Keihin jets flow rates?

[jetski9010]

I know mikuni bases they jetting sizes on different flow rates but I have read that keihin jets are different. Does anyone know if the jet size of the keihin jets have to do with flow rate or just the diameter of the jet or something like that?

 

[egbrig]

Neither base their jet sizes on flow rates. The sizes are listed by the orifice sizing.

 

[jetski9010]

I thought I read something somewhere that mikuni wrote that there jet sizes where based on flow rate. I guess that info was wrong or I misread it. So the jet sizes for both mikuni and keihin are just based on the orifice diameter?

 

[kevbo]

Mikuni round main jets and keihin CDK mains are sized in MM, a 150 main = 1.5mm hole. Older mikuni hex mains are based on flow rather than size. They provide more linear progression that is easier to work with in smaller sizes. Pilot jet numbers seem to be proprietary between brands and styles. For watercraft carbs, a single Keihin 40-42 would use a 80-85 pilot. A single Mikuni 44 will often use 120-135 pilots while both use similar numbered mains.

 

[jetski9010]

That makes sense. So im a little rusty on my math skills. If I am trying to figure out how to get 30% more fuel then jetting that is currently in the carbs how would I go about figureing that out?

 

[kevbo]

If all things are equal then just shoot for 30% more area. Pilot jets are much easier to predict because the vacuum signal is always similar between all sizes of carbs at low throttle openings. With this variable out of the way you can figure (more/less) a direct line between butterfly diameter and pilot jet size. Predicting main jets are much much more difficult, especially with large carbs. When carbs get oversize signal falls off really fast. Its a lot easier and safer to start with what should be way too big and work down.

 

[jetski9010]

I know you also have to factory in surface volume as a 30% bigger hole does not mean 30% more fule. but for the most part if I have a 100 jet size and put a 130 or 135 jet in there that would get me close to the 30% more fuel mark?