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Help troubleshooting 87 XR600

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Hey guys, 

 

I have quite a bit of experience working on bikes and cars, but my friends 87 XR600 is proving to be difficult to diagnose. Any help is greatly appreciated.

 

He just bought an 87 XR600 with the dual carbs a month or so ago, and it was running great.  the previous owner took good care of it, and it looks to be in great shape. It ran well for a week or so after he got it, then on his way to work one morning, it started bogging. he let it warm up for a couple minutes, and started riding, once he hit the highway it started to bog badly for a few minutes, then the power returned. We figured it was a fuel delivery/carb issue, so we pulled out the carbs and thoroughly cleaned them out and inspected them. all the jets looked clear, and the needle valves looked good. We blasted carb cleaner through all the passages, adjusted the float levels, put in a new inline fuel filter, cleaned out petcock/screen, changed the oil, adjusted the valves, and put in a new spark plug. It starts first kick, but the bogging didn't change. he cruised around at full throttle, and it is backfiring badly on decel, and the plug was white. The jets are the stock jet size (we have the factory manual) and the bike is bone stock other that the spark arrestor has been removed, and he has a K&N filter. We live in Tahoe, so all the riding has been at 6200-7200ft. We figured it should not need richer jets than stock? we started with the air screw at the stock position, then ended up screwing it all the way in, and there is little improvement.

 

The carbs looked a little worn, and the previous owner stripped a screw that prevented up from removing the needle jet, so he picked up a new(used) set of carbs on ebay off of a running bike, they looked to be in great shape, but we pulled them apart and did a similar cleaning. they have a larger main jet.  Still no change, bogs when cold, and randomy runs better. plug still seems to show that it's running lean.

 

Lastly we checked the compression, and it is somewhere between 130-140psi (measured with carbs on, warmed up, throttle held wide open). Seems like the motor is getting a little tired, but he just wants to get it running better for the rest of the summer, and we'll rebuild the top end this winter. anything we should check. this has us scratching our heads a bit.

 

Thanks, Ryan

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Did you check the carb boot between the carb and head? It sounds like an air leak.

Thanks for reminding me, yes we did check there, and the orings were pretty shot, so we replaced both of the orings and the gasket between the carb boot and the engine. we also sprayed starting fuild around that area while the bike was idling to see if the RPM's changed and they did not.

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If you feel positive that the boot isnt leaking and this problem has stayed constant through two sets of carbs, i am thinking it is time to start looking elsewhere. But, i still have to ask, did you check the air cut off diaphram on both sets of carbs for pinholes or tears?

Next question, is there an exhaust leak at the header? A missing copper gasket or loose header nut can cause the lean condition and backfiring on decel although i wouldnt think it should cause it to bog like you are describing.

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Thanks for your help, I Just checked out the rubber diaphragm in the original carb behind bright light, and didn't show any signs of pinholes or cracks ... Exhaust hasn't been removed, and bolts are tight. Any other suggestions?

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And yeah.. We're pretty sure the carb boots aren't leaking. They look to be in great shape, and we replaced both orings and the oring style gasket. We thoroughly cleaned the surfaces and did the starter fluid test. Pretty stumped at this point... Should we re-jet. You guys have any suggestions on jet sizes for 6-7k ft?

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I can certainly understand why you are stumped. Lets go back a step if you are game and talk about the stripped screw on the original carbs. Are you talking about a carb bowl screw? If you can post a picture of the stripped screw someone may be able to offer a solution. The reason i am suggesting fixing and using the original carbs is because the bike ran good with them at first and there are lots of versions of these carbs that look identical but are in fact slightly different.

When you say you were unable to remove the needle jet, are you refering to the brass emulsion tube that the main jet screws into? I think the clymer manual refers to the emulsion tube as a "needle jet" or was it a typo and you meant the jet needle?

Typically the complaint we see on this forum is " my bike starts good and idles fine but bogs and sputters when i give it gas" and 9 out of 10 times the problem is either a clogged main jet or the little holes in the sides of the emulsion tube are plugged. Anyway, if the original carbs are able to be repaired, I personally would use them over the replacement unless they are jetted the same as the originals and have the exact same serial number on the right side of them. The serial number should begin with "PH 52 A" for an 87 xr600r.

Ok, lets just say carbs are ruled out since a different set has the exact same symptoms, no air leaks and exhaust is tight, bogging and backfiring are not usually symptoms of an ignition issue so the next thing i would look at is valve timing. It is not unheard of for these bikes to jump a tooth on the timing chain and exhibit the issues you have. Pop the inspection plug on the stator cover, put it at tdc on the compression stroke and see if the timing mark still lines up.

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Thanks for all your replies... I didn't actually take the original carbs apart myself, my buddy did with the help of a friend who works as a part time motorcycle mechanic and also owns a later model xr600. My understanding is the screw that was stripped was inside the slide, preventing them from removing the jet needle and checking the clip position. The carbs he bought were listed on ebay as off another 87 xr600, and appear identical, but I'll look at the serial #'s. We were able to remove the tube, and all the holes appeared to be clean. Both sets of carbs were very clean when we took them apart, and we tore into them pretty far... I might just tear into them again and see what I can find.

The timing is a good point, I was thinking of checking into that next. I'll have him check it out and post our findings.

I think the most frustrating part is that it appears to be somewhat inconsistent, and seems to run better after riding it for a few minutes.

Stator or ignition could certainly explain the bogging and lack of power, but I would expect the plug to be black.

It just has to be a fuel issue, but for the life of us we can't figure it out. Petcock flows great, gas cap isn't clogged, floats are adjusted to spec, and we blasted out all the jets in both carbs.... Hmm

We'll check the timing. Thanks again for all your help!

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