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Carb Main Jet Options

  • Thread starter Thread starter justanother54321
  • Start date Start date
J

justanother54321

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I have a 79 GS850 with stock carbs VM28, and stock exhaust.

I'm curious about what size main jet would provide the best fuel economy for trips that vary between sea level and 4500ft.

I don't remember what size the stock main jets are but I also have a set that are the next size or two up, that I bought for higher elevations 6,500 (the size numbers that come to mind are 90 and 105)

I will be riding below 2500ft most of the time.

Any thoughts are much appreciated.
 
justanother54321 said:
I have a 79 GS850 with stock carbs VM28, and stock exhaust.

I'm curious about what size main jet would provide the best fuel economy for trips that vary between sea level and 4500ft.

I don't remember what size the stock main jets are but I also have a set that are the next size or two up, that I bought for higher elevations 6,500 (the size numbers that come to mind are 90 and 105)

I will be riding below 2500ft most of the time.

Any thoughts are much appreciated.
Click to expand...

Yours stock should be 102.5, don't go any smaller for the higher elevation unless you won't be riding at sea level, as running too lean will do damage.

Mostly the VM carbs ran pretty good with the stock settings, unless the intake or exhaust has been modified.

What I do is jet my bike for 5,000 where I live, and remove the airbox lid to help it keep from going too rich when I go up in the mountains.
Another idea is to carry a set of 100s for the mid elevations and maybe a set of 97.5 for if/when you go way up high.
Takes a couple minutes to change them out if you have tools that fit in there correctly, I put Allen head screws on my float bowls to make it easier.
Depends on where you will be riding, is it up a mountain and right back down, or go high and stay there all week?
If the first case, just leave the stock jetting and accept the reduced power.

Actually, if most of your riding is below 2,500', just leave it alone. It will run great, mileage will be fine.
 
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Personal opinion...leave them stock or change them out for the higher elevation rides (but that seems like a lot of trouble to me) as you may see some plug fouling when you hit the lower elevations... :-k
 
Dave8338 said:
Personal opinion...leave them stock or change them out for the higher elevation rides (but that seems like a lot of trouble to me) as you may see some plug fouling when you hit the lower elevations... :-k
Click to expand...

Um, the plug fouling happens from being too rich, up high with stock jetting, running at sea level with smaller jets would be too lean, and could burn valves or pistons.

But a GS with stock jetting won't generally be so rich that it fouls plugs unless you go way up high and stay there quite a while.
 
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