springs to break. Hope this helps. Good luck!

On a side note, if you getting into dual sport, I'd recommend looking into some sm wheels if thats something that interests you. I have some warp9's on my xr400 and it handles great on the street. Tons of fun and leans a lot harder than you'd expect out of a dirt bike.

Edited by Greg Motard

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Greg, Thank you for the tip.  Just last evening I checked for compression at TDC.  I managed to get TCD, even though the auto-decompression makes that a bit of hell (how is one to account or that not being a cause of issues?).  I was able to remove all the rocker covers, get TCD and then forced air into the spark plug hole with a compression tester and it held perfectly.

 

What I'm wondering at this point, is if the valve timing is off a tooth, there doesn't appear to be a way to check this without pulling the valve cover. This leads me to new questions to all your Thumper Heads out there. 1) Can you pull the valve cover without taking the engine out of the frame? 2) If it's a tooth off, wouldn't all the valves be closed at TDC? 3) Would it be wise to disable the auto-decompression and how is that accomplished?

 

I'm at my wits end, this bike is cherry but for the "running like crap" part, it's like having a hot fudge sundae in front of you that you can't eat... With a cherry on top.

Have you checked for any bent valves? try forcing air into the intake and see if anything comes through the exhaust when its at TDC.

Also, have you check your cam chain tensioner? I'd recommend a manual tensioner if one isn't installed already. I've heard its fairly common for the auto tensioner springs to break. Hope this helps. Good luck!

On a side note, if you getting into dual sport, I'd recommend looking into some sm wheels if thats something that interests you. I have some warp9's on my xr400 and it handles great on the street. Tons of fun and leans a lot harder than you'd expect out of a dirt bike.

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I would start checking in he basics like electrical connections, proper fuel flow from the tank ,make sure the breather tube on the gas tanks not plugged. And check all electrical components tolerances. Also check valve clearances. Always check the easy/cheap stuff first before tearing into the engine

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I'm not sure if you'll be able to do this on yours, but assuming its similar to mine, you should have room with the engine in the frame to take off the entire valve head and rocker assembly. The service manual should tell you how to do this. This will let you see the cam and tops of the valves. With the engine at TDC on compression stroke make sure both your crank and cam sprocket are at TDC. Both should have a mark to indicate. If the crank is at TDC and the cam sprocket isn't (another way to check is make sure the cam lobes are facing down) then your cam chain skipped a tooth. Also with the cover off you can check your cam chain tension (my auto tensioner locked on me in full extension and gave me huge issues, replacing it with a manual tension did the trick). Beyond that I would go with davids suggestion and check electrical, spark plug, breathers etc. I know it sounds like a lot but should only take a day. Youtube also has a lot of videos with these types of engines, and from what i've seen all the xr's have the same basic design aside from a few minor variations

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I've replaced the CDI box this morning, same issue...  :(  For all the basic checks and fuel flow, all that checks out, new fuel line, new carb, tested carb on another bike.  The stator tests fine (it's been replaced), new coil, new CDI at this point.  All that's left is the crank sensor and it tests okay, maybe I should just replace it to rule it out.  lol  I guess it's time to pull the valve cover.  I'll order up a gasket.  (sigh)

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Hello All,

 

Just to circle back and close out, it turned out to be the stator. I installed a 200w Ricky Stator and it made all the difference. I was able to test the old stator on the floor of the garage and I got OL (open loop) on both the green and white wires, 8 ohm on the black/red. What's amazing is that it ran at all.

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I've replaced the CDI box this morning, same issue...   :(  For all the basic checks and fuel flow, all that checks out, new fuel line, new carb, tested carb on another bike.  The stator tests fine (it's been replaced), new coil, new CDI at this point.  All that's left is the crank sensor and it tests okay, maybe I should just replace it to rule it out.  lol  I guess it's time to pull the valve cover.  I'll order up a gasket.  (sigh)

But you said it's already been replaced & checks out fine :excuseme:

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I did replace the CDI a while back, this was about replacing the stator (alternator).  That was ultimately the real issue.

 

But you said it's already been replaced & checks out fine :excuseme:

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