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HELP! 1981 XS650 Carbs, Floats, Valves, etc.

Discussion in 'The Garage' started by TheRunaway, Jun 30, 2014.

  1. TheRunaway

    TheRunaway XS650 Enthusiast

    MY 1981--/--650 was running a little rough last week. It had all the signs of needing a float height adjustment (from what I gathered), so I decided to spend the weekend fixing it.

    It started when I noticed gas leaking out of the left side carb (view from saddle) right where the float bowl attaches. I researched RESEARCHED RESEARCHED. Thought I had it all figured out. Took a sh_t load of notes too.

    Based on some videos I saw on youtube, I decided to go ahead and do my Cam chain tension adjustment and my valve adjustment. Cam adj. went fine as far as I know. Valves gave me a lot of trouble. Every time I set them to .06 and checked to make sure .07 wouldn't go it would be fine. UNTIL I WENT THROUGH FULL ROTATION AGAIN. Sometimes all the way up to .10 would fit! ugh. I tried at least 6 times with the intake valve to no avail. That's when I decided to leave it be and move on before I messed something up that I thought was supposed to be pretty simple. What is going on there??:shrug:

    (Oh yeah, my spark plugs were black. Internet told me that also alludes to a bad float height adjustment. Is that true? Do I need new spark plugs? I thought that might be the problem too.):confused:

    Which brings me to the carbs. Mikuni BS34s. I took everything apart, got the carbs out (fun stuff), and saw that whoever had them before me had stripped the screws for the float bowl. So I had to jimmy those out and get new ones. Got the carbs pulled apart. Everything looked great! Like they had been rebuilt and cleaned. So that was good! Checked my float height. It was at about 20mm. So I fixed them both exactly at 22mm. Made sure everything was moving and smooth, clean and free of debris. Tightened my sync screw all the way down then brought it up 2.5 turns based on more info I had found. Put it all back together this morning and when I put my tank back on and attached my tube (luckily I hadn't put my filter back in the box) the left side carb started pouring gas out the back into the air filter box!!! I unhooked the tube, waited a minute and wiped the gas, then put it back on. Nothing happened that time so I put the filter in and got everything ready to fire it up for a test. It started (sounded off, not sure why) then turned off. It wouldn't start again after that until I had waited about thirty minutes. I gave it one last shot and gas came out of the left carb again so I called it quits. :banghead:

    So there's everything! Lot of info but I wanted to be thorough to help with troubleshooting. I need help! I'm taking my carbs back off tonight. So any help today would be awesome. Thanks guys.:(
  2. LukeN

    LukeN XS650 Member

    ive never had that issue with the carbs leaking out of the intake before, so i cant answer that. but as far as the plugs go, just hit em with a wire brush until you get all of the crud off, and then check for spark. they should be fine though if they were working prior. sorry i cant help more
  3. weekendrider

    weekendrider Iron Horse cowboy Top Contributor

    S.W. MO
    Runaway have you printed the carb guide?
    Or at least read it through several times.
    The bowl seal would need to be addressed first imo.
    Then the sticking float(the gas pouring out off the intake bell.
    Bench sync your carbs by throttle plate position instead of a set number of turns.
    You can fix all these things it just takes more reading and doing to understand it.
  4. TheRunaway

    TheRunaway XS650 Enthusiast

    Thanks weekendrider. I did print the carb guide, along with diagrams and multiple sources of info to compare and double check. The bowl seals were fine when I checked them. Maybe I put them back wrong? I didn't think I had but I can check. The float didn't really seem to be sticking either. It was free and clear. I can check that again too though. I'm not sure what the throttle plate position is. Would that be in the carb guide? I'll read through again.
  5. 5twins

    5twins XS650 Guru Top Contributor

    I think you're mixing up some of your terminology. That "sync screw" you say you set at 2.5 turns sounds more like the mix screw. On a BS34, it should be set to between 3 and 3.5 turns out initially. The actual sync screw is something different. It's on the linkage between the carbs and allows you to match the butterfly plate opening on one carb to the other. That makes the two carbs "synchronized" so they are open the same amount at any given throttle opening. You need this so both cylinders run at the same speed.


    A common problem with the BS34s is a slow leaking float needle seat. The seat is a push fit into the carb body. It is sealed in there by an o-ring. The o-ring can go bad and then gas can leak around the outside of the seat into the bowl, not just come through its center hole as it should. The bowl overfills even if the floats are set right. If you pull the carbs again, you might want to check that o-ring.
  6. TheRunaway

    TheRunaway XS650 Enthusiast

    Thanks 5twins! I read the carb guide you helped put together, and I learned a lot from it. I'm grateful for your input. However, I wasn't getting my terminology confused. Maybe I did when I read that the factory setting was 1-1,5 turns out but 2.5-3 turns out enriches it and helps improve idle. I'm guessing that was talking about the sync screws (< right here! I said sync screws! I meant mix screws) and not the sync screw. One question does come to mind though. When I was looking at the mix screws there weren't any actual screws? I just glanced because I didn't really mess with them. Are they inside those holes?

    I will definitely check the O-rings though. A picture of where those are located would be extremely helpful though if you have access to one. Thanks again.
    Last edited: Jun 30, 2014
  7. TheRunaway

    TheRunaway XS650 Enthusiast

    I just realized that I got my terminology confused in that last post! Irony... wow. Haha. I promise I wasn't confused earlier though.
  8. itsatoy

    itsatoy XS650 Addict

    There are plugs you have to drill out to get to the mix screws.
  9. 5twins

    5twins XS650 Guru Top Contributor

    Yes, on the '80 and newer BS34s, the mix screws come capped over from the factory. You need to pull that brass plug out in order to adjust them, also to properly clean the carbs. You won't get far trying to tune the bike if the plugs are still in place. The screws were set at the factory to meet emissions standards and are set too lean for proper running.

  10. TheRunaway

    TheRunaway XS650 Enthusiast

    Aha, awesome info. I remember reading about that now, thanks.

    On a side note, I pulled my carbs back off last night to take a look at them and the diaphragms weren't staying put. That's definitely ONE of the issues. Is there some type of adhesive or something that I should be using?
  11. itsatoy

    itsatoy XS650 Addict

    Vaseline helps, but you still need to kinda slide the cover on and keep the diaphragm pressed into the channel at the same time. If the diaphragms have dried out it also helps to soak them in a little gas first so they will expand.
  12. TheRunaway

    TheRunaway XS650 Enthusiast

    Awesome thanks! Made my day.
  13. TheRunaway

    TheRunaway XS650 Enthusiast

    :shrug::confused::(https://www.mikesxs.net/product/48-9000.html <<< I found these on MikesXS and I was curious if its just bolt on and go? I'm still having trouble with my carbs. Ordering a rebuild kit and pulling them off to check them over again, but I can't see what is wrong unless the float needle isn't seating. I've also rebuilt the Petcock to get rid of the gas flow problem, but I'm still getting flooded. Any more tips would be great, and believe me I've been scouring this forum and others to try to find an answer but it seems like I've been told everything already. That's why I'm curious about the performance carbs. If I can't fix the issue that seems like it would be the best bet. I'm a tad bit frustrated because it ran fine after I spent a few days toying with the carbs but now it's back to cutting out on my and dying at stoplights plus an added problem of the rpm's staying up when it should be decelerating. I'm printing the carb guide again and I'll be reading thoroughly it tonight.
  14. ippytattoo

    ippytattoo Just another grumpy hack.

    Short answer: no those are not a bolt and go affair. Long answer: as with any performance upgrade or carb swap (from a different style and/or model ie. ex500) there will be some tuning and jetting that will need to be done. Those carbs have a bit of a bad reputation or at least the earlier runs had serious issues, mrriggs did an extensive thread on his trials and tribulations with those carbs. Since you have an '81, I am assuming you have the BS34's, a set of carbs that many including myself have had good results from are the CVK34's from a Kawasaki EX500. There is a thread specific to this swap around just do a search for ex500 or cvk34 and you should find it. They fit in the oem carb holders but I can't comment on what modifications are need to make them fit the stock airboxes if you are running those although I can say that the same pod filters that fit the BS34's fit the CVK34's so it will most likely be a matter of length that will be the limiting factor on fitment to the stock airboxes.

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