tuning Keihin CV carb for altitude: Sprockets forum: Grassroots Motorsports Magazine

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Sept. 8, 2009 6:03 p.m. benzbaron Reader

Well in a few weeks I'll be heading to Reno for street vibes and need to go through my bike(2000 buell blast) to make sure it runs good. My issue is that tuning carburetor mixture sounds like one part craft to one part witchcraft. I read something about checking plugs after riding a certain way to see if the mixture is alright. This part of the job sounds like witchcraft.

My bike's only modifications are an uncorked exhaust, foam air filter, and next largest size pilot and main jets. The bike runs good at sea level but last time it didn't like idling once up to altitude(2-3000ft). Reno is around a mile high and Virginia City is 7500ft.

I have an easy idle adjust screw and the idle screw needs to be further out than the 2.5 turns out the jet manufacturer suggests. It is like 4-5 turns out to have the bike idle alright. I was thinking after reading about carb tuning that the pilot jet might need to be the next size up, but I'm a little nervous of taking my bike further away from stock. I also have to check the automatic choke to see if it works.

Just curious to see if anyone knows of the hoodoo voodoo involved in carb tuning.
Sept. 12, 2009 5:43 p.m. 4g63t Reader

If you're too lean in the pilot jet now (as evidenced by your four turns out) you'd probably be a bit rich at altitude.
Sept. 13, 2009 7:23 p.m. ManofFewWords Reader

If I were you, I would call a dealer that's located at the altitude you're headed to. That's what we did when we lived in Aspen (8000ft+).
Sept. 15, 2009 6:52 p.m. andrave Reader

If you can get your hands on a dial a jet for that carb they work wonders when trying to tune for different altitudes. I ran one on my TRX450R four wheeler with great results. They make different models for almost any carb.
Sept. 15, 2009 9:32 p.m. Jensenman MegaDork

When a bunch of us flatlanders were headed for 9-13K feet elevation, Moose Racing (now called A Loop Racing) recommended we use our existing main and pilot jet numbers and multiply by .90. As in, a 158 main jet x .90 = 142.2, so a 142 main would work well at those altitudes. I was a bit skeptical but it worked. The bikes were down on power compared to sea level but they started and ran well. You won't be going quite so high, maybe use a .95 correction factor. Always remember that air temperature plays a big part as well (cold air denser, hot air less so).

Here's a temp/altitude jet correction table I found: http://www.nightrider.com/biketech/main_jet_correction.htm

Man, I lubs fuel infection.
Sept. 16, 2009 3:34 p.m. benzbaron Reader

Thanks for the information folks, I really appreciate it. I'm going to just pack the stock jets and head up. I'm going to go up to my moms a couple of days before to test the bike and make sure it doesn't live up to its nickname, the punky buellster.

I doubt the bike will see over 7000ft but who knows. Thanks again!