jetting?

[thrownchain]

From sea level jetting specs to 4000 ft, what could you expect to change jetting wise? Just ball park, could you expect a big jump jet size wise? Or minimal? ( Wombat motor)

[matt glascock]

Same fuel with less air = rich condition. Its been a long time since I had a bike up that high but from here (880' ASL) to Durango, CO (6500' ASL) MJ down 2, dropped needle 1 notch. 1978 RM 250.

[ossa95d]

The trails near the farm here in Vermont can vary as much as 3500 ft from the valley floor to the tops of the mountains. The bikes run noticeable richer on the higher trails. We jet for the lowest elevation in an abundance of caution, but if the trails were consistently at the higher elevation I would expect to at least drop one size down on the main jet.

[bobwhitman]

Thus the joy of reeds. Later (01, Toads, 03s) are much more tolerant & flexible than earlier motors.
Bob

[taber hodaka]

I don't change my jetting?? Is it the reeds? -------------- Clarence

[Bullfrog]

Reeds do indeed make the engines more tolerant of elevation changes regarding jetting. But carb size is easily as important in that regard. A rule of thumb I've learned is that carbs smaller than 50% of cylinder bore diameter are pretty tolerant of all sorts of ambient effects on jetting. But carbs larger than 50% of cylinder bore diameter tend to get more finicky. By the time you get to a 36mm carb on a Hodie 125 (56mm bore) - which calculates out to 64%, you'd better have a reed AND a fully stocked carb tuning box, and lots of tuning experience, and time to do the tuning each time you go out riding (and every time a cloud a goes overhead, and whenever anyone spits on the ground within 10 feet of the bike (humidity change) and so on . . .) OK. OK. I'm overstating the case. But it is true that small carbs (relative to bore size) are much easier to live with than really large carbs (relative to bore size).
Ed

[Joe Ormonde]

Bullfrog is 100% correct! I live at 4000 ft above sea level and moved from 1000 ft. My CR 500: Zero changes to the Jetting. On my 71 Super Rat to get it super clean: 1 step leaner on the Needle. I did however find that with the lower atmospheric pressure at 4000 ft, one step hotter on the Super Rat Spark plug was required. I do find it odd just how massive some guys want their carbs. My 88 CR500R has a 38mm Keihin Flat slide carb. Crisp, clean and Violent! Joe.