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High altitude jetting

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I just moved to Colorado, and where I am riding it is 8000 feet to about 10000 feet (2004 Crf150F). What do I need to do to make sure it is jetted properly. I have never even attempted to jet, so kinda lost on what to do. My manual has nothing on jetting and I can't find anything else online. The temperatures range between 40 and 80, and it is usually pretty dry here.

Thanks for the help.

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You must change your A/F roughly 1% for every 1,200 feet and/or every 9 degrees F.

 

For altitude:

  • 8,000 ft = 93.3% or 42 pilot and 112 main
  • 10,000 ft = 91.7% or 42 pilot and 110 main

I'd start with a 42 pilot and 112 main.  Better off a little rich than a little lean.

Edited by VortecCPI

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You can change the main via the bowl drain at the bottom of the carb.  It's a bit cramped but it can be done.  The main jet holder / Emulsifier tube may come out with the main.  That's okay - Just don't tighten it too much or the main jet will shear.

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make sure it needs re-jetting before you do any thing....

 

my daughters TTR125 ran just fine to the top of engineer pass (12900ft) with no jetting issues.

I had purchased some just in case, but it never needed them..

 

Due to EPA, most bikes are coming from the factory too lean in my opinion, and that trip proved it to me.

 

ride your bike for a while and then perform a plug test. BEFORE YOU BUY JETS...

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make sure it needs re-jetting before you do any thing....

my daughters TTR125 ran just fine to the top of engineer pass (12900ft) with no jetting issues.

I had purchased some just in case, but it never needed them..

Due to EPA, most bikes are coming from the factory too lean in my opinion, and that trip proved it to me.

ride your bike for a while and then perform a plug test. BEFORE YOU BUY JETS...

Ok how do you perform a plug test?

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Forget about a plug test.  It's a pain and not reliable.  Use the Mikuni "Roll-Off" method:

"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). Fiticon1.png 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."

Edited by VortecCPI

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Forget about a plug test. It's a pain and not reliable. Use the Mikuni "Roll-Off" method:

"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). Fiticon1.png 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."

Ok I will try this, if I need to re jet do I need to buy new needles or do I just move the position they are in? If I need to re-jet my bike will most likely be running rich right?

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