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Crf150f no top end power

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So I recently got a chance to ride my bike again for the first time since fall after New Yorks terrible winter and I found that my bike has very low top end power. It seems fine in 1,2,and 3 gear but when I shift into 4 it can't gain any more rpms. It just stay at a pretty much constant rpm until I shift into a lower gear. I cleaned out the carb and Jets twice and it's spotless. The plug looks perfect color. There it no Bogging that I would usually associate with a rich main jet. The top end was replaced last summer (valves, piston, etc.). I checked the timing already and it seemed perfect. I am really stumped at what the problem could be. Could it be that the spell is weak under hasn't loads being put on the engine? Maybe the stator is going bad? I do have tall gearing on it but it was flawless last year and ran perfect.

Please help me figure this out. All help is appreciated.

Fox

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I just checked the float level yesterday. It was set at 12.5 mm. I will check the fuel lines today. It's probably not it but what else could it be?

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And it doesn't sound like it's starving for fuel, like there's no sputtering or anything. Could it be something worth the stator going bad and it losing spark under heavy load? Just throwing out ideas..

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It gets worse. But Y would this only be happening in higher gears under more load.?

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It gets worse. But Y would this only be happening in higher gears under more load.?

 

The higher the gear the higher the load on the engine and the higher the demand in fuel flow.  This is why we dyno test with a 1:1 gear.

 

A flat top end is usually a sign of a lean condition.  A missing or sputtering top end is usually a sign of a rich condition.

 

A lack of total timing can also cause a dull, flat, or weak top end.

 

Did you check valve clearances?

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I just checked the float level yesterday. It was set at 12.5 mm. I will check the fuel lines today. It's probably not it but what else could it be?

 

I always check float level externally.  Even though the measurement is correct there is still the possibility of issues such as bad seat, bad needle, bad float(s), sticking, etc.  Put a piece of clear line on the drain nipple and run it up vertical to a point above the top of the carb.  Open the carb bowl drain and the fuel will climb to the same level it is in the carb.  it should be just at, or just a hair below, the float bowl gasket position.  If it is not you have a problem.

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Ok thank you very much I will try that. I just checked the valves last night and the intake clearance was .0025 and the exhaust was .004. Is that right? I got it off the manual but what is the best to run them at. When I checked the timing I got the engine to TDC and I put the index mark on the case in the center of the two marks on the flywheel and the lines on the cam were parallel with the head surface. I also have a hot wire plug wire and an iridium plug, along with a thunderjet from the same company as the dial_a_jet (works amazing). In regards to jetting I should I try a bigger main jet? I think I'm at a 105 or maybe even a 102 cause of the thunderjet.

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Ok thank you very much I will try that. I just checked the valves last night and the intake clearance was .0025 and the exhaust was .004. Is that right? I got it off the manual but what is the best to run them at. When I checked the timing I got the engine to TDC and I put the index mark on the case in the center of the two marks on the flywheel and the lines on the cam were parallel with the head surface. I also have a hot wire plug wire and an iridium plug, along with a thunderjet from the same company as the dial_a_jet (works amazing). In regards to jetting I should I try a bigger main jet? I think I'm at a 105 or maybe even a 102 cause of the thunderjet.

 

I use a Dial-A-Jet on my CRF230 and I went down from a 120 main to a 112 main per Lonn's advice.  It works awesome.

Mike Coe recommends a 110 main for the CRF150 and Thunder products states "With the Thunder PowerJet installed, the main jet is decreased by 20% to 25%."  That means you should be running an 82 to 88 main jet.  If you're not sure give Lonn a call so you can get it dialed in correctly.

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I actually just checked and I'm at a 95 main with about a turn out with the thunderjet. What and the jetting gets worse (to lean, popping) when I screw it in. So what should I do with the jetting?

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I actually just checked and I'm at a 95 main with about a turn out with the thunderjet. What and the jetting gets worse (to lean, popping) when I screw it in. So what should I do with the jetting?

 

I have been using Mikuni's "Roll-Off Method".  It is very fast and easy.

 

"Roll-off Method:

This method is a good way to get the main jet either correct or within one size of correct. It is based on the fact that that as the throttle is closed, the air/fuel mixture richens momentarily. This normal enrichening can be used as a diagnostic tool. The test is started with the engine running at an rpm high enough to ensure that it is “on the cam.” Open the throttle fully and let the engine pull for several seconds. Then, quickly close the throttle to about the 7/8ths position. If the engine seems to gain power, the main jet is too small (lean). Fit a larger jet. If the engine hesitates as the throttle is rolled off, the main jet is too large. Fit a smaller one. When the main jet is correct,  the engine will continue to run smoothly and evenly as the throttle is closed. Note that a main jet that is far too rich or lean may cause the engine to misfire at full throttle."

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So I checked the fuel line and made sure it was flowing ok and I also draw ed the tank and put in a fresh tank of 93 (what I always run) since it's been sitting all winter even though I put stabil in it before winter. Waiting on a new smaller main jet to arrive cause the smallest my dealer had was a 92. Any other ideas of what could be causing this? Could it be something electrical? What is the best valve clearance to use?

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Anyone have any ideas?. I'm really confused as to what could be causing my bike to not gain any rpms in higher gears. It just says at a constant rpm at full throttle, like it has no power to accelerate. Please I need help to fix this.

Thanks

Fox

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