Xr vs Xl carbs

I put an 83 xl motor in my xr 600, it has a wiesco piston, uni filter and an fmf full exhaust. I am currently running the smaller 87 carbs because the xl carbs are set up for remote choke. I am going to swap in the 30mm 83's but i'm wondering what jetting to start with. It runs good now with the small carbs but i had to turn the air screw out 2 3/4 turns to get it to run ok. I know this beast has more and i am looking for a baseline to start with. What jets should i buy and where should i buy them?

It has the short screw type mains. Theres a few reasons im messing with it,

1. With a fresh high compression piston, big port heads, an fmf full exhaust and uni filter it seems counterproductive to run the small carbs, especially since i already own a set.

2. Im not going to mess with the current carbs at all so i can always just put it back the way it was, So this is a fairly low risk endeavor.

3. It runs ok now but is really lacking in the top end. The current jets in the 27mm carbs are as follows: primary 122, secondary 122 and slow is 45. Everything im reading makes these all seem small.

4. Im really not excited about the air screw being that far out (2 3/4 turns), I dont know if it matters at all but i do know that the small valve 87 xr600 ran these same exact carbs at about 1 3/4 turns. I might be way off, but that tells me that this thing wants more air...right?

5. And last, check out the size difference in these things in this pic! i put the coins in because it made the size difference stand out better.

Im just hoping that someone with an 83 with similiar mods could let me know what works for them so i have somewhere to start.


if you have to run with the air screw out more it wants more fuel or is calling for bigger jets if im not mistaken , not air

Sorry that pic is so small, the 87 is on the left and the 83 is on the right. I cant figure out how to upload anything except thumbnails for some reason??

if you have to run with the air screw out more it wants more fuel or is calling for bigger jets if im not mistaken , not air

I may have this all wrong but as i understand, it is an air screw and not a fuel screw so it works backward from a traditional carb. If you turn the screw in (clockwise) it will result in a richer mixture and lower the rpm at idle as opposed to a traditional fuel screw where turning in the screw would lean out the mixture and increase the rpms.

Air/Fuel or whatever screw it's called depending on position on the carb all gets darn confusing..




In the above I think those are the mains you want..I am not 100% sure on that,,Pay to measure what you have and compare it to them

to confirm..Either that or get them direct from Honda if they still have any for sale..I say get 2 122s two 125s and two 128s..Should cover the bases

with the mains..Pilots an N424-74C..size..at a guess something between 55 to 65 should be about right..


Ah..You already have 122s,,,get the others..You shouldn't need to go above 128..Don't think I ever did..Course I may be wrong and someone else

with some super hot twin carb 600 may have gone bigger..I never found the need..Same size mains in both carbs is how I ran mine when I still

owned one..nothing above 128s..

Edited by Horri

If you want to upload actual large images here I'm pretty sure you have to host them somewhere else like Imageshack

then copy the url from there to here and the picture is presented in full size. What I have noted with places like Imageshack

is they all start out with a hiss and a roar and uploading photos etc to there is easy..As soon as they get bigger they want you to register

and the whole issue of uploading to them becomes more complicated and problematic. They upgraded there site so much it has now

become semi unusable as I found when trying to upload a pic through them to here the other day. I'm on the lookout for another

site now who doesn't go to technical or cover there site in bs when they get bigger..A previously simple operation has now turned

into a nightmare..I couldn't even find an upload button to click even when I was logged on as a user..Useless sods..


You just don't need bs when trying to upload ,,bad enough with a poor connection without throwing a more complex web site

at you to deal with as well..

Edited by Horri

I'm running 128 mains in my 640cc XR600 with mild cam, porting, big tube headers, XRs Only Comp series and big XL carbs. Runs great up top but may be richer than needed. Ran the same with the 617cc XL600 engine with the same mods. I forget what I have in the pilot but it's horribly rich and I can't get a clean idle less than 2500 rpm. set your needles a notch richer. Got to fix that still but it's such a pain to get in the carbs I'm waiting to put on a different head.

Edited by valvesrule

Awesome, thanks for all the advice and the link! I'll probably toss in the 125s first and go from there. Sounds like a reasonable plan. Damn! Local honda shop is closed tomorrow, stupid sunday. Thanks again.

Has anyone ever encountered a stuck choke valve on an 83-87 dual carb bike? I got it out, but what a head scratcher. Im only asking because i could easily see how this process could go horribly wrong in a hurry. If anyone cares, i will snap some pics and post the process i used to get it out. It worked well.

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now