• Required reading for all forum users!!!

    Welcome!
    Register to access the full functionality of the GSResources forum. Until you register and activate your account you will not have full forum access, nor will you be able to post or reply to messages.

    A note to new registrants...
    All new forum registrations must be activated via email before you have full access to the forum.

    A Special Note about Email accounts!
    DO NOT SIGN UP USING hotmail, outlook, gmx, sbcglobal, att, bellsouth or email.com. They delete our forum signup emails.

    A note to old forum members...
    I receive numerous requests from people who can no longer log in because their accounts were deleted. As mentioned in the forum FAQ, user accounts are deleted if you haven't logged in for the past 6 months. If you can't log in, then create a new forum account. If you don't get an error message, then check your email account for an activation message. If you get a message stating that the email address is already in use, then your account still exists so follow the instructions in the forum FAQ for resetting your password.

    Have you forgotten your password or have a new email address? Then read the forum FAQ for details on how to reset it.

    Any email requests for "can't log in anymore" problems or "lost my password" problems will be deleted. Read the forum FAQ and follow the instructions there - that's what we have one for...

  • Returning Visitors

    If you are a returning visitor who never received your confirmation email, then odds are your email provider is blockinig emails from our server. The only thing that can be done to get around this is you will have to try creating another forum account using an email address from another domain.

    If you are a returning visitor to the forum and can't log in using your old forum name and password but used to be able to then chances are your account is deleted. Purges of the databases are done regularly. You will have to create a new forum account and you should be all set.

Drilling main jets

  • Thread starter Thread starter Ric
  • Start date Start date
R

Ric

Guest
So I found a page that cross references different brand jets to each other. This page right here - http://www.historicmotorcycle.org.au/Adler/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?.
 
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.
 
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.
 
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...
 
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.
 
GregT said:
I do mine in a lathe - and when changing to genuine jets have never noticed any difference.
Click to expand...

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.
 
You must log in or register to reply here.
Back
Top