2strokeworld jetting poll on google docs

[Hawaii-Mike]

This poll is several years old, at least it provides info for jetting Yamaha twins.

https://docs.google.com/spreadsheets/d/1cWqeS9eIAdSZYOhNPKVySceG4PFovT54PIcUks58OpM/edit#gid=0

[scully]

That'll be handy :) thanks !

[buffalorider]

I saw this a while back but it did not address my elevation. Here is what I have to get me started.

34mm Mikuni's
1.25 overbore with ProX pistons
So far using oil injector
6DH3 needle on 3rd clip down
159 P-8 needle jet
270 main
45 pilot
2.5 slide cut out
1 1/8 out on air screw

I live at near '6900 but will be riding bike from around '4500 to maybe '10000 feet above sea level when hitting mountain passes.

[teazer]

I suspect that you might be rich in the midrange ie mid throttle.  Others use P-2 or P-6 needle jets with 6DH7 needles which are leaner.  6DH3 taper stats earlier and therefore is smaller from that point on, even with the same taper angles.  But try it and see how it runs.

#270 is a fairly small main jet, and maybe it's compensating for the needle and jet combo up to flat out. Or maybe it's the right amount of lean out to match the lower altitude.  Honda suggest one jet size decrease for that altitude, but that's for their generators with tiny jets. ROtax suggests a 7% reduction for that altitude, so that's say 320 down to 300 (298 to be precise, but jets that size don't exist).

The thing is that cold air is denser and high altitude air is less dense, so it's always a compromise.  At 60 degrees it's about a 7-8% reduction and above 12,000 at say 80 degrees it's a 13% reduction.  In a low pressure trough, it will be an even larger decrease. 

I'd start with 300 mains, P-6 needle jets and 6DH7 needles but start rich and see how it runs and then try to lean it out a little at a time.