1. Dismiss Notice
  2. Hey Facebook people... We've created a group for XS650.com members to connect. Check it out!
    Dismiss Notice

Carb jetting

Discussion in 'The Garage' started by dlabkeeg, Dec 29, 2009.

  1. 5twins

    5twins XS650 Guru Top Contributor

    20,403
    12,625
    813
    Yes, use the 24mm float setting. The '78-'79 carb set pretty much always needs the needles leaned when larger mains are installed. I'd just do it as part of the jet install. Don't do it before you get the larger jets in there. That would make it too lean now. Look at that carb circuit drawing I posted a couple posts up. Your new larger mains and pilots are going to make the midrange too rich because of the circuit overlaps.

    The usual scenario is you do the mains first because they will effect all the other settings. In your case 2 to 3 sizes up should do. That will probably induce the upper midrange break-up I mentioned. You fix that by leaning the needles a step. Now, the stock pilot is usually fine as long as you haven't leaned the needles. Once you do though, due to the circuit overlap, you lean the upper part of the idle circuit as well and that usually creates a flat spot or hesitation just off idle. The larger pilot is then needed to cure that.
     
  2. nooch

    nooch XS650 Enthusiast

    30
    0
    6
    canada
    Awesome!! thanks so much 5 twins! I really appreciate the help! as far as my fuel mixture screw goes, im assuming I will start at the 2 1/4 turns out(stock setting) and adjust from there also right???

    when my parts come in and I get everything set up to try again I will let you know how she goes! ...or I will ask you a million more questions :laugh: thanks again!! much appreciated!!:D
     
  3. 5twins

    5twins XS650 Guru Top Contributor

    20,403
    12,625
    813
    Yes, start the mix screw at the factory spec of 2.25 turns out, then fine tune from there. There is a range of adjustment, called the "sweet spot", where the engine will idle the fastest and smoothest. This is what you're looking to find and place your final setting in. Start by choosing a direction to turn the screw, in or out, and adjust 1/4 turn at a time, waiting a few seconds after each quarter turn to see/judge the effect, until the motor starts running rougher and stumbling. Note how many turns that happens at. Now set the screw back at 2.25 and repeat the process turning the screw in the opposite direction. Once you establish these lean and rich "roll off" points, you've bracketed the "sweet spot" between them. Your final setting will work best somewhere in that range. If you gets lots of popping out the exhausts on decel, setting to the rich side of the "sweet spot" can help with that - but only to a certain extent. If the mix screw won't tune that out, you may need to go up another size on your pilots.
     
  4. nooch

    nooch XS650 Enthusiast

    30
    0
    6
    canada
    thanks 5 twins! very much appreciated. one more quick question, I clicked the link for the uni filters you have posted. I noticed it doesn't carry "our model/year" of bikes. Any certain size(bigger/smaller) or model type would you say would work best on an xs650? or just what ever will fit in the space available on the bike? thanks.
     
  5. nooch

    nooch XS650 Enthusiast

    30
    0
    6
    canada

    http://www.unifilter.com/wp-content/themes/uni/images/clampon/podkits_lg.jpg

    5 twins, will these be ok??
     
  6. 5twins

    5twins XS650 Guru Top Contributor

    20,403
    12,625
    813
    If you have a hardtail bobber, those are the usual choice because they're shorter and space behind the carbs is sometimes limited. If your bike is stock, the usual choice is the UP-4200 or UP-4200ST (dual layer), both of which are a bit longer. The BS38 intake bell measures about 2" across and the 4200 mounting flange is that size exactly. They fit like a glove. If you get those shorter ones, you would want the PK-82 kit.

    http://www.unifilter.com/clamp-on/

    These are the UP-4200's on my '78 .....

    [​IMG]
     
  7. nooch

    nooch XS650 Enthusiast

    30
    0
    6
    canada
    I got my uni's on, and instantly a difference. I went up one size on my primaries and up 3 on my mains, and leaned my needle one notch. fuel screws at 2.5 turns out. she screems and pulls hard from 2000rpm and up, but still having problems getting a low idle it idles at 2000rpm but any less anything it slowly just drops off and stalls. any suggestions?? Thanks!
     
  8. 5twins

    5twins XS650 Guru Top Contributor

    20,403
    12,625
    813
    Sounds like the floats may be off. Have you carefully re-set them to 24mm? Also, when setting them, you need to measure each one twice, once from each side. You want to check each float "bulb" individually. Many times the floats are twisted or tweaked on the mount and sit at different heights. You won't catch that unless you check each side individually. And you measure from the gasket surface on the carb body, not off the gasket.
     
  9. nooch

    nooch XS650 Enthusiast

    30
    0
    6
    canada
    ok great thanks! ill reset them again tonight after work. I did adjust them after I put them on, with fuel in them, that way I could see when they were actually shutting off the fuel. but only from the outside bulb... what are your thoughts on this method?? any other (better) ways of setting the floats? if there is please let me know cuz the way I am doing it is a pain in the ass! haha I do think my floats are a little tweaked also, so hopefully that IS my problem. ill set both sides this time. thanks again!!!
     
  10. 5twins

    5twins XS650 Guru Top Contributor

    20,403
    12,625
    813
    I simply measure with a 6" steel rule graduated in MM on one side. If you get a thin one (many are very thin), it will stick down in between the gasket and the lip on the side of the carb body, reaching the gasket surface. The only "gotcha" with this method is you must site directly from the side (just eye up the tops of each float "bulb" in line) or you'll introduce a parallax error and your reading will be incorrect. Here's some examples. This is the same float ('77 carb set at 25mm) measured (sighted) correctly, from too low an angle, and too high an angle. As you can see, it really throws the reading off if you look at the wrong angle .....

    [​IMG]

    [​IMG]

    [​IMG]

    The bowl design on the 38s, with the jets mounted in the bowl and no jet tower sticking up between the float "bulbs", makes setting them with a ruler very easy. You set the ruler on the far side and site directly across the tops of both "bulbs".
     
  11. nooch

    nooch XS650 Enthusiast

    30
    0
    6
    canada
    perfect!! I will adjust them as you have recommended. thanks again 5twins, I really appreciate the help! ill let you know how I make out!
     
  12. XSLeo

    XSLeo XS650 Guru Top Contributor

    Often when an engine won't idle well below 2000 rpms is the idle circuits may be plugged or very restricted. I might suggest checking this by using spray carb cleaner and spraying it in the pilot jet and watching the tiny holes by the throttle plates. You should get a good flow out all three holes. Also removing the air mix screw and spraying in there should get good flow out the pilot jet and all three holes.
    Leo
     
  13. nooch

    nooch XS650 Enthusiast

    30
    0
    6
    canada
    leo, I have cleaned my primarys multiple times I have good flow coming though all three holes and pilots are clean and I was using carb cleaner. I have done this everytime I have had my carbs off which is a lot haha. 5twins, as for my floats I set them to 24mm bang on, I did have to tweak them a bit to get them even, bench synced my carbs using the light method as well as checking with a feeler gauge put everything back on the bike and ran like sh*t. I had to drop my floats 3mm more to get it to quit back firing and popping. for some reason it was way to rich, I don't get it. when I reset them I did it on the bike with fuel after I evened them out. I did get it to idle around 1300-1400rpm but mainly off the right cylinder. I am running rich on the left side now and perfect on the right but my sync screw is unreasonably touchy, when I say this I mean if I "touch" it with a screw driver it goes crazy... so I am having a hard time to say the least with syncing the carbs. but now I have bigger issues, I had a "hot" wire break coming off my fuse panel and lost all spark, I reconnected it and now my spark is irratic and will barely run(popping sputtering etc etc), but if I hook it up to a battery(I am running a capacitor) it runs fine... any ideas?? I suck at wiring... thanks guys, sorry for the long ass post.
     
  14. Quinn_McLeod

    Quinn_McLeod XS650 Addict

    Sorry to jump into this thread, but I'm having (what I assume) is some carb issues.
    I've got '78 carbs on an '81 engine. Shouldn't be a problem. I'm running HHB PMA and Pamco ignition. It's batteryless kick start only with a Sparx capacitor. Simplified wiring (headlight, taillight) and I'm running open headers with velocity stacks on the carbs (no "filters").

    Before you start yelling at me because of no air filters and no muffler, hear me out.

    It idles GREAT. I've tuned the carbs (air pressure hose gauge thingy) with the help of a friend, and the timing has been set based on Hugh's suggestions with Pete's suggestions. My problem only comes at mid-power. With acceleration, but not full-throttle. Full on, it revs and then holds steady. But at mid way it bogs down really bad, and backfires some too. The bogging down is what's got me confused.

    .....AAAAAAAAAND, fire away :yikes:
     
  15. TwoManyXS1Bs

    TwoManyXS1Bs BBQ Hunter Top Contributor

    Yeah, mid way is where things can get complicated. It's influenced by the needle jet, jet needle, slides and their diaphrams. Could be something physically wrong, like needle retention and leaky diaphrams, or could be just a tuning issue. Have you been thru the carb guide and done the servicing?
     
  16. Quinn_McLeod

    Quinn_McLeod XS650 Addict

    I haven't taken the carbs off yet. I did put in some SeaFoam hoping it would help burn/clean out the fuel system . . . but now the bike runs to rich with that stuff in it and it's revving really high. I thought that stuff was safe for these kinds of applications. But it's making it run weirder.

    After running that through the tank and putting in some fresh gas, if that doesn't do the trick, I'll pull the carbs off the bike and open them up to see what's going on.
     
  17. Quinn_McLeod

    Quinn_McLeod XS650 Addict

    I'm running velocity stacks on my '81. There is a screen to keep leaves out, but that's about it. Any input on performance with these?
     
  18. TwoManyXS1Bs

    TwoManyXS1Bs BBQ Hunter Top Contributor

    Could be that it's cleaning out the tank, petcock(s) and fuel lines, and that stuff is gettin' into the carbs.
    Could also be that it's cleaning the carbs, revealing the true jetting.

    Yep, gotta learn about them carbs, and crack 'em open...
     
  19. Quinn_McLeod

    Quinn_McLeod XS650 Addict

    Yeah, that could be. It's just that they've been running fine. All Spring and Summer I haven't had any issues until just recently. That's why it's weird. And I've been running it pretty consistently. Enough to prevent the fuel from gelling.
     
  20. TwoManyXS1Bs

    TwoManyXS1Bs BBQ Hunter Top Contributor

    Inlets, carbs, head porting, cams, and exhaust are usually a package deal. All selectively fit and tuned together to make a total performance package.

    By itself, any one of those can help, or hurt, the performance.

    Gives it that racy look, tho...
     

Share This Page