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Thread: GN250 running too rich?

  1. #1
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    GN250 running too rich?

    I have a 2007 GN250, i Have swapped out the exhaust on it for an aftermarket one. I am finding that if i have the idle around 1500rpm (as is meant to be) it bogs when warm and i open the throttle to pull away lights etc. I have screwed in the a/f screw as much as possible as I believe it is running rich (the choke is fully pushed in also). I believe it is running rich as the stalling problem gets worse when the a/f screw is turned out to 2+ turns, and when I fire it up after it has bogged (or if I manage to catch it before it stalls) then the smoke coming from the back is black. This generally happens when the bike has had chance to warm up. If I wind up the idle screw (the one that adjusts the resting position of the accelerator) it seems OK but of course it is revving its tits off at 2500-3000rpm.

    I tried removing the air filter and twisting the throttle then to see if that was preventing the air getting through the system but made no difference.

    ANn suggestions or is my next stop a carb removal and clean (checking the pilot jet thoroughly),

    Cheers

  2. #2
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    you're going to want to rejet that motherfucker

  3. #3
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    Quote Originally Posted by Akzle View Post
    you're going to want to rejet that motherfucker
    You reckon they're rooted or too large?

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    you're going to want to take it to someone who knows what the fuck they're about.

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    Quote Originally Posted by Akzle View Post
    you're going to want to take it to someone who knows what the fuck they're about.
    You mean to say messing with the exhaust, which alters the stoichiometric ratio beyond what current jets can be adjusted to, requires a bit of clever thinking...

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    Quote Originally Posted by caspernz View Post
    You mean to say messing with the exhaust, which alters the stoichiometric ratio beyond what current jets can be adjusted to, requires a bit of clever thinking...

    well i don't know about any of that fandangled stoichiometry, but i do know that the carburettor pixies get upset when you change the flow and will often go on strike. and if you have to ask "why" then you probably need a professional pixie-whisperer.

  7. #7
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    Quote Originally Posted by Akzle View Post
    you're going to want to take it to someone who knows what the fuck they're about.
    IS that what your wife said to you mate?

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    Quote Originally Posted by globe View Post
    IS that what your wife said to you mate?
    She said "I'm not touching that" ... so it was close ...
    When life throws you a curve ... Lean into it ...

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    Quote Originally Posted by globe View Post
    You reckon they're rooted or too large?
    Sounds like another "it was running fine until I changed the wiper blades" type issue ... How old is it and how many km's .. ???
    When life throws you a curve ... Lean into it ...

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    Quote Originally Posted by FJRider View Post
    Sounds like another "it was running fine until I changed the wiper blades" type issue ... How old is it and how many km's .. ???
    Yeah nah, it was a bit rough before I changed the exhaust, I always ran it at uber high idle (like 2500rpm) - now i'm trying to get serious and learn some shit at the same time, which is fun, but 10 years ish, done 23,000kms according to carjam. Cheers

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    Quote Originally Posted by globe View Post
    Yeah nah, it was a bit rough before I changed the exhaust, I always ran it at uber high idle (like 2500rpm) - now i'm trying to get serious and learn some shit at the same time, which is fun, but 10 years ish, done 23,000kms according to carjam. Cheers
    Put the old exhaust back on and see if that changes anything.
    When life throws you a curve ... Lean into it ...

  12. #12
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    Quote Originally Posted by FJRider View Post
    Put the old exhaust back on and see if that changes anything.
    Good idea, however.....yup......it is no longer in my possession.....bugger

    yeah i know dumb move

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    Quote Originally Posted by globe View Post
    IS that what your wife said to you mate?
    no. but your wife did.
    i think being over 3", she just didn't know what to do with it...

  14. #14
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    Idle speed at 1500rpm seems high to me, despite what your manual says. Uber high idle speeds of 2500 are ridiculous for a single four stroke like this.

    Idle speed is whatever setting allows the engine to maintain flywheel rotation and overcome flywheel inertia, and provide a clean response when you open the throttle.

    I think your idle is too high and you're not actually running on the pilot circuit at all when at idle. Probably running on slide cut away.

    Warm the bike by riding down to your local bike shop. Buy a new spark plug. Come back and leave the bike running. Drop the idle as low as you dare, then tune the air/fuel screw for best fast running at idle. Drop idle some more, fine tune air/fuel screw again. It may not change. Small fractions of a turn are possible of making a big difference. Now you should be running on the pilot circuit.

    Stop bike. Fit new plug. Bike should start and idle well. You can repeat the above process again with the new plug if you like but probably not necessary. If it doesn't idle something is probably blocking air or fuel feed. Could also be valve clearances but if it was running okay before you changed exhausts then they should still be okay.

    You need to understand what twiddling with the air/fuel screw actually does. Does it control air or fuel feed?

    Your new exhaust may be relevant further up the throttle position range but on this motor I doubt it will change much in the idle circuit.

  15. #15
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    Quote Originally Posted by Ginge09 View Post
    Idle speed at 1500rpm seems high to me, despite what your manual says. Uber high idle speeds of 2500 are ridiculous for a single four stroke like this.

    Idle speed is whatever setting allows the engine to maintain flywheel rotation and overcome flywheel inertia, and provide a clean response when you open the throttle.

    I think your idle is too high and you're not actually running on the pilot circuit at all when at idle. Probably running on slide cut away.

    Warm the bike by riding down to your local bike shop. Buy a new spark plug. Come back and leave the bike running. Drop the idle as low as you dare, then tune the air/fuel screw for best fast running at idle. Drop idle some more, fine tune air/fuel screw again. It may not change. Small fractions of a turn are possible of making a big difference. Now you should be running on the pilot circuit.

    Stop bike. Fit new plug. Bike should start and idle well. You can repeat the above process again with the new plug if you like but probably not necessary. If it doesn't idle something is probably blocking air or fuel feed. Could also be valve clearances but if it was running okay before you changed exhausts then they should still be okay.

    You need to understand what twiddling with the air/fuel screw actually does. Does it control air or fuel feed?

    Your new exhaust may be relevant further up the throttle position range but on this motor I doubt it will change much in the idle circuit.
    Cheers mate, great response. I cleaned the carb thoroughly and it seems to have made a difference. Haven’t had chance to take it for a proper spin yet but here’s hoping 👍

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