Jetting for Altitude

[BrianM]

This is more for the archives incase someone searches, but I *Finally* got the damn jetting at least in the ball park for 5600 feet in elevation (I'll know better once it gets over 50 and the snow melts from the passes).  Pilots were the last piece to fall into place and were the smallest made with 30.s (factory is just one step up with 32.5's), but the mains I have at 95's currently, which is Several steps down and may need to even go one more.  

The needle is still stock, raised about 1.5mm (2 2.5mm washers), and obvious...  but it's rideable with only a slight dip in power.

I'd hazard to say that going to 30 pilots and down a couple mains would be of benefit even as low as 2~3k feet in elevation.  For $17 and 30 minutes of time, it's certainly worth trying them out.

[gsxr400 racer]

interesting mine came stock with 35's for pilots. speaking of  i have 8 un molested pilot jets first offer takes them.

[PitterB4]

Good stuff.  Thanks, Brian!  

[duane]

So to follow your reasoning here:
At higher elevation there is less oxygen, thus the bike runs richer, so you reduced the amount of fuel in the mixture by using a smaller pilot jet?  
How did you know to use a smaller pilot jet, why not main jet?
(hope this doesn't sound contentious, only curious)
Air-fuel idle mixture screws have always been the ones that have given me fits.

[BrianM]

Now that I've got some time on it, it's still too rich on the pilots (I can start it from Dead cold, 30*F, no choke and one blip to 4k will allow it to settle back to my normal idle)....  so it's time to lower the floats a touch or mess with the air/fuel mixture screws.

And this bike was a recovery from being abandoned in Denver ~ a shop there may have dropped one pilot size from the factory before I get my hands on it ~ though the fiche (IIRC) listed 32.5's as one of the stock options.

Duane:  You've got the basic idea, less atmospeheric pressure up here at altitude, so less pressure squeezing air into a cubic foot of space and thus less O2 for the engine.  Cold changes that a bit as cold air is more dense (which is why some bikes that get tuned in the summer will run lean through the winter) as well.  And this has been an on-going process where I started with the air/fuel mixture screws by setting them to a universal 2.5 turns out.  this is Pretty standard for a baseline adjustment, and on most bikes I've owned, it's been a final adjustment too.  I dropped the Mains 4 sizes at the same time and shimmed up the stock needles.  I made a Lot of big adjustments, but the bike Barely ran before I did that, and ran much better (but no where near perfectly) afterwards.  

I've also got a decade of time under my belt fiddling with carbs and adjusting them every couple of years as I move to different places that were more than 2k feet different in elevations.  The best time was durring my dynojet dyno training when I Really got to see what changes felt like and looked like on the dyno.  I got to calibrate my butt-dyno then.  :D

Anyway, the deciding factor will be as soon as I get out for a longer ride.  Last time it loaded up so bad at low throttle openings (cruising down the highway) that if I'd close the throttle and pull in the clutch, it'd die from fouled plugs and be a Bitch to get started again.  I hope to get a thousand mile trip in the last weekend of this month (with a lay-over at a friends house were I can tinker a bit as needed).

[gsxr400 racer]

Are you sure your rubber plugs for the pilots are good?
And just one more note , it has been said on here several times that if you can start your bike cold with no choke you may have oring issues?

[BrianM]

I doubt those are an issue as I replaced all of them with Factory items last season.

The issues I've got now Feel like just tuning issues, nothing at all like the flooding issues I was having with the old/rotten stuff last spring.