The effects of weather, elevation, on


6 replies to this topic
  • rdrata

Posted June 15, 2009 - 06:08 PM

#1

Jetting...
Anyone have a cliff note version on how seasonal and elevational changes effect your jetting and why?

  • grayracer513

Posted June 15, 2009 - 08:12 PM

#2

Temperature and altitude affect jetting by changing the density of the air. The engine is, in part, an air pump, and moves air by volume (cubic feet). Fuel mixture is done on the basis of mass, not volume, and 14.7 pounds of air is required to oxidize one pound of air.

The trouble is that when air is less dense, their is less of it in any given cubic foot (it weighs less per volume).

So, when temperature rises, or the altitude is increased, the air becomes less dense, and the jetting becomes richer because a smaller mass of air is flowing through the carb than it was calibrated for.

Conversely, if it gets cold, or the altitude is lower, the air is denser, and the carb becomes lean.

  • rdrata

Posted June 15, 2009 - 08:56 PM

#3

Thanks for that concise explanation Gray; on my 45/168 set up, is their a general rule of thumb on changing pilot/main relative to a set temp/elevation, or rather, how much "fudge" can you get away with before it is imperative to change? I ride in the 600~4000 ft range almost year round (not the hot,hot months!:thumbsup:)

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  • grayracer513

Posted June 16, 2009 - 05:16 AM

#4

I ride in the -80~4000 foot range without changing jetting except to go to a 165 in around May sometime, and back to the 168 in November.

It's obviously not going to be perfect everywhere, but 4-strokes have a great deal more flexibility with their fuel mixture than people sometimes think. If I was campaigning a flat tracker, I'd be more precise, but I'm not.

The "rule" is roughly a 1% reduction in jet number for each 1000 ft increase in altitude, or 11 ℉ increase in temps, or the reverse if you go lower or colder.

  • rdrata

Posted June 20, 2009 - 09:53 AM

#5

Gray
Higher elevation, Higher temp = Leaner jetting
Lower elevation, Lower temp = Richer jetting

What happens if your in Higher elevation w/ Lower temp; or
Lower elevation w/ Higher temp?

  • grayracer513

Posted June 20, 2009 - 03:04 PM

#6

Gray
Higher elevation, Higher temp = Leaner jetting
Lower elevation, Lower temp = Richer jetting

What happens if your in Higher elevation w/ Lower temp; or
Lower elevation w/ Higher temp?

To clarify:

Higher temps or elevations result in RICHER jetting if you change nothing, and require you to jet LEANER to compensate.

Just so we're talking about the same thing.

If you went up 1000 feet, but it was 11 degrees cooler there, it would be roughly a wash.

  • rdrata

Posted June 20, 2009 - 03:22 PM

#7

Yeah, I got you; that's what I meant (require you to compensate). I thought that they may wash each other, but I had to ask.:worthy: Thanks:thumbsup:





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