Carb jets on a motorcycle

[Stepside]

Are the Jet numbers the same as number drill sizes? This would be on an older Japanese bike.

Thanks in advance.

Pete

[topct]

Are the Jet numbers the same as number drill sizes? This would be on an older Japanese bike.

Thanks in advance.

Pete

The hole is a drill size. The number is theirs. I think you can find references as to their number to drill size.

[Black_Moons]

In my limited experiance, its in 100ths of a mm
ie a 74 jet is 0.74mm

[lakeside53]

Yes, that's what I typically see also.

You're better off reaming jets than drilling them, especially on the the very small jets. Of couse.. you have to first find a set of jet reamers!

[Willy]

It's been a while since I was fluent in jet-speak but I will give you a little bit of a heads up.

Mikuni and Keihin carbs, which are the most common on Japanese motorcycles, typically use different numbering systems for jet sizes...but not always. There are round and hex jets in I believe both brands, this is key to jet size nomenclature per brand.

Some use the diameter of the metering hole for jet size, some use flow rate for jet size.
Also be aware that even if two jets use the same numbering system and have identical size metering holes they may still have different flow rates. If one jet has a different length metering hole or taper leading up to or away from that hole, effective flow will vary. In other words...they are different.

Wish I could give you more info but my carb manuals are in the dark recesses of my shop and I would have to move heaven and earth just to find them.
But I'm sure with the right info you should be able to Google up the answers before I could find it in the flesh.;)

[nitsuj]

The short answer is no, the jet sizes aren't the size of the holes. Every carb maker uses their own system, maybe one of them just use the actual measurement, but that would mean the others don't since their system doesn't jive. What I mean is, a Keihin carb and a Mikuni carb and an S&S carb main jet all having the same size hole will all have different numbers.

[Yow Ling]

The main Jap carbs are Mikuni and Keihin, Mikuni 6mm hex Jets are numbered by flow, Keihin are by jet size 134.5 is 1.345mm

[firbikrhd1]

As lakeside53 mentioned in an earlier post, drilling isn't the preferred method of enlarging jets. Drilled jets don't flow like factory jets even if the holes are the same size due to minute imperfections left in the hole by drills. Usually jets are available somewhere on the Internet for not too much money, even for older carbs. Ordering them in a couple of sizes larger than you have would probably be your best option.

[EVguru]

I have a set of Stahlwille reamers that I use for re-sizing jets.

http://www.ebay.co.uk/itm/Stahlwille-11085-Carburettor-Jet-Reamers-0-33-2-0mm-/310268586291

For tuning purposes you can soft solder up jets and re-drill and ream them.

Once you have got things running well the flow of the temporary jet can be measured and an equivilent permanent jet substituted.

There's some interesting stuff on carb jets and how to measure their flow here;

http://www.saltmine.org.uk/martin/jet_sizing.html

Not all jets are equal!

[Black_Moons]

As lakeside53 mentioned in an earlier post, drilling isn't the preferred method of enlarging jets. Drilled jets don't flow like factory jets even if the holes are the same size due to minute imperfections left in the hole by drills. Usually jets are available somewhere on the Internet for not too much money, even for older carbs. Ordering them in a couple of sizes larger than you have would probably be your best option.

Yea, I considered making my own jets then realised that these carb makers have been making jets since before I was born and likely know a thing or two about making them properly. (or maybe they just make them cheaply and have no clue since they fired the one guy who knew what he was doing, but whatever)

Now I have like 20 jets from 0.64 (stock size was 0.70) to 0.96 (Running a 0.90 I think now?)

Thankfully they where like $2 each and the local motorcycle place does not charge shipping if I was OK with waiting 3~6 days for them to order something else.