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Old 04-21-2008, 05:18 PM   #1
BrandonR OP
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Anyone want to Diagnose this? (1987 XL600R)

OK, let me explain the symptoms...

1: loss of power, ~80% of the time, gets up to about 65-70MPH WOT.
2: But sometimes there's lots of power (about 20% of the time). gets up to about 90mph WOT.

When the power is "off" the pipe gets to a dull glow just past the heatshield when the power is "on" the pipe cools off, you can only see this in the dark, it's not bright enough to see in daylight.

I can be riding along at a constant throttle position and the power will either come on all of a sudden or drop off all of a sudden (depending on whether it's currently on or off) for no apparent reason.

Other symptoms:
1: hard starting... well it never was easy but it used to run strong last fall when I bought it.
2: Stalls at traffic lights untill it's been run for a while, (like 1/2 hour or so), short trips have me keeping the revs up at lights to keep from stalling. It'll idle for a while then just "poof" it shuts down, or it'll just die when I pull in the clutch to stop.
3: crappy gas mileage, it used to get ~52, last weekend it got about ~41 without any notable reason (headwinds hills etc...)

The bike has 3400 miles.. yes "hundred" not "thousand" on it, valves are in spec, compression seems good.

I don't feel like shotgunning this problem since replacing parts quickly gets to the "just sell it off and buy something new" point. I don't know where to start for likely issues. I've cleaned the carbs and played with the jets but can't seem to affect the problem with that.
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Old 04-21-2008, 05:39 PM   #2
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You said something about the pipe glowing. The exhaust pipe "glows"? Like red hot? Sounds like a blockage in there. Carbon can build up and should be cleared out. If you look at your exhaust pipe, there is a little panel under the rearmost cylindrical part. If you remove the screws to the panel and stuff a shop rag in the usual hole exhuast comes out of and rev the engine a lot, carbon buildup is supposed to blow out of there. These are instructions straight out of the factory manual. I hope that gets it, otherwise try a new pipe or aftermarket pipe while you're at it. I can't think of anything else that would lead to your problem as you described it, but I may not be as experienced as I think in diagnosing problems. Please elaborate on the "glowing pipe".
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Old 04-21-2008, 05:57 PM   #3
gboezio
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High EGT usually are late ignition, you should have some advance mecanism on your bike, check for proper operation, second cause can be lean mixture, a lean mix burn slower and may finish combustion in the exhaust pipe.
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Old 04-21-2008, 06:45 PM   #4
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You probably have a leak at the intake manifold. There is also an oring gasket between the manifold and the head. When it starts sucking air you will go lean, lose power and cause the exhaust to glow.
The ignition timing is all regulated by the cdi unit. If the bike is running, I doubt the CDI is bad.
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Old 04-21-2008, 07:04 PM   #5
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My vote is for a plugged carb, perhaps some crap in the bowl that sometimed gets picked up and plugs the main jet. Or an intake air leak as mentioned. Last idea, although probaly far fetched, make sure the timing chain hasnt jumped a tooth.
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Old 04-21-2008, 07:12 PM   #6
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Quote:
Originally Posted by G-Force Junkie
My vote is for a plugged carb, perhaps some crap in the bowl that sometimed gets picked up and plugs the main jet. Or an intake air leak as mentioned. Last idea, although probaly far fetched, make sure the timing chain hasnt jumped a tooth.
Sounds like the float in the carb is sticking. Try tapping on the side of the bowl a few times with a ratchet handle or a 6 inch extension. Ride the bike and see if it works any better. If it does, clean and rebuild your carb when the problem comes back. You could run some carb cleaner through a tank of gas or two, but I think a rebuild is in your future.
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Old 04-21-2008, 07:18 PM   #7
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Something is causing a lean condition. Most likely it is something like crap causing a partial or sporatic blockage. It could be somewhere in the carb or it could also be in the fuel petcock. Pull the fuel line off of the carb and check for good solid flow. Pull the carb and clean it well, blowing all of the jets out after they have been removed. While you have the carb off, pull the manifold and check for damage and see if the "o" ring at the head is intact.

What does played with the jets mean?
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Old 04-21-2008, 09:19 PM   #8
BrandonR OP
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Quote:
Originally Posted by Zombie_Stomp
You said something about the pipe glowing. The exhaust pipe "glows"? Like red hot? Sounds like a blockage in there. Carbon can build up and should be cleared out. If you look at your exhaust pipe, there is a little panel under the rearmost cylindrical part. If you remove the screws to the panel and stuff a shop rag in the usual hole exhuast comes out of and rev the engine a lot, carbon buildup is supposed to blow out of there. These are instructions straight out of the factory manual. I hope that gets it, otherwise try a new pipe or aftermarket pipe while you're at it. I can't think of anything else that would lead to your problem as you described it, but I may not be as experienced as I think in diagnosing problems. Please elaborate on the "glowing pipe".
I saw that part in the manual but when I pulled off that plate there's no hole under there. I just pulled the exhaust off, there's no blockage in there. On the exhaust there's a guard to protect your right leg which is mounted over the Y section of the exhaust, just past this (toward the back of the bike) the exhaust bends in to go inside the frame, at this bend the pipe will get to a dull red which you can only see in the dark.
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Old 04-21-2008, 09:30 PM   #9
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Quote:
Originally Posted by Strong Bad
Something is causing a lean condition. Most likely it is something like crap causing a partial or sporatic blockage. It could be somewhere in the carb or it could also be in the fuel petcock. Pull the fuel line off of the carb and check for good solid flow. Pull the carb and clean it well, blowing all of the jets out after they have been removed. While you have the carb off, pull the manifold and check for damage and see if the "o" ring at the head is intact.

What does played with the jets mean?
I guarantee there's no intake leak, I had one for a while after re-installing the carbs but not getting the clamps on right. I had thought I found my problem but although sealing it up improved things the main problem hasn't gone away. I've tested the intake hot, cold, in between, at various RPM's and it isn't sucking air.

By "played with the jets" I mean I've tried various sizes, made sure they're clean etc... thinking it was just too lean or they might be clogged but I can put the biggest jets available or any size in between in it and still have the power on/off symptom.
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Old 04-22-2008, 01:13 AM   #10
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Quote:
Originally Posted by rdnakx250
You probably have a leak at the intake manifold. There is also an oring gasket between the manifold and the head. When it starts sucking air you will go lean, lose power and cause the exhaust to glow.
The ignition timing is all regulated by the cdi unit. If the bike is running, I doubt the CDI is bad.
I agree with the intake manifold(insulator) leak with the following qualification regarding something else, completely different.

You likely have a bad CDI ..

Assuming
1: You've built the carb correctly, and are honest about new jets etc..
(using a published fuel schedule meter for your temp and altitude, and exhaust IE: FSM specs Honda)
2: No intake leak at o-ring, airbox boot, or carb insulator (general intake tract overall)
3 : experiencing 'hard start' or 'no start' conditions, usually when cold..
4: Your spark is weak and white or yellow and thin...(if you get one)
5: Your valve clearance is set correctly


The CDI controls timing issues...i had a similar issue, and it was cured by replacing CDI.
Also, if the exhaust valves are not sealing/a exhaust guide is cracked you will also experience a glowing pipe, in addition to being caused by a lean carb/air leak intake side..
If the CDI is bad, it could have a odd resistance firing when the exhaust valves are already opening, blowing hot compressed gas out the pipe as it lights...

I don't think valve clearance would be intermittent, but your problem sounds eerily like when i had to replace a CDI...

What's your spark look like?
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Old 04-22-2008, 03:04 AM   #11
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A timing light will tell if the spark is too late.
The intermittent nature of the problem is ruling out the valves question, exept if an exhaust valve is sticking open but the engine would rattle like hell and die.
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Old 04-22-2008, 05:28 AM   #12
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+1 on the CDI

My '86 XL600R had a very similar intermittant loss of power/running hot problem that went away when I replaced the CDI black box.
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Old 04-22-2008, 10:20 AM   #13
BrandonR OP
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Quote:
Originally Posted by coppertop
My '86 XL600R had a very similar intermittant loss of power/running hot problem that went away when I replaced the CDI black box.
Thanks, I've been thinking electrical problem but wanted to hear that someone had seen similar symptoms before.

I haven't looked at the spark recently but when I did previously it was a nice blue color. It seems like a CDI would be pretty safe to buy, try out, and if its not the solution sell on Ebay for only a small loss.

Given the low mileage it is hard to believe there'd be anything wrong with the engine itself, and the 20% of the time it runs strong seems to back that up.

edit: just ordered a CDI, it'll be here Friday, stay tuned for updates.
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Old 04-22-2008, 04:34 PM   #14
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Cool, let us know if it's saving the week end
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Old 04-22-2008, 04:43 PM   #15
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Originally Posted by gboezio
Cool, let us know if it's saving the week end
The weather looks like it might be decent this weekend so I might actually get out for a ride, if it runs right I'll be sure to thank everyone profusely but I might just have a spare CDI for sale on Monday and more questions if it doesn't work out.
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