PW50... won't rev... lots of things tried

Discussion in '2 smokers' started by ripcuda, May 6, 2015.

  1. ripcuda

    ripcuda Adventurer

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    Oddometer:
    10
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    Hey Gang,
    I recently picked up an old but decent condition '83 PW50 for my youngest son. The Zinger runs but won't come off idle. And it's not the Stop-Run-Start switch. Appreciate any ideas.

    Here's what it has and has been done:
    - good compression (120psi)... engine turns over smoothly
    - new Miniky (clone Mikuni) PW50 carb. It has the right jets in it ~#67.5 main and #40 pilot (jets measured).
    - mixture screw changes making no difference
    - reed valve good, new gaskets
    - new NGK plug
    - new aftermarket coil... lots of strong blue spark when cranking with plug out and grounded.
    - coils in magneto ohm tested in spec
    - exhaust de-carbon'ed (burned out)... clean and free flowing. I've even tried running engine w/o exhaust on... no difference (other than louder). No restrictor washer.

    I ohm-tested the Stop-Run-Start (SRS) switch... tests fine per wiring diagram. Bike starts and idles in both Run and Start position... but won't rev any higher. I can see the slide/needle in the carb (airbox off) lifting up... but engine won't rev. Sounds like it's being held back like a rev-limiter. I've disconnected the SRS switch and the bike will still start, but not rev. I've even removed the entire ES Control module (the Start mode rev limiter, right?) and the bike will start and idle, but not rev.

    I'm stumped. I'm down to the stock CDI being possibly bad... or the stock ES Control module... or the magneto (which tested in spec)... or possible wire short?? (Anyone know which wire could be shorting that would cause this?)

    Appreciate any ideas!
    #1
  2. Luke

    Luke GPoET&P

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    General advice, not knowing this particular bike:

    Install an in-line spark tester and see if you are actually losing spark when you try to rev it.

    Also check the timing- pull the flywheel and see that it is keyed properly to the shaft. I don't know how you would use a timing light on this particular engine but verifying that it is firing in the vicinity of TDC would be helpful.
    #2
  3. Wolfshirt

    Wolfshirt 2nd degree Adventurer

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    Sounds like it could be a bad crank seal. I had a PW 80 give me the same issues and I rebuild the top end and new reeds and changed carb. No luck then I changed the flywheel side crank seal. Just pulled off the flywheel and changed it (no case splitting).
    It ran perfect after that.
    #3
  4. ripcuda

    ripcuda Adventurer

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    Thanks for the ideas!

    I'm going to pull the flywheel and check the key and crank seal (probably replace it while in there). I've also got a replacement CDI coming... so hope to get it sorted out soon. My boy is antsy!

    Cheers!
    #4
  5. stainlesscycle

    stainlesscycle Long timer

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    need more information:

    what does it do when you open the throttle? nothing? bog? or just die/shut off like a switch? can you very gently open throttle to bring the rpm up?

    couple of things.
    carb:
    check and make sure the needle is coming up with the slide. have had a needle clip come loose and slide would open, but needle just sat in the needle jet..
    make sure throttle cable is fully opening slide.
    make sure main jet hasn't fallen out.

    it's not the magneto, unless it crackles/breaks up and just shuts off...

    could be cdi. have never fooled with the limiter module.
    #5
  6. ripcuda

    ripcuda Adventurer

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    Dec 22, 2012
    Oddometer:
    10
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    Tried a few more things today.

    I previously ohm checked the pickup coil and charging coil... both perfectly in spec. Coil, wire, cap and plug are new. Lots of blue spark.

    I've confirmed the throttle is working... carb slide moves up/down and the needle is moving also with it. Choke is working also... not sticking.

    I've tried a new CDI... no change.

    I had the exhaust off (after I burned it clean and repainted) so I blocked the exhaust, kept the spark plug in, pulled the carb and rigged up a sealed cap with nipple on intake. The system holds 10" vac just fine. So that seems to rule out a leaking crank seal to me.

    Bike starts and "idles" fine... but will not rev any higher than idle. Both in "Run" and "Start" positions... even with Stop-Run-Start switch unplugged. Even with the whole ES Control box removed. Choke doesn't help any either. Putting my fingers over carb intake as a sort of restrictor makes no difference either. Having the exhaust completely off... or just the pipe and expansion (no silencer)... neither makes a difference.

    I'm left with timing. I *hope* the flywheel key sheered. I have a flywheel puller on its way... so that's next to check.

    I'll post my results.

    Cheers!
    #6
  7. Suzuki Phil

    Suzuki Phil Been here awhile

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    I would try to find the correct carburetor. If it's a clone it's probably made in China which means junk. Just because you put in the correct jets doesn't mean the rest of the carb is ok.

    Did you use the old throttle valve or the clone? After playing with two-strokes carbs on many bikes, the factory carbs always work the best.

    Keep us informed,

    SP
    #7
  8. ripcuda

    ripcuda Adventurer

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    Thanks for the advice. I might very well come back to the carb again. I'm using a new throttle cable with the new Miniki carb throttle valve. It slides up and down with the needle fine. When it opens wide, the engine still refuses to rev. :huh

    I got the flywheel pulled tonight. The key is fine and flywheel looks in fine condition as well. It has the stock magneto stator, but I ohm checked it again. The pickup coil measures 20 ohms... and the charging coil measures 302 ohms... right in the middle of the specs I found online.

    I'm going to replace the mag-side crank seal while in in there... even though the engine holds pressure fine. Then I'll button it up and continue my search.

    I'm wondering about a possible short in the wiring harness. Guess I'll start tracing wires.

    This is the most confounding moto I've ever worked on.

    Cheers!
    #8
  9. willis 2000

    willis 2000 neo-quixote

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    ensure the exhaust restrictor is not installed
    the start/off/run switch is lame
    it retards the ignition for starting
    rewire it with a kill button
    ensure your bars are grounded
    might need a frame strap
    i did this on my daughter's bike
    with poor grounding
    the kill button shocked
    lotsa laffs
    #9
  10. willis 2000

    willis 2000 neo-quixote

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    repair and reinstall the mikuni
    #10
  11. stainlesscycle

    stainlesscycle Long timer

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    what happens when you open the throttle? is there absolutely no change? or does it bog or sputter or shut down?

    reed valve issue or intake manifold issue ????? a bad reed could cause this...
    #11
  12. alansz400

    alansz400 Long timer

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    My first thought as well. I have tried china carbs on crf50s and KLX110 and they never work right. Close but never as good as a stock carb.
    #12
  13. ripcuda

    ripcuda Adventurer

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    Well... spend some more time on this old peewee. Still no dice... bike will NOT rev above idle. But idles nicely.

    I had the flywheel off and pulled the stator out. Double checked all 3 coils in the stator and they spec'ed perfect for the ohm check on each.

    I replaced the crank mag-side seal for good measure.

    I cleaned and double checked the Miniky carb... it is clean and has the right stock size jets in it. Slide works fine with throttle. Needle is rising/lowering with slide as expected. Enricher is full down/off. It is a new carb... yes, it's Chinese. No... I do not have the original anymore.

    I rewired the mag direct to the CDI and coil. All back together... it runs exactly the same. So it's not a wiring issue.

    Recap: exhaust is clean/clear, no restrictions, no airbox, good compression, tight vac-holding engine, good reeds, new gaskets, new carb, new coil/wire/plug cap/plug, new CDI, ES unit (rev limiter) removed from bike, stop/run/start switch removed.

    I'm down to the stator... even though it ohm tests perfectly in spec. And the carb... even though it's new, clean and has the right jets in it.

    So much fun...


    [​IMG]

    [​IMG]

    Cheers!
    #13
  14. DisorganizedVince

    DisorganizedVince Wanderer

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    How did you measure the jets? Are they the jets that came with the carb or have you replaced them with Mikuni jets? From past experience despite the numbers stamped on them, Chinese jets are never what they claim to be
    #14
  15. Wolfshirt

    Wolfshirt 2nd degree Adventurer

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    If it has stock jets and no airbox it probably won't run right. the stock airbox is very restrictive. Try starting it and put your fingers over the intake of the carb and vary the size of space between your fingers. Then see if if will rev that way.
    #15
  16. ohgood

    ohgood Just givver tha berries !!!

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    the airbox makes a huge difference on these small engines. it needs some restriction (the air filter) and resonance to make power and rev proper.

    without air filter = barely pull up a hill
    with air filter = pulls like a (50cc) beast !

    just my experience from a 50cc yamaha jog :)
    #16
  17. ripcuda

    ripcuda Adventurer

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    Thanks for the ideas gang.

    The Miniki (Mikuni clone) is new, clean and I measured the jets size against real Mikuni jets and they're the same size. #67.5 main , #40 pilot.

    I have an airbox, but have it off for testing. I have put my fingers over the carb inlet to give it more restriction. It still will not rev above idle. That's actually how I kill the engine... since I hard wired the Mag to CDI/Coil... so no kill switch.

    I've also held my hand on the expansion pipe opening (don't have silencer on it... just header/expansion). This also makes no difference. Still will not rev above idle.

    I've ordered a new magneto. Even though the current one ohm checks in spec. I'm crossing my figures. This little pee-wee will not get the better of me. It's going to have every new part on it until it runs!

    Cheers!
    #17
  18. Gamequeazy

    Gamequeazy Curbjumper

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    And you're *sure* the reeds are good?
    #18
  19. stevo7706

    stevo7706 Been here awhile

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    Give us a picture of the reeds.
    #19
  20. stainlesscycle

    stainlesscycle Long timer

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    and you measured these jets how? go/no go plug gauge?
    #20