Drilling main jets

[Guest]

So I found a page that cross references different brand jets to each other. This page right here - http://www.historicmotorcycle.org.au...carby/jets.txt . It explains the different ways companies derive their jet sizes. Mikuni sizes theirs by flow - a 120 jet flows 120cc/min at a given vacuum/pressure. Dynojet and most others are numbered by the actual metric orifice diameter(a 120 Dynojet has a 1.2-mm orifice).

Now in there it shows a .062" drill bit can make the equivalent of a Mikuni 138.8(I know they don't make a 138.8, but it's what they show). Well a 1/16" bit measures .0625", so the 130s I had in the 300 before, ummm... aren't exactly 130s anymore. Figured that extra .0005" would get me close to a 140 which should fix the somewhat lean high-rpm WOT condition, especially considering I'm making pipes for it as we speak.

Well? I converted the 1/16" to mm and came up with 1.58***mm. Now THAT size translates to more like a 147.5 Mikuni, assuming the cross-reference charts on that page are accurate. They aren't on the bike yet, so no need to panic. Just figured I'd give it a shot before dropping the coin for some proper 140s & 142.5s

What yall think?.

[Datsa Noydb]

I think the next person that goes into those carbs is going to be VERY puzzled when be tries to fine tune the jetting , then will cuss you out vigorously when he finds the jets are drilled.

Do yourself and your bike a favor. Just buy the proper jets, they are not that expensive.

[Who Dat?]

I agree. Somebody is going to see a 130 on the jet and wants to richen it up just a tad, so they install a 132.5 and wonder why it all of a sudden runs leaner.

[GregT]

Drilled jets many times - but I do have a full range of drills in metric size from 1.0 to 3.0 in .05mm steps...
And I ALWAYS grind off the size markings....
Good way to arrive at your final jetting - but then replace with genuine jets...

[Ric]

That somebody will be me. This is my wife's bike, and unless she takes a liking to mechanicking on her bike then I'll be doing the upkeep on it. Besides, I DO plan to get some 140s and 142s still. Likely won't use these unless I give in and swap some 250 cams in. In which case I'll have the proper size scribed onto them and the original 130 removed with a file.

[Who Dat?]

I would still be leary of trusting the actual size of a home-drilled jet.

[GregT]

I would still be leary of trusting the actual size of a home-drilled jet.

I do mine in a lathe - and when changing to genuine jets have never noticed any difference.

[zuluwiz]

I do mine in a lathe - and when changing to genuine jets have never noticed any difference.

Even in a lathe, I wouldn't trust a drill to keep to size . A reamer might do better, but still wouldn't trust the hole to be to size.

[chuck hahn]

Heres a chart. Dont know if it will be of any use as far as pilot jets goes though.

http://www.jockeyjournal.com/forum/showthread.php?t=64769