Needle Jets need to match Needes?

[KEITH KRAUSE]

Sam, I'm off on a vacation for the next week or so, just so you know. Sounds like Chuck can help you out if you need it. Good luck with it!

[Chuck78]

Chuck, I visited Mikuni Corp in California in 1980 to ask some questions. During my conversation the Mikuni rep' said all of their jets (back then) were in 2.5 increments, with 2.5 and 7.5 representing a "step" and whole numbers as a "full" size. I often heard the term "half size" used instead of a step when referring to Mikuni jets. I've always used the same terminology since. The terminology was common back then.
I've never given it much thought since. Now you bring it up so I did some looking thinking it would be easy to find info on how Mikuni sizes their jets. I found only a couple of sites that mentioned the word "step" and nothing as concrete as my conversation that day.

Interesting read, thanks for clearing that up. I was curious as I've never heard it referenced that way before, but typically only on the number of jets larger, not counting whole numbers vs half steps. Not trying to argue with you, but was just curious where the origins of that reference came from.

Sam, all I can say is go with Chuck's advice on the mains since he's worked on 750's much more than me.

Every bike is different... even side by side GS750's of same model year may have noticeably different camshaft timing and engine traits due to widely varying machining tolerances. And old used bikes add a whole lot more variables to this...

I have no idea why a very similar motor 3/4 the size of a 1000 with nearly identical carbs would require much leaner main jetting to run correctly, while the other jetting circuits appear to require similar changes. I also have no idea why a 750 comes with larger mains than a 1000 from the factory.

The difference in 750 vs 1000 is due a lot to significantly different air jet sizes in carburetors which externally appear identical, camshafts with more valve lift and less opening duration, and different bore and stroke...

I can tell you a typical 1000 needs to go from 95 to 130 mains with identical mods. Why a 750 would require a change of only half that is strange to me.

I'll keep this in mind for my two 1000 engines! I have two sets of Keihin CR31's, however, which are very likely to end up on those two bikes... And I have several GS 1100 GK Deport top ends. I suppose the jetting on that is a little less commonly known, but advice on a good starting point there would be very appreciated!