Mikuni TMX Main Jet Numbers, What do they mean?

[XXX Adrenaline Spo]

Im jetting my 01 CR250 and have ran out of sizes of main jets so Im afraid Im going to have to resort to drilling one out (Just for this weekend 'till I can get more sizes) Im up to a 460, Stock is a 420. I can get the low to mid jetted OK but if I get it on the pipe (Main Jet) It gets WAY too lean, Everyone keeps telling me Ive got an airleak or something but I dont buy it (Or see it) everything is clean and dry including under the LH Cover. Plus if it were an airleak causing this then I think it would make the "ying ying" sound and it would be lean from bottom to top NOT just on the top end right?, Right??

My reason of writing is to try to find out what these numbers mean (420 cannot be Thousands of an inch, Get a vernier caliper and open it to .420" and you'll see what I mean) Sooo, Can anyone enlighten me as to how to know what size drill I need to goto a 470 then a 480 then a 490 etc. Surely there is a chart out there somewhere that someone has formulated for this (I Hope).

Well, Once again Help will be greatly appreciated.

Thanks, Stuart

 

[Rich Rohrich]

Mikuni hex head jets are numbered by bulk flow, while Mikuni round head jets and Keihn jets are numbered by inner diameter in mm.

The flow difference between a Mikuni 175 hex jet and a 180 is about 6cc ~3%) . Keihn jets are numbered by jet id, and the difference between a 178 and a 180 is about .0008" (I'll let those so inclined do the math to determine the flow difference) .

Now to further confuse the issue keep in mind that air/fuel ratio is based on weight of fuel and weight of air, but jets essentially meter by volume. So we really need to know the weight of the fuel flowing through a jet to understand all this. In simplest terms fuel weight is a function of the area of the jet multiplied by the value of the square root of the fuel head pressure multiplied by the density of the fuel.

It looks like this:
weight of fuel = jet area * ( SQR Root (head pressure * fuel density)

None of the above takes into consideration the changes in fuel vaporization characteristics as you change altitude, which can have a profound impact on the final air/fuel ratio available in the combustion chamber ( the only place a/f ratio is really significant anyway) when the sparkplug fires.

As fun as it might be to look at jetting this way, the sad fact is the cross sectional area of two jets marked with the same number can vary FAR MORE than the difference in flow due to small changes in density . Good jets can vary as much as 5% cheap jets can be closer to 10% variance.

I hope this helps.

 

[IndyMX]

Nothing like oversimplifying the answer there Rich.. ;)

 

[XXX Adrenaline Spo]

Thanks for the scientific answer Rich (And I mean that with No Sarcasm, I like to Learn) But....... Can you tell me how to get to a "490" or so out of a junker "200" I have?? I was hoping to get a drill bit diameter, Also, Ive heard that drilling jets ruins the jet due to changing the shape of the hole, Is this true?? I heard something about the hole is tapered all the way thru on a microscopic level maybe??

 

[Rich Rohrich]

If you need a 490 BUY a 490. Simple as that. ;)

 

[whenfoxforks-ruled]

Even if you find the seemingly hidden secret of the jet sizes, they are not drilled holes. They are machined holes. They will never flow the same. You can not engineer jets, unless you invest in the proper size broach or ream. Buy what ever larger jets you need.

 

[rmc_olderthandirt]

I can believe that drilling a jet is not the best way, but it is a technique that used to be very common, at least back when cars had carburetors.

The first thing you need to do is get a set of drill bits that will do the job, and something to hold/turn the bit. Forget the number on the jet, find the largest drill bit that will fit into the jet now and then go up one or two sizes.

Note that you can only go larger and there is no going back so unless you have a pile of too small jets then you would probably be better off finding new jets. If you found new jets instead of drilling out your old ones you can always go back, which you might need to do later if you change altitude or make some other change to the bike.

I just went through a re-jetting exercise on my bike which also has a Mikuni carburetor. I had the opposite problem, I needed to go a lot leaner than the jets available from OEM dealers would get me. I contacted JD Jetting and although they didn't have anything for my bike they pointed me in the right direction and I finally found a website that sold jets. Lots and lots of jets.

Try looking at www.cyclewearables.com Do a site search on Mikuni. I bet that they have what you need, probably about $5 each. You will be able to buy a range of jets for less money than a set of drill bits that you would need.

Warning: their website sucks, their communication is terrible, and they take their time shipping but I did get the jets I wanted.

Rod

 

[IndyMX]

looks like that site is not in action any more Rod..

cyclewearables.com is just a placeholder search page.. Not an actual company..

I had pretty good luck with www.motorcyclecarbs.com

 

[rmc_olderthandirt]

Sorry, mis-spelled it.

http://www.cyclewareables.com/

Their ordering system isn't the most obvious but it worked for me.

The motorcyclecarbs.com website looks pretty good too. Prices are a bit higher but their order system is a lot easier!

Bottom line; The jets you need are probably available.

Rod