04 yz450 hi-alt./lo-mid-temp. JETTING?

1 reply to this topic
  • tommymac

Posted April 20, 2005 - 09:57 AM


I originally posted this in the Jetting Forum, but go no responses at all - would really appreciate any insight as to the following:

Question for JD or any other TT jetting guru out there:

Ride in and around Calgary, Alberta which has an elevation of approx. 3500 ft. Rides are typically from this elevation up to around 4500-5000. Average relative humidity in winter is about 65% - not sure what it is in the summer - but I'm guessing no more than 75% (its pretty dry here). Temperatures currently are about 10-15C (approx 55-60F ??) and in the summer are usually around 20C

Have been experiencing mild to moderate sputtering and popping on decel - as I understand it this is either to be expected with a 4-stroke, or may be symptomatic of a lean condition. Installed a WB Carbon-Pro exhaust which I would assume would exacerbate a lean condition, and therefore took to tinkering with the jetting.

Given that the popping occurred while off the throttle, I assumed that the first course of action would be to go to a larger pilot jet (stock is a 45). I did this but when I went to kick it over it let forth a HUGE backfire and the whole garage reeked of gas. Was unable to get it to fire. I figured I fouled the plug - but checked it and it was fine, and once I replaced the stock PJ - she fired up easy.

Instead I raised the stock needle by dropping the clip to the 4th pos. and went to the next largest main jet that I had, a 178 (stock is a 172). Took it to the track and this setup seemed to remedy any popping/sputtering. I'm wondering now though whether or not it's too rich.

Just received the JD jet kit, which suggests a main jet of 168 and the blue clip in the 3rd pos. While I understand that these are merely guides and that actual conditions will dictate changes +/-, does anybody have any recommended jetting setups for the aforementioned conditions? I would like to have the best throttle response possible, but do not want to sacrifice engine reliability to get it (I'd rather be safe and ride a little rich than a little lean), which brings me to my next question: How dangerous is it to run a 4-stroke on the lean side. I have heard that this is more a danger for 2 strokes, but I haven't yet heard a convincing argument as to why this may be so. Thoughts?

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  • grayracer513

Posted April 20, 2005 - 11:06 AM


First, popping on decel is not always a result of incorrect mixture. It can just as easily be caused by an air leak in the exhaust, and although you can sometimes cover the condition up with richer idle mixture, you'd be running a overly rich mixture in reality by doing that. Leaks most commonly occur at the joint of the header to the midpipe, but they can happen at the head and the can/midpipe joint as well.

Typically, more altitude, and lower temperature make you richer, and I believe that higher humidity has the same effect because the extra water in the air displaces that much of the air itself.

Are you good with Excel? I have a formula that can be used on its own or plugged into a spreadsheet to calculate jetting changes based on temp and altitude.

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