Where did I go wrong?

Discussion in 'Motorized Bicycle General Discussion' started by Chase Spellman, Nov 7, 2018.

  1. Chase Spellman

    Chase Spellman New Member

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    Hi y'all, new to the forum thing in general, I've got a 2 stroke grubee Skyhawk that I rebuild mostly with a Chinese clone. The clone I ran for about 30 miles but decided it would be better for parts considering the head and cylinder were one piece (ofc the seller online didn't show that). And I'm looking to go with a high compression head, etc.

    ANYWAYS, the clone with just a larger "performance" carb and a 36t rear sprocket topped out at about 27mph.... Not bad but it usually averaged around 24 and with a 14 miles round trip commute it was painfully slow. Now I'm not shooting for record numbers or ridiculous speed. I know that's not what these were made for. But since I rebuilt the Skyhawk and have gone over it half a dozen times, adjusting spark plug gap, clutch tension, recleaned the carb, greased my wheel bearings and just about any other moving component, I took the rebuilt motor from 18-20 mph back up to 24 at WOT. I can actually feel a huge difference in power between the two motors!

    My problem... Or ignorance... I see the motors guys build on YouTube that easily cruise at 35 and peak around 40, **** I've seen 70mph bikes... I just wanna do mid thirties, what am I missing?

    What all I've done:
    Port matched intake/exhaust
    Ngk spark plug
    Larger carb
    36t sprocket
    Lubrication
    97octane w/propper oil mix
    I'm sure there's other things I'm forgetting.. Just help a brother into the thirties!!
     

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  2. MacZulu

    MacZulu Member

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    You mention a larger carb, have you changed jets? I would suggest carb tuning as a first step.

    Stock exhaust? A bigger carb is only going to help if your engine is moving enough air. Intake and exhaust need to be tuned to your carb.
     
  3. Chase Spellman

    Chase Spellman New Member

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    Not really sure on how to go about the Jets like you mentioned, and I'm planning on welding up a new exhaust soon. I'll look into the changing the Jets like you mentioned
     
  4. MacZulu

    MacZulu Member

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    I dont know what carb your running, but if you went to a bigger carb and no other changes you might be rich.
    Does your carb have a air/fuel mixer? Does it have an adjustable needle height?

    For jetting there are tons of Write ups. Basically if your increasing the air in and out of the engine (intake and exhaust) you need to increase the fuel to maintain proper a/f ratio. Sounds like you may have increased fuel with out increase air.

    It you are doing an exhaust, I would wait until you install that and then tune the carb.
    And increase air flow in, to match your increase air flow out. Then match the fuel (jet). Once it "feels" pretty good, you see if your lean or rich by warming up the engine, find a good stretch of road to WOT for a bit and kill the engine while throttle is wide open. Pull the plug and take a pic, then find online help through pics or posting your here and asking.

    I think tuning the carbs is one of the best skills to have on these set ups, I have a long way to go. But I understand the underlying principle which guides the process, and I like to learn and tinker.

    Oh and Chase I have no idea of your experience or skill level, so I dont know if I'm over or under explaining.
     
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  5. Chase Spellman

    Chase Spellman New Member

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    Nope, prefect explanation. Thanks! Will make that happen asap!
     
  6. crassius

    crassius Well-Known Member

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    sometimes with a 36T rear, the motor never turns fast enough to reach its power band - you might try a 41T on it
     
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  7. Chase Spellman

    Chase Spellman New Member

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    Thanks! I don't have a 41.. but I'll try it with the 44!
     
  8. Chase Spellman

    Chase Spellman New Member

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    So I tried the 44t sprocket... Lost 5 mph. Topped out at 20 :(
     
  9. crassius

    crassius Well-Known Member

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    wouldn't have expected that - 44T used to be the standard sprocket for all kits

    somewhere in mix/match of parts, I'd guess you've gotten a bad port mapping or bad compression fit or maybe just wrong main jet size

    probably too late to try to get it back to stock
     

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