Drilling out jets, where can I find size chart?

Guys I think you are looking at this the wrong way just a little. You guys are listing all the reasons not to but the reason to do it is greater. That reason is that there is nothing to lose by trying it and if I do mess one up all I have to replace is a jet. I could even soldier the opening shut and try again if I really screw up. Or I could use the jets that I have ordered when they finally come (special orders take time). I want to be able to ride my bike until they come.

If you guys don't know of any charts could you tell me what the numbers on the keihin jets correspond to? Are they millimeters or just some arbitrary numbers?

jason8887 wrote:

I don't know if a chart exists since the 'number' on a jet does not mean the same thing between different manufacturers. Keihin jets are numbered different than Mikuni, etc.

I've found them all to be in hundredths of a millimeter. Mikuni, Keihin, Teikei, Dynojet, K&L, Keyster all follow the convention based on orifice diameter. However, they may have different flow rates if the jets have different length orifices or if the overall jet is of different lengths (including the head). As long as you are comparing similar type jets with similar orifice placement, they should flow about the same.


100 = 1.00mm
92 = 0.92mm
70 = 0.70mm
36 = 0.36mm

#36 drill bit is hard to find.
The one I use is .0145" and is closest to the #37 jet size. When you drill it, it's easy to have a variance of one jet size anyway.

Drill chart:
bobmay.astronomy.net/misc/drillchart.htm

Tiny drill bits on ebay
It's a great deal! You get four sets for less than $20.

Post edited by: loudhvx, at: 2007/10/06 13:39

Dynajet main jet sizes are not based on the size in hundreths of a millimeter and a conversion formula needs to be used to convert their sizes to millimeters. I suspect they do this to keep their main jet sizes unique as this is essentially one of only two real rejetting components in their jet kits... the other being their jet needles.

wiredgeorge wrote:

Dynajet main jet sizes are not based on the size in hundreths of a millimeter and a conversion formula needs to be used to convert their sizes to millimeters. I suspect they do this to keep their main jet sizes unique as this is essentially one of only two real rejetting components in their jet kits... the other being their jet needles.

If you mean Dynojet, all of the Dynojet mains I have are in hundredths of millimeters. I actually measured them, and because of this rumor, I actually called them, and they confirmed that they are hundredths of millimeters.

I don't know why this rumor persists, of the orifice diameter being other than hundredths of millimeters, but I can't find any that are different from hundredths of millimeters.

I just wanted to tell you guys that I did this and that it made a large difference. I drilled out the pilot, main and secondary. I didn't have the absolute exact size I needed for the pilot jet. I have to say this made a huge improvement in the bike. It isn't perfect and I think I feel a flat spot but it is at least ridable now. I had no problem with burs and the amount you are enlarging the holes is very small. I still plan on putting in new jets when they arrive but because they are special order they will take some time to get here.

I just wanted to tell you guys that I did this and that it made a large difference. I drilled out the pilot, main and secondary. I didn't have the absolute exact size I needed for the pilot jet. I have to say this made a huge improvement in the bike. It isn't perfect and I think I feel a flat spot but it is at least ridable now. I had no problem with burs and the amount you are enlarging the holes is very small. I still plan on putting in new jets when they arrive but because they are special order they will take some time to get here.

I found out the hard way on Mikuni jets that the # is not in hundredths of mm. It is close for the smaller jets but WAY off for the biger ones. The # is the flow rate in cc/min. This is varified by the chart first provided in the begining of this post. The flow rate is only partly affected by the Diametter of the hole but also by the length of that diametter. Small changes in a jet are OK like, #100 to 102 or 104 but large ones could be quite detremental. I have a complete Cobalt drill set from #80 to #20 and can drill a very smooth hole but I agree with most of you that say that it is not worth it. You can buy any jet you want from Sudco, they have everything in stock. Last time I ordered from them it took 3 days to get. They are also one of the least expensive.

Otto Kudrna