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PostPosted: Mon Sep 20, 2010 11:44 am 
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Joined: Mon Apr 19, 2010 12:00 pm
Posts: 1
Hello, I have an '89 RM 125. I've searched the forums, but haven't found any conclusive answers to my questions.

Problem:
The bike has no power at wide open throttle. In all gears, it will run through the first 75% of the power band fine, but during the last 25% (especially if you open the throttle all the way) the engine studders. It doesn't bog down, just has no power/accelaration.

What I've done so far:
Rebuilt the top end (piston - .25mm over, ring, gaskets, etc.), cleaned all jets in the carb (all jets are stock sizes), adjusted float level to stock height, floats don't have holes in them, needle clip is in stock position, exhaust/power valve is at stock setting, reeds are fine, repacked muffler, new air filter - not over oiled, running 20:1 premix because thats what the manual calls for, new spark plug looks fine (may lean toward the rich side, but not that bad), changed transmission oil, and changed coolant.

So....at this point I don't know if its a carb/fuel problem or an exhaust/power valve problem. For what its worth, it seems to run great right before it runs out of gas. Not sure what that means though.

Any help would be great, thanks in advance.


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PostPosted: Wed Sep 22, 2010 5:53 am 
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Joined: Thu Jan 05, 2006 1:35 pm
Posts: 344
I would normally suspect power valve not opening, but you noticing it runs great when nearly out of gas leads me to believe it may be your main jet or you still have too high of a fuel level despite setting float height. Are you leaking any fuel out carb vents?? a stuck float needle can cause this. If not, try a couple sizes smaller main jet for just a quick run and see if that makes any difference at all. But be sure and run a plug chop if you notice a difference. You should always step down main jets one at a time, but I get impatient with troubleshooting and will try two sizes at a time to see if that is the problem or not.


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PostPosted: Mon Oct 18, 2010 11:57 am 
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Joined: Fri Apr 04, 2003 11:34 pm
Posts: 43
Location: Daytona Beach, FL
I would look at the main jets 1st. The 20-1 is a little much, IMHO. Also, take a good look at the float needle. It may be worn or sticking, resulting in too much fuel at full throttle. This would also account for the reason it runs good just before it runs out of gas. I had this happen to me with my '77 RM125B. It would fire right up, rev while sitting in neutral, but feel like someone stuck a banana in the silencer when I went to open it up full throttle while riding. I cleaned the carb many, many times, set the float height, etc, etc... I was about to replace the carb with a new one and decided to replace some of the jets first. When I replaced the float needle valve, it came to life and ran like it should. Good luck!


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