Loss of top end power
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  1. #1
    Junior Member DesertMonkey's Avatar
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    Mar 2011
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    Alright here we go. Long time forum stalker, first time poster.



    I have a '91 dub with 9500 miles on it. I was out on a ride a few months ago and out of the blue my bike began sputtering, choking for life, and finally died. I started her up again and was able to run it in first and second gear to get home, though there was still sputtering, engine revving, and a little smoke. If I passed second gear the bike would die.



    All I had changed on the bike prior to this ride was I lowered the back sprocket from 54 to 50 teeth, hoping for a little more top end speed.



    I began the process of fixing or at least diagnosing:



    1. Changed the air filter (was quite dirty at this point)

    2. Changed the spark plug (looked a little lean, but not too bad)

    3. Changed the oil

    4. Ran a tank of gas with Sea Foam mixed in

    5. Cleaned out the carb (was very clean, no clogged jets. I have an in-line fuel filter so that probably helped. DID NOT adjust the float as I don't know how).

    6. Adjusted mixture screw to 2 3/4 turns to achieve slightly richer mixture

    7. Ran a compression test and got 120 (later a shop told me they read only 100???)

    8. Adjusted the valves



    Adjusting the valves seemed to help. Both valves were out of spec, intake was a little tight and exhaust was very loose. After the valve adjustment the bike will now idle fine, the engine no longer dies in higher gears, and no more smoke. I can now ride the bike in all gears, but I have a serious loss of power on the top end. Top speed with the 50 tooth sprocket is around 50 MPH and I have trouble getting the RPM's up enough to shift from fourth to fifth gear.



    I finally brought it to a shop and all they could tell me is that the "engine is tired" and I should pull apart my cylinder/piston. Or just put the 54 tooth sprocket back on as it was "making up for the tired engine".



    Does this sound like the piston to anyone? I was under the impression that the piston would affect the performance of the entire motor and no just the top end.



    Any thoughts, ideas, suggestions would be appreciated!



    (p.s. I have read TONS of threads on this forum already, but if there is a pertinent thread please send a link)

  2. #2
    Banned qwerty's Avatar
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    Jul 2010
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    NW Tennessee
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    Sounds like maybe a burned intake valve, since it was too tight.



    Anywho, try working through the troubleshooting section of the shop manual. I've switched between 15/50 and 15/54 sprocket sets and noticed no difference in top end. That is not your problem.




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