jet size vs. elevation

[dirtdawg]

someone had a post in the how to section a while back how to jet your bike based on your elevation, but there is no chart that tells you what jet you need for your elevation. i am at 85 feet above sea level but i was wondering if there is some kind of equation. i have been looking online but can't find anything. the jetting process is annoying and not that accurate. there is no sure way of telling you are getting the optimum performance besides the plug, which just changes color as it burns. there must be some kind of calculation that is more accurate and lets you unlock the performance of the engine. there is a calculation that tells you what size carburetor you need by figuring the cfm's needed for your application based on normal operating RPM, cylinders, and displacement. just need to find the relationship between the cfm size and the size of the jet (not the jet number but the diameter of the opening.) food for thought but i was wondering if you guys have seen anything like i am talking about. sorry for the long post, don't want to bore you.

[v0lcom13sn0w]

lower elevation you wanna run fatter jets. on my 400ex i really only ride the dunes at the oregon coast its 0 to 250ft approx... stock the jetting is a 148mj and a 38pj so i bought a dynajet ket and am now running a 170mj and a 42pj... also took my choke off because with that fat of a pj you dont need the choke no matter how cold it is. i noticed a HUGE difference and with simple bolt ons... jetting, exhaust, k&n with no airbox, and bored out carb i beat my friends all stock 400ex by about 10-15 bike lengths in a straight drag

[GPracer2500]

Originally posted by dirtdawg
...just need to find the relationship between the cfm size and the size of the jet...

It's not that simple. There are too many things involved.

A main jet's orifice size is not the end-all-be-all that decides how much fuel that particular jet will deliver (even given a specific cfm). It is carburetor, engine, and setup specific. Two jets of identical size can deliver different quantities of fuel depending on other things.

Differential air pressure is what pushes fuel through a carb's fueling circuits. Lots of things can influence that differential. A change in compression ratio can change the signal strength seen by the carburetor, for example. Main jet circuits also have main air jets--they are air bleeds that have just as much influence on fueling as main jet size. On many carbs the main air jet isn't a removable jet--just a fixed orifice. But that would still need to be taken into account to come up with some sort of universal equation. Also, the air pressure present at the opening of the main air bleed can be effected by intake boot design--yet another variable.

I'm just scratching the surface here. I'd be hard pressed to come up with a comprehensive list of all the things about the carb, engine, setup (intake, exhaust, etc), and environment that would need consideration in order to develop a jetting equation that was trustworthy. But if such an equation was developed, it would be pretty specific to a particular machine, me thinks.

If you want your jetting to be as close to perfect as possible, have it tuned on a dyno with a wideband a/f meter and a tuner that knows what he's doing. Otherwise, trial and error combined with your own experience will likely yield the best results.

FYI, as a very generalized rule of thumb a ~2000ft change in elevation may be 1 jet size (e.g. 155 to 152) while a 20*F change in temperature will also be one jet size.