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Surging at low RPM's

Discussion in 'The Garage' started by Kinetic, Aug 25, 2020.

  1. Kinetic

    Kinetic XS650 New Member

    Hey forum'ers, I recently bought a non-running 1980 XS650 and have been bringing it back to life. First of all I want to say that this place has been extremely helpful for getting informed on all aspects of the bike, and most problems I've encountered there is usually a thread with great advice. So thanks to all of you for sharing!

    The bike i purchased was sitting in a garage for almost a decade. It has great compression (140psi in both cylinders) and a new battery. I pulled apart the bs34 carbs and followed the carb guide on this site to clean everything out, and check all the screws (everything in the carb is stock). After re-assembling i got the bike fired up, and after tweaking the mixture screws (both currently sitting around 3 turns) it idles fine with the choke off.

    Now to my current problem which i was hoping to get specific advice on: I took the bike out for a ride around the neighborhood and when i'm in the 1/4 throttle range 1k-3k rpm, the bike surge's and bucks as if the motor is cutting on and off. Once i pass 3k it opens up and pulls smoothly (sounds great too). After parking the bike i tested the exhaust headers with water and while both are hot the right side cylinder is much cooler then the left. Furthermore, there seems to be fuel leaking from the pilot air screw passage on the right side carb into the airbox.

    The fuel leaking will stop when i lean (screw in) the mixture screw on that carb, however from research it sounds like the cylinder running cooler is due to an already overly lean mixture, so theoretically i would want to enrich it?

    Not sure if these two issues are related but any advice would be much appreciated. Thanks for reading!
     
    Paul Sutton likes this.
  2. 5twins

    5twins XS650 Guru Top Contributor

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    A lean cylinder would run hotter, not cooler. The difference in cylinder temps may be caused by the carbs being out of sync. One cylinder is running faster than the other. That might also be contributing to the surging. It's also possible you didn't get the idle circuits totally clean, or maybe they have partially plugged back up.

    Often when you clean a really dirty set of carbs, try as you might, you don't get all the gunk out. Once you put them in service, the bits you missed can break free and plug things up again. This happened on the '83 I'm fixing up. Like yours, it sat for many years and the carbs were pretty gunky inside. I thought I had them really clean but once I started running it, the right pilot jet plugged up again on me not once but twice. After that second time it's been good now for quite a while. Hopefully it will stay that way and the carbs are really finally clean inside, lol.
     
  3. Paul Sutton

    Paul Sutton Still Looking Good Top Contributor

    When I have seen fuel leaking back into the airbox it has been due either to a sticking fuel level valve or the fuel level being set too high i.e. float levels wrong. I suggest you recheck the carbs as per 5twins suggestions and also double check the fuel level in each carb.
     
    Last edited: Aug 29, 2020
  4. grizld1

    grizld1 Grumpy old man Top Contributor

    Fuel leak stops when the mix screw is turned in? That's a new one on me. Open that carbie again, as 5twins suggests give it another cleaning, check to see that the washer and O-ring are in place at the bottom of the mix screw casting (and that one and only one of each is in place), that the plug is in place at the bottom of the pilot jet casting, that the float has not taken on fuel if you still have the OE 1980 brass floats, and that float level is set correctly for the float set you have. Then synch.
     
    Paul Sutton likes this.
  5. Kinetic

    Kinetic XS650 New Member

    Update: Pulled the carbs and re-cleaned the pilot circuit. I also realized my rookie mistake of failing to clean the emulsion tubes. They were so crusted up that i actually thought they couldn't be removed the first time. Re-set both mixture screws to 2.5 turns.

    Next i tested if the old float valves/o-rings are actually sealing. Set the gas tank feeding the carbs with the float bowls removed allowing fuel to drip. They both stop the flow of fuel when i lightly raise them. The seals appear to be doing their job, so adjusted the floats from 27mm up to 28mm. After re-assembly and test driving this has seemed to solve the fuel leak issue.

    I thought my surging problems were solved by the clean emulsion tubes, however, instead of surging it now has a flat spot as the throttle is opened. To my knowledge this indicates leanness, so i attempted to screw out the mixture screws 1 turn (now at 3.5) and re test. The bike now surges as before. My thoughts are that it is still too lean? I just ordered a #45 pilot and a #135 main.

    Some last notes: i tried finding the optimal mixture screw settings by listening for an increase in idle, but for the most part there seemed to be no audible change as I turn the screw. Turning the screw on the right cylinder (cooler exhaust pipe) did make the rpm bump up temporarily, followed by a return back down after 2 seconds. It then oscillated like this a few times in 2 second intervals. My thoughts are that the right cylinder is half-assing it, and the rpm increase is it coming to life finally. What do you guys think?
     

    Attached Files:

  6. Paul Sutton

    Paul Sutton Still Looking Good Top Contributor

    Just you wait til your lady spots what you have done with her best stainless steel mixing bowl.....:guns:
     
    Mailman and timbeck like this.
  7. Someone corect me if im wrong but what we commonly refer to as mixture screws are an air bleed adjusment. They work in reverse of what you would find on an automotive carburetor. Turning the screw out leans the mixture ratther than enriching it by allowing more air through this circut . Have to agree with 5twins, even after a thorough cleaning or two gunk can re-lodge in these passages. Especialy.Small ones like the air bleeds. As you said adjusting these screws had little effect it sounds like this could still be the case. And as he said, lean= hot. If the bike is in stock form I doubt that the larger jets are neccessary. Just get what you have dialed in. These bikes are the most fun to ride when they are running tip-top. So while repeatedly removing and going through carbs can be a pain, it goes faster and smother each time and can really pay of. Thats my two cents. Good luck and have fun.
     
  8. 5twins

    5twins XS650 Guru Top Contributor

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    No, the mix screws on these aren't air or "air bleed" screws, they control true fuel or, more correctly, fuel/air mix flow. But yes, turning them in does lean the setting. It lets less of the fuel/air mix in.
     
    Paul Sutton likes this.
  9. Thanks for clearing that up. Thats why I love this forum.
     
    Paul Sutton likes this.
  10. grizld1

    grizld1 Grumpy old man Top Contributor

    Rule of thumb: if the mixture screw is located in front of the slide tower, it controls vacuum fed to the pilot jet: out=more vacuum=more fuel=richer mixture. If the mixture screw is located behind the slide tower, it controls air feed to the pilot circuit: out=more air=leaner mixture.

    The above is true of Japanese carburetors and of every motorcycle carburetor I've seen. I haven't seen everything, so if anyone knows of exceptions, please jump in!
     
    Paul Sutton likes this.
  11. Hey thanks. Makes sense.
     
    Paul Sutton likes this.

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