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Ask Jeff Policky Your Own Question
Jeff Policky
Jeff Policky, Motorcycle Mechanic
Category: Motorcycle
Satisfied Customers: 2425
Experience:  Yamaha Gold, Suzuki Gold, Honda Bronze, Polaris Bronze, BRP
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I have a 2008 yz450f that I have rebuilt the top end on, and

Customer Question

I have a 2008 yz450f that I have rebuilt the top end on, and rejetted to the high altitude recommendations of jd jetting. I have been experiencing a stalling problem after about 15 min of running. I have been leaning out the fuel mixture which seems to help, however I then need to raise the idle to keep it running. Any ideas?
Submitted: 5 months ago.
Category: Motorcycle
Expert:  RSRBOB replied 5 months ago.
Hi and thank you for your question.You mentioned having to raise the idle. Are you setting it with a tachometer and what spec are you setting it to?Also, what is the elevation where you are riding?What are you changing to lean it out?The problem you are experiencing is strictly at idle?ThanksRSRBOB
Customer: replied 5 months ago.
I did set idling speed with a tach initially, 2100rpm, and before I ride for 15 min it seems to work ok. Then I find when I am cruising on easy terrain it seems ok. Once I have to come into a sharp turn, or get to a more tech section it stalls. Then it quickly goes into a trend of stalling often and also going down hill even on easy terrain. I'm riding mostly from 8000 -10000 Ft. I have been using the air fuel screw on the bottom of the carb. Although I have messed around with the throttle control cable adjustment at the grip, I usually adjust the idle speed with the screw on the side of the carb
Expert:  RSRBOB replied 5 months ago.
Thank you for the additional information.I would like to know exactly what jets you changed also.Also, how many turns out is the fuel screw right now?Have you confirmed the fuel level also? Spec is 8mm,While the bike is still idling fine, if you hit the throttle wide open, like you wanted to blast from one corner to the next, does the bike instantly respond or is there a hesitation of any kind?Lastly, is this only happening at 0-1/4 throttle position settings? Once you get into mid-range all is well?I apologize for asking so many questions but without being able to feel it, I need to know a lot of details about what you see and hear.Thanks,RSRBOB
Customer: replied 5 months ago.
No problem with questions I'll do my best to answer. Main jet#160 3rd clip pilot 48. Screw is at one turn now. And when it is cold it idles lower (no tach right now) no problem, and no lull in throttle or catch... however once it warms up, the idle has to be increased or it stalls and not all the time but offen and increasingly more noticeable there is a bog in acceleration as it warms up. Even when I try to charge a difficult section with high throttle and try to use clutch to control power it often stalls. Again all bad signs show up after its warm... generally starts pretty easy... increasingly difficult as symptoms worsen
Expert:  RSRBOB replied 5 months ago.
I noticed you are up 1 size on the PJ. Stock is a 45. Yamaha offers a 42, and KH probably offers a 40. With your fuel screw all the way down to 1 turn, that tends to indicate the PJ is too rich. Right off the bat I would throw a 42 PJ in it and adjust the fuel screw (out) from there. If it gets better but not 100% yet, drop down to a 40. the PJ and Fuel screw don't meter flow identically, although they do overlap at transitions. You don't want to try to correct for one being wrong by over compensating with the other.Another thought came to me too. What kind of fuel are you using? We ran into a problem at Yamaha one time where an ATV was boiling the fuel in the carb and causing it to go rich once it got hot. Just a thought, but, if you aren't using race gas, it is possible. The subtle caveat to that was that it was also spring time and the ATV's were ending up getting winter fuel which tends to boil at a bit lower temp than normal (summer) fuel. That is definitely a bit of a long shot but something I thought about when contemplating what else could change when the engine/bike gets hot. If you aren't using designated race fuel, I would try that before too long. I suspect changing the PJ is going to help no matter what, but the race fuel may boost overall performance.Another point to consider if acceleration becomes the sole issue, is the amount of fuel pumped through the accelerator pump is also adjustable (jetted) too. Right now it sounds a bit more serious than that, but keep it in mind as you are testing different settings and when the conditions the symptoms occur.RSRBOB

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