300 jetting advice for colorado??

[celler]

Originally Posted by GGRider01

Just curious, why are a lot of guys going up on pilots at high altitude? Is the lower air density causing less fuel to be pulled through at idle RPMs?

I would like to hear some thoughts on this. I have done this to many bikes successfully.

[coopernicus]

You know, I really did not want to believe that going up on the pilot at our altitudes would help but it does. I conducted repeated tests with and without RB mods on a 250 and 300 using multiple needles. Surprisingly, on my 250, the best low end was with a 52 pilot (AS1), an N1EE needle, number 8 slide, a Boyesen Rad Valve with carbon reeds, local tuner head mod, and the reed spacer removed. It jumped off the bottom as strong as a 300 and then fell flat on it's face due to the rich taper and could never tune it out. I believe the lesser atmospheric pressure at our altitudes will not "pull" enough fuel at a given RPM through a pilot jet (remember, it is all about orifice size, pressure, atomization, and timing of the atomized fuel getting to the combustion chamber). A larger pilot gives it access to more fuel to offset this weaker vacuum in the carb. The end result is that a larger pilot does work at higher altitudes.

Eric

[GGRider01]

Originally Posted by coopernicus

Surprisingly, on my 250, the best low end was with a 52 pilot

This was done in Denver? You're only 500 or so feet higher than me and I'm running a 42 pilot.. That is the fattest I have, I believe. Time to pick up some more jets and see what happens. Although the 50cc less displacement of the 250 could probably have a measurable difference on how strong a vacuum it generates.

I have no intention of having a carb mod done, but I won't be surprised now if a fat pilot gets me hopping along even faster than before.

I'm curious, if several step ups are taken, if I end up back on a W diameter needle to get a better transition coming off of the pilot. I know after going up and down from the 172 main the NEDH and 172 are very happy together so I'd hate to start over on all that.

Anyway, didn't mean to hijack, but this definitely provided some interesting insight.

[adriano.ro]

Originally Posted by coopernicus

You know, I really did not want to believe that going up on the pilot at our altitudes would help but it does. I conducted repeated tests with and without RB mods on a 250 and 300 using multiple needles. Surprisingly, on my 250, the best low end was with a 52 pilot (AS1), an N1EE needle, number 8 slide, a Boyesen Rad Valve with carbon reeds, local tuner head mod, and the reed spacer removed. It jumped off the bottom as strong as a 300 and then fell flat on it's face due to the rich taper and could never tune it out. I believe the lesser atmospheric pressure at our altitudes will not "pull" enough fuel at a given RPM through a pilot jet (remember, it is all about orifice size, pressure, atomization, and timing of the atomized fuel getting to the combustion chamber). A larger pilot gives it access to more fuel to offset this weaker vacuum in the carb. The end result is that a larger pilot does work at higher altitudes.

Eric

Removing reed spacer,gain some quick response,torque? Where do you feel more difference,bottom,midrange...? I have 0,5mm one.

[coopernicus]

Originally Posted by GGRider01

This was done in Denver? You're only 500 or so feet higher than me and I'm running a 42 pilot.. That is the fattest I have, I believe. Time to pick up some more jets and see what happens. Although the 50cc less displacement of the 250 could probably have a measurable difference on how strong a vacuum it generates.

I have no intention of having a carb mod done, but I won't be surprised now if a fat pilot gets me hopping along even faster than before.

I'm curious, if several step ups are taken, if I end up back on a W diameter needle to get a better transition coming off of the pilot. I know after going up and down from the 172 main the NEDH and 172 are very happy together so I'd hate to start over on all that.

Anyway, didn't mean to hijack, but this definitely provided some interesting insight.

I had the carb mod and I don't think I would do it again. Please remember that all bikes will differ a little if they are the same configuration. Additionally, we don't have much riding at Denver's altitude as we tend to ride at 9000 feet +/- 2000. I ride with Frankinberg and our bikes our tuned and geared differently. We differ most with pilot and main but I ran the JD Blue needle most of the time on my 250 with a 50 pilot, #8 slide, and 165 main.

Eric

[coopernicus]

Originally Posted by adriano.ro

Removing reed spacer,gain some quick response,torque? Where do you feel more difference,bottom,midrange...? I have 0,5mm one.

The only difference that I was able to feel was on the bottom as soon as I opened the throttle and nothing more. I called it "improved throttle response" but I would not say it made any difference anywhere else in the powerband.

Eric

[Zman]

While the specific jetting suggestions may be close, you need to remember all bikes are slightly different and you need to jet your specific bike for the riding conditions. I am pretty sure your base jetting is different from other bikes so you need to work from your base line.

Assuming similar riding temperature ranges, I have found that dropping the pilot and main jets 1 size worked well for 6 to 9k ft worked well and drop 2 sizes and drop the needle one clip for 9K to 13K+. After you reject you will need to adjust your idle and idle mix as well.

Regarding the question on increasing the pilot size. It helps compensate for a weak vacuum signal like many GG engines have with large squish and big 38mm carbs on 200 cc and 250cc engines. If you already have the squish and carb mods done it should work out well as indicated above.

You can also help increase the vacuum by adding a little ignition timing.

[FastFast]

40 Pilot
NEDW on the 3rd clip
165 Main
Air screw is 2.5 out.
50:1 with Opti2 and 91 Octane.

I use this combination riding in the Rockies. Very pleased.