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Jetting for altitude

Spyder

Spyder

Forum Sage
Past Site Supporter
Trying to help a friend out. He has moved to Colorado, and is having huge running issues. I know we have had quite a few peoples here having to rejet for moving to higher elevations. I believe I am correct that a bike will run richer at higher elevations? Anybody have a rough figure on how many jet sizes he will need to drop for moving to 3000+ feet? Just trying to get him in the ballpark for a start.
 
What's he riding? My 850 with CV carbs and stock jets has run fine from 3,000 feet in Southern New Mexico up to Wolf Creek pass in CO at over 10,000 feet. The only difference I noticed was better gas mileage up there.
 
He's on a '93 GSXR 750. He said it was running fine before moving. He went from sea level to 3000+ feet. All of the symptoms he has listed indicates very rich. I am only assuming it is due to the altitude near the continental divide.
 
I have ridden a normally aspirated CV carb bike up & down every mountain in Colorado (nearly)

2 things come to mind --

1) A sever lack of power when at altitude (specifically above 6000-7000 ft)
2) They sell 85 octane rated fuel in Colorado to accomodate the change -- could that be it ?

The CV carbs do a pretty good job of working through the altitude changes in my experience-
My stock jetted 850 was at the top of the Cherahola Skyway last year (5390 ft) with no ill effects ?
No lack of power either really ---

Just my experience
 
Thanks Carl. This guy has stock jetting, and he seems to be running rich. But, as you know with forum diagnostics, it can be difficult to obtain all facts.
 
With sea level jetting (central CA coast) that was rich for max power , I have gone through Tioga Pass going into Yosemite from the East with ill effects. I'm pretty sure the road is over 9000 ft.
The bike will load up but, will clear on the way down. The point is the bike continues to have plenty of power, although the decreased AFR probably does reduce max power. But nobody is doing 1/4 mile trial at 10K ft on mountain passes.

I would guess that the friend's bike had the carbs dry out and clog up the pilot jets of something equivalent.
 
For some closure. He dropped a jet size, lowered his needle, and put new stock pilots in. carbs are running great now. Now he is having fuel issues from the tank I think. Sounds like he ran the bowls dry. He's almost there. Thanks for all the info. I haven't had to deal with altitude vs. carbs before, and I am one of the few on the GSXR site that still knows what a carb is, LOL.
 
Spyder said:
For some closure. He dropped a jet size, lowered his needle, and put new stock pilots in. carbs are running great now. Now he is having fuel issues from the tank I think. Sounds like he ran the bowls dry. He's almost there. Thanks for all the info. I haven't had to deal with altitude vs. carbs before, and I am one of the few on the GSXR site that still knows what a carb is, LOL.
Click to expand...

As long as he stays at some altitude that should work fine --
But if he comes down to the flat lands -- it may run like crap again


 
Got to wonder if there isn't some confusion as to cause and effect here. I've never had problems with a CV carb at altitude, seems to me the best course of action is to go 100% stock as a baseline and work from thee.
 
He has a petcock issue right now as well. Once he gets that replaced, I will have him start to read the plugs. It's running well on prime right now. Once all else is correct, he can begin the fine tuning process. Had to get it to a consistent running state to iron out all of the other issues. I am thinking he may need to bump up one size to stack once it is all said and done, but we'll see.
 
Yes, performance drops a bit as you go up in elevation. Simple physics, lack of oxygen and all that. Somehow, the CV-type carbs tend to compensate for that. Maybe not completely, but mostly. "Mostly" enough that most people don't need to adjust their jetting. Only if you are racing and NEED that last 0.1 horsepower will you need to change the jetting.

That is great for all of us flat-landers. :clap: :clap:
For those that live at higher elevations and ride to even higher elevations regularly, there is still not much need to adjust. But if you DO adjust, just make sure you don't go to lower elevations without re-jetting back to stock. Your "higher-elevation" jetting will be too lean for lower elevations and you will stand the risk of overheating, possibly to the point of damaging the engine.

.
 
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