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Carb jetting

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

  1. Herper

    Herper XS650 Member

    Just wanted to stop in and say 'thanks'. Picked up an '80 Special that was sitting in a shed for who knows how long. Have gathered a wealth of information here on this site. Rebuilt the carburetors using the Canadian needles and jets but wasn't sure where to start for jetting using pod air filters. Sundie hit it right with the quote above. :thumbsup: After some tuning, took it for a first ride around the block and it's running GREAT! :bike:

    Now to build an engine stand, strip it down and get ready for a nice winter project. :) Thanks again for what everyone does here on the forum.

    Regards,
    Ed Nied Jr.
    Southampton, MA
     
    gggGary likes this.
  2. Buddy,I love the term"long-neglected"! Yeah, that describes the "76 I got, that wasn't indoors for about 20 plus years! Tahnks for a smile! Take care!
     
  3. mason_burke

    mason_burke XS650 New Member

    Is it possible for anybody to link me directly to a location where I can buy these Canadian needles and jets?
     
  4. ryan_mac18

    ryan_mac18 XS650 New Member

    In need of some advice. Just bought an 80' G model with upgraded mikuni 36 carbs. Previous owner had stock jets but I'm losing powerand bogging when hitting higher revs. In need recommended of jet kit size
     
  5. 5twins

    5twins XS650 Guru Top Contributor

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    They don't make jet kits for these bikes. In order to tune your carbs, you first have to identify exactly what they are. "Upgraded Mikuni 36 carbs" doesn't even begin to I.D. them. Post some pics if you can.
     
  6. ryan_mac18

    ryan_mac18 XS650 New Member

    Okay. Mikuni VM. But they are 36mm carbs. Unable to take pics until the weekend, I'm working out of town.
     
  7. 5twins

    5twins XS650 Guru Top Contributor

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    If you're sure they're VMs, then we don't need pics. Find one (or more) of the VM carb guides available on-line and study up. There are specific jetting "baselines" that have been proven to work with the 650. They are listed in some of the 650 VM guides and you should probably follow one of them. Most of the guides pertain to the VM34s as they are the most popular size for the 650 but the 36s can work with very similar jetting.

    The "off the shelf" VMs are basically set up and sold to bolt on 2 stroke dirt bikes. All the jetting is wrong for 650 use. You'll need to disassemble one and record all the jet sizes installed along with what needle and needle jet combo is in there. Then start buying 650 specific replacements as listed in those "baselines".
     
  8. ryan_mac18

    ryan_mac18 XS650 New Member

    Thank you. I'll study up and write again when I've figured it out.
     
  9. wilbur60

    wilbur60 XS650 Enthusiast

    Question: Is it possible or reasonable to run two different size pilot jets? I switched to 45's when I put on the new air filter. Did not work well on the left, worked great on the right. Switched back to the 42.5's and now my right side is popping on decel like it is starving for fuel... With the 45's is was working fine.
    I do not know if that is ok to run two different size pilots or not? I may find out here soon. I just cannot locate the reason for the popping. Every since I changed the jets back to 42.5's it has done that. Otherwise it runs great. (Better than it did on the 45's)
     
  10. Morgue13

    Morgue13 XS650 Enthusiast

    I would try and run the carbs with the pods first without a jet change see if you have issues then go from there I've had many 650's and 400's run great without a change to the jets for just a pod filter mod including my own 650 but after I chopped the pipes is when I started to have issues (but like i tell my customers EVERY BIKE IS DIFFERENT) so start simple and test them without a jet change first

    :bike:just my 2 cents:bike:
     
  11. XSLeo

    XSLeo XS650 Guru Top Contributor

    The popping can be an air leak some where, often in the exhaust. Going one up on the jets is trying to compensate for the air leak.
    I might suggest checking for leaks in the carb holders, exhaust. Maybe even pull the carbs and be sure they are perfectly clean inside.
    Leo
     
    gggGary likes this.
  12. figure8

    figure8 XS650 Guru

    Ill throw my situation in here. 81 special II with 34 carbs. I had the bike professionally worked on. Did my homerwork, had cam chain adjujsted, valves and timing done. Uni pods and cut pipes with shorty mufflers, gave the mechanic stock main and two sizes up. Stock pilot and one size up. Bike was given to him with the biggest of both jets in,also two rebuild kits and had him adjust float height. Went to get bike and it takes forever to kick it over. Finally get it going, take it for a ride and its completely bogged down. Take it back and he says it must be running to rich. Says try the smaller jets. Limp it home and start trying different jets. I can barely keep it running. It needs the choke out, dies when giving throttle. What am I missing here? Dissapointed with what I payed for and my lack of ability to figure this out. I have the carb guide in my garage and have read countless posts's. Also I am noticing the right side pipe is healthy sounding and the left is flat and barely warms up. Suggestions please. I feel I could have done all the work this guy did myself but wanted the comfort of knowing is was absolutely done right. Now I have a bike that runs worse than before. HELP!
     
    gggGary likes this.
  13. 5twins

    5twins XS650 Guru Top Contributor

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    If your "mechanic" used the specs from a Yamaha shop manual for setting the floats then they are probably wrong. The manuals were not updated with the new float setting spec for the plastic floats. 27.3mm is the spec listed in most of the manuals but that only applies to the 1980 BS34 carbs with brass floats. Your plastic floats should be set @ 22mm. If your plastic floats are set to 27mm then they are way too lean and the bike is starving for fuel. That would explain your problems like the hard starting and needing the choke all the time.

    Yes, you probably could have done a better job yourself. It's hard to find people who know how to work on these (or any old) bikes. It's becoming a lost "art". You did some homework, just not enough it seems.
     
    gggGary likes this.
  14. figure8

    figure8 XS650 Guru

    5twins, thank you for the reply. I was aware of that and specifically told him 22+/-1. I have seen little bills video on how to do so as well. If I am recalling correctly Bill says set the height from the top point of the arc in the float, not the the flat spot. Maybe I have it backwards. I'll have to recheck it. I am also still at a loss from the difference in the exhaust, a leak somewhere?
     
    gggGary likes this.
  15. 5twins

    5twins XS650 Guru Top Contributor

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    Check your carb sync. If it's way off, that weak cylinder could be effectively shut off at idle. Your ignition uses a single coil and fires both cylinders from it. If it fires one, it should be firing the other. It can't go "half bad" and only fire one cylinder. What could be at fault is the plug wire or plug cap (or their connections) on that weak cylinder.

    Yes, your float height is measured to the rounded part of the float, not the higher flat area near the pivot.
     
  16. figure8

    figure8 XS650 Guru

    Hey 5twins, I noticed in the rebuild kit there were two plugs that I think go over the pilot jet opening. My "mechanic" didn't think they should have been in there, I think they were there before and should be? Well?
     
  17. gggGary

    gggGary I'm listening, change my mind XS650.com Supporter Top Contributor

    Yup BS34s have to have the rubber plugs in place.
     
  18. 5twins

    5twins XS650 Guru Top Contributor

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    Yes, put those rubber plugs in or the bike won't run right (as you've noticed). Your pilot jets are sucking too much fuel without the plugs.
     
  19. wilbur60

    wilbur60 XS650 Enthusiast

    I am surprised it runs at all without those plugs. Or does it? If you have had it running without the plugs, what were the symptoms? (Always curious to see what happens if you leave a part out)
     

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