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Linkmeister Supreme
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7,960 Posts
Discussion Starter #1 (Edited)
Can we create "Carb Jet Chart" for common modifications and elevation changes

Every time someone asks about carb jet sizes needed after an ear shave or exhaust modification, we say "contact lance328".

Perhaps we can get lance to post a chart or table with his guidelines for choosing sizes needed at various elevations on a stock bike, and for the common intake and exhaust modifications. This will save him time in the long run from having to answer every individual question.

IF I understand carburetion principles correctly, a bike running in thinner air at 2,000 or 5,000 feet elevation needs correspondingly smaller fuel jets than one running at sea level. Is this right?

Conversely, if a bike has a freer breathing engine due to an intake ear shave (and installing K&N filter pods or the like) AND an aftermarket exhaust, the carb needs bigger jets at a given elevation,
(for a richer mixture to compensate for increased exhaust scavenging efficiency of the cylinder).

Does anyone know what elevation the OEM stock jets are calibrated to on a US, California or Canadian model VN750? No info about this that I can find in the service manual, or here. I ask because it could be operated at any elevation from sea level to an upper extreme of perhaps 11 or 12 thousand feet in mountain passes. I suppose the majority of motorcycles in North America are operated between sea level and 3-4 thousand feet of elevation. A few more at 5-6 thousand feet around The *Mile High City* of Denver, or other mountain areas.

So, can we create a "rule of thumb" saying something like- "You increase/decrease jets by one size for every 1,000 (or 500, or 1,500?) feet difference in elevation?"

Likewise, is it as easy as upping the jets one or two sizes for specific intake and/or exhaust mods?


Extreme demo example: An OEM VN750 carb is calibrated for sea level, and it is now in Denver at 5,000+ feet, earshaved and V&N pod filters, with V&H exhaust.

Can I add 2 or 3 sizes for the intake/exhaust mods, and subtract X number of sizes for increase in elevation?

Is this too simple an equation to work?
 

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Old Truck Junkie
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4,133 Posts
I am at 1962 ft. above sea level. Lance said to use 40 pilots, 140 mains and 2 shims on each needle.
I had jardine exhaust and ear shave with the k&n rc-2340 air filters.
 

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Linkmeister Supreme
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7,960 Posts
Discussion Starter #3
I am at 1962 ft. above sea level. Lance said to use 40 pilots, 140 mains and 2 shims on each needle.
Niterider, is that with stock intake and exhaust?
 

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Giggity!
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4,307 Posts
Hoss, Drinking tonight? Pssssst... I am lol. Read this again. Your answer is here.

I am at 1962 ft. above sea level. Lance said to use 40 pilots, 140 mains and 2 shims on each needle.
I had jardine exhaust and ear shave with the k&n rc-2340 air filters.
__________________
 

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Linkmeister Supreme
Joined
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7,960 Posts
Discussion Starter #5
Hoss, Drinking tonight? Pssssst... I am lol. Read this again. Your answer is here.

I am at 1962 ft. above sea level. Lance said to use 40 pilots, 140 mains and 2 shims on each needle.
I had jardine exhaust and ear shave with the k&n rc-2340 air filters. __________________
Niterider must have added the second line after I quoted him, because it wasn`t there before.:)
 

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Registered
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2,687 Posts
This is a GREAT idea. Been wanting to know what jets to use myself. Even just a list of what jets people are running would be good since some people will have about the same setup and/or be at around the same elevation.
 
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