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Thread: Main jet on small single 4stroke

  1. #16
    Should Get Out More a_morti's Avatar
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    Default Re: Main jet on small single 4stroke


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    Maybe it's previously been run on an open exhaust and/or pod filter? The jetting was increased for that but forgot to return to stock when took the silly parts off?

    Not many other reasons to go that much bigger on the main jet than a pod filter.

  2. #17
    A Bit Bored mala's Avatar
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    Default Re: Main jet on small single 4stroke

    Sounds like overjetted so maybe go back to stock.
    Would explain the sooty plug.
    At high throttle openings the main jet is very dominant so needs to be right.

    ATB

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    Default Re: Main jet on small single 4stroke

    Well, put in stock main jet #122.5, and no disernable change in performance! Haven't checked the plug yet.Might get a slightly more open air filter and go back to the #135. And then forget about it. Get myself a 80mph 250+.

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    Should Get Out More maccecht's Avatar
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    Default Re: Main jet on small single 4stroke

    Questions you say the carb is clean? How long has it sat? The small drillings in the carb clog easily from the krap fuel we have these days and sitting for a few months is all it takes. Had similar issues to you on many bikes and 90% were solved by cleaning the carbs. Soaking in Coke overnight is my preferred method. Having a compressor is also a boon as you can them blow out all the orifices. Have you checked the float height is correct? What year is the bike? Do you have a manual? Eliminate all the easy things first.

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    Default Re: Main jet on small single 4stroke

    Update: small oil leak from front left of head gasket, tightened some head bolts and a little nut at the front, recon idle has improved but maybe not. Oil still leaking (tiny).
    Top end performance the same.
    Put stock 122.5 main jet back in and no real change in performance but now new plug is reading fairly lean- (this is the important part-time does this suggest?) and lots of backfire on deceleration.
    Took off carb and float level is spot on. All the Jets I can get to are clean.
    Will soak in cola overnight and see.

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    Should Get Out More Julian_Boolean's Avatar
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    Default Re: Main jet on small single 4stroke

    Quote Originally Posted by Lalababa View Post
    Update: small oil leak from front left of head gasket, tightened some head bolts and a little nut at the front, recon idle has improved but maybe not. Oil still leaking (tiny).
    Top end performance the same.
    Put stock 122.5 main jet back in and no real change in performance but now new plug is reading fairly lean- (this is the important part-time does this suggest?) and lots of backfire on deceleration.
    Took off carb and float level is spot on. All the Jets I can get to are clean.
    Will soak in cola overnight and see.
    You've got an air leak on the exhaust fairly close to the head

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    Default Re: Main jet on small single 4stroke

    Soaked in cola, blew out all holes and jet holes. Filed a notch on needle below the only notch that was there.put circlip on lower notch. Put 122.5 jet back in, no difference save maybe less backfire.
    Tried to tighten exahust manifold nut(nut rusted to thread) sheared off. Whey hey!

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    Default Re: Main jet on small single 4stroke

    Julian_boolean ....you say back fire most likely caused by exhaust leak close to head....would this also account for a v. lean looking spark plug?

  9. #24
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    Default Re: Main jet on small single 4stroke

    Quote Originally Posted by Lalababa View Post
    Julian_boolean ....you say back fire most likely caused by exhaust leak close to head....would this also account for a v. lean looking spark plug?
    It could be the cause, but it might not be the only cause.

    Sent from my SM-T560 using Tapatalk

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    Default Re: Main jet on small single 4stroke

    Time for a compression test methinks if not your carb something else is amiss

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    Default Re: Main jet on small single 4stroke

    Apologies, am using incorrect word backfire, when it should be afterburn or bangs from the exhaust. Have sent off for compression tester.

  12. #27
    Should Get Out More maccecht's Avatar
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    Default Re: Main jet on small single 4stroke

    Quote Originally Posted by Lalababa View Post
    Update: small oil leak from front left of head gasket, tightened some head bolts and a little nut at the front, recon idle has improved but maybe not. Oil still leaking (tiny).
    Top end performance the same.
    Put stock 122.5 main jet back in and no real change in performance but now new plug is reading fairly lean- (this is the important part-time does this suggest?) and lots of backfire on deceleration.
    Took off carb and float level is spot on. All the Jets I can get to are clean.
    Will soak in cola overnight and see.
    If you still have an oil leak it might be your head gasket compression test will tell as you said you have done the valve clearances. Had a GPz750 same symptoms turned out one of the head bolts had stripped fixed that and away she went

  13. #28
    Should Get Out More a_morti's Avatar
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    Default Re: Main jet on small single 4stroke

    Revisit the head gasket. You've nipped it back up but pennies to pounds it has a hole in it from being run loose.

    If your head gasket is leaking on an air cooled bike, it would not only let oil weep out, but could let air straight into the combustion chamber. This would cause both the lean condition you're seeing, and the popping on the overrun. It could also let oil into the combustion chamber which I guess would make it run lean, but you've not complained of a smoky exhaust so I guess it's not terribly bad yet, if it's that.

    Sort head gasket or the lean condition will be what kills your engine. If not running out of oil.

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