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Old 04-30-2013, 04:56 PM   #9
because I can
Joined: Sep 2010
Location: San Francisco Bay area
Oddometer: 9,116
Originally Posted by disston View Post
I said I thought there was a correlation between engine size and jet sizes. I was looking at the info wrong I think. There seems to be a correlation between age and jet size. The older bikes are jetted richer. But then on reflection I don't think it's the age but the compression ratio. The higher compression bikes seem to have bigger jets.

SS does much more rejetting of Airhead carbs than I do so I appreciate his input.

The later bikes also had leaner jets because of emission concerns.

This whole subject is very confusing because of the numerous variations BMW used. I just spent an hour trying to find the part numbers for every main jet BMW ever used on the Airheads. There are 14 main jet sizes. There are 5 needle jet sizes. At least 2 idle jet sizes. (These numbers are for machines sold in N America. There are some other sizes for European machines)

Anybody want me to type up the list for reference?
Higher compression does call for richer jetting sometimes but the main difference between the older bikes with 9.0:1 CR and the newer ones with 8.5:1 CR is that the earlier bikes were jetted too rich. Jetting mains? My bikes and most all setups I have worked on will go faster and faster doing tests somewhat like BCR's while you are jetting them leaner and leaner. Up until the point of not running at all at higher rpm. Personally, I don't think BCR's tests work that well for the other jets. I don't think the manual's theoretical throttle position/jet parameter works out in reality. What does work? Trying different jetting combo's until you find what works best as lean as it will work best.
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