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Jetting for altitude

Fuel System: Gas (Petrol) tanks, Carburators
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Frank in Wyoming Member
Posts: 15
Joined: Sat Jun 02, 2018 9:07 pm

Jetting for altitude

Post by Frank in Wyoming » Wed Apr 22, 2020 8:27 am

Good morning, just wondered what some suggestions would be for jetting for my altitude. My city is at 6000 feet in Wyoming. I recently installed new jets from the ScramblerCompany for my CL77. I matched the jet numbers to what was originally found in the carbs. 135 and 38. Bike seems to be running a bit rich. Thank you! Frank

redblk63 Member
Posts: 101
Joined: Mon Oct 15, 2007 5:16 pm
Location: pasadena, california

Re: Jetting for altitude

Post by redblk63 » Mon May 04, 2020 10:02 am

You can calculate the jet by using the fact the scale height of the atmosphere is 30,000’. At 6,000’, your city is above 6,000/30,000 = 20% of the atmosphere. That means the atmospheric density is 80% of sea level.

If the 135 jet is correct for sea level, you need to reduce the jet area by a factor of 0.8. Since the jet size is proportional to diameter, you need to replace the 135 jet with a

Jet needed ~ 135 * sqrt(0.8) = 121

Round up to 125, as “a bit rich” is safer than too lean. This calculation is an approximation, but is a useful rule of thumb to get in the ballpark. You should read the plugs as a check.

Larry -Pasadena

Frank in Wyoming Member
Posts: 15
Joined: Sat Jun 02, 2018 9:07 pm

Re: Jetting for altitude

Post by Frank in Wyoming » Mon May 04, 2020 5:28 pm

Thank you Larry. That makes a lot of sense, and have not seen the calculations presented that way. Appreciate your help!

marzini Member
Posts: 50
Joined: Thu Nov 08, 2007 3:54 pm
Location: Santa Fe, New Mexico

Re: Jetting for altitude

Post by marzini » Sat Jun 19, 2021 6:22 pm

I'm at 6300' in Santa Fe, New Mexico.

It depends on your air filters and exhaust too.

I have big K&N RU-0175 filters and relatively loud EMGO Dunstall replica mufflers.

My formerly trashed engine with no compression needed stock #135 main and #42 slow jets; needle at middle slot.

On a fresh engine I'm surprised that #130 main and #42 slow jets and needle at middle slot are sufficing.

I'm going to try #40 slow jets soon with that combination. As the engine gets broken in, perhaps the stock jetting will work again after all.

Who knows…
'62 CB77, '76 XT500

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