honda305 Home honda305 Auctions honda305 Gallery honda305 Forum Electronic Ignition honda305 Forum honda305 Forum honda305 Forum honda305 Racing honda305 Literature

Auctions  Mailing List    Registry   Marketplace

honda305.com Forum - FULL SCREEN

honda305.com Forum

Login
□ Search
  □ CLASSIFIED_ADS
□ FAQ 
  □ SEARCH_ADS
□ 
  □ MY_ADS
Vintage Honda Owners,
Restorers, Riders and
Admirers

Jetting for altitude

Fuel System: Gas (Petrol) tanks, Carburators
Post Reply
Frank in Wyoming
honda305.com 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
honda305.com 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
honda305.com 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
honda305.com 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

Post Reply
cron

 


 

CB-77 | CYP-77 | Road Test | Riding Log | Literature | Zen | Marketplace | VJ Survey | Links | Home