Do main jet sizes correspond do something? Perhaps diameter in thousands, or drill bit size, or....
And is it the same for Keihin and Mikuni?
If not, does anybody know that answer for both?

I want to do a few math games with volume and etc... and need to better understand the opening size as it relates to the numbers. (yeah, yeah, bigger is bigger.... I know)

Other oddities: I'll move a keihin from a 152 to a 165 and make what amounts to a summer to winter jet change (I know simplistic, but it IS easy this way). A similar change in a mikuni be from a 240 to a 280.

Now, in commonly available ranges that's about 4 steps each. Course, if that was thousands - diameter it would be 13 thou vs 40 thou, a rather large difference, and the increase in volume flowed between the two would be even greater still. So, as you can see, I need to understand what the numbers mean.

Help!!??

Thanks guys.

dan

 

Dan, you pose a good question that I was wondering about myself. Doing the drag racing in cars, the main jet corresponds to actual thousandths number, but obviously you cant have a 250 thousandths hole in a Mikuni main jet because it would be the same size as the jet. Maybe someone can shed some light on this, as I would like to have an answer as well.

 

Mikuni and Keihin both use diameters on some style jets and flow rate or area numbers to number others .

Mikuni slot main jets and Keihin slot jets numbers are a diameter in millimeters for example a 110 Keihin slot or a 110 Mikuni slot jet both have a hole size of 1.1 millimeter.

Mikuni hex jets like in most polaris 2 cycles are calibrated in flow rate or area .

What I really wish the carburetor manufacturers would do is to rate the carburetors in CFM like car carburetors . *34 mm 36mm 38mm 39mm* is not the best way to classify a carburetor . Without a flowbench you don't know what you have got .

 

What I was wishing I could find is some kind of a cross reference for different jet sizes from the different carb manufacturers and/or after market jet manufacturers....for instance, a 145 Mikuni stock main = 144 Dynojet, etc....I have no idea how that would be done...mm size or flow rate? Of course, Dynojet would never let any of their special design venturi info out or they wouldn't be able to charge an arm and a leg for their product....

Has anyone ever seen any kind of cross reference info on which jet sizes correspond to another??? Or even corresponding needle sizes???

 

Thanks guys. Rick, as usual, appears to have nailed it. I did some research in the interim.

It "seems" Keihin uses 1000's of MM to ID theirs. So a 145 is 145/1000 MM.

Mikuni, in our carbs, uses a flow rating that is percentage based.

Therefor a 120 flows 20% more then a 100, and a 200 flows 33% more then a 150 ((200/150) - 1).

I can live with either, and now it makes some sense as mikuni's "steps" were so large it was a bit hard to make sense out if. Given the large flow increases for small diameter changes (ala kehin style) I suppose it all makes.

A scary thing of note: If what I read is true, Mikuni specifies a +/- 10 on their jet accuracy. Hence a 240 could really flow 250, while a 250 could really flow 240. In a worse case scenario you could pull a 240, install a 250 and be 1 step leaner! In fact, I bumped in to a factory Polaris 700 sled warning on just such an event.

Thanks again guys.

On cross reference, its hard to know why dynojet would screw with flow rates. Perhaps they think their solutions need to be protected more then you need to be able to tune your bike. I suppose a 100cc syringe + water, with rubber tubing to a jet, flowed for 100 (you pick the time) seconds would give you a baseline. Then, as mikuni's are percentage, you could swap to another jet, measure again, and do the math...

 

dfischer,
maybe this will help out..it compares dynojet to mikuni, goes up to about 200 dynojet..it is from a friend o mine's V max site
Terry is real sharp on the v max stuff, (& Royal Star as well) I can't get the link to work but cut & paste this in the browser...
http://www.users.qwest.net/~tcamp30/jets.htm