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High altitude jetting


CB750 Member
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Hello all,

Ill keep this simple, moved from 500 foot elevation to 7200 feet. Bike is running really rich now due to thinner air. I would like to fix that problem. Most riding will be done between 6000-7000 feet i should note.

I have all stock setup with 105/70 jets. Bikes a 1979 Cb750K

What jets would you guys recommend I put it to have it run just right? I have quite a few sizes to play with but as you probaly know removing the carbs is not something you want to do more then once, so I’m hoping I can get this in one shot.

Let me know what yall think.
I'd like to help as I have a work order for my bike that says that "high altitude" jets have been installed. Unfortunately, the parts list on the work order shows 1 each of 2 different part numbers and 2 each of another part number. Thus, 3 different sized jets for 4 carbs.
This leads me to believe that it depends on the individual bike and engine as my bike runs just fine and the plugs all look similar when I pull them. Entire bike is stock, including the air box.
Makes sense, I have that tool that you use the sync up the carbs. The one where you hook up all the rubber hoses on the intake…god its been years since I tuned her up though.

So far I’m at 50 miles riding at this high altitude and its really not as bad as I thought. Yes it’s a little slower but that could be a combination of high altitude and it running a little bit to rich. Most likely I’ll just pull the plugs after 100 miles and see how black they really are.

If it’s really that bad maybe I’ll just experiment and throw a 100/65 in and see how she likes it.
I bounce back and forth between Arkansas at about 800' in the winter and California at about 7200' in the summer. Now that I've ridden it at altitude I do believe it was running a bit lean in Arkansas, but once the bike was warmed up back there it ran fine.
Seems to be a old post, but is there a chart for jet size versus altitude? I have read that go down one jet size for every 2,000' of elevation. I foul plugs consistency after moving from 700' elevation to 6,800'. Any thoughts?