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Thread: GN250 strange behavior from 6k rpm and up

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    GN250 strange behavior from 6k rpm and up

    Hi
    I have a GN250 1994 model that behaves oddly. It has the expected pull from low rpm and power gets feelable lower from 6k rpm and up.
    I have cleaned the carbie, cleaned airfilter, adjusted valves cleaned and changed fuel, new CDI.
    Compression is around 185 PSI and should be fine ?

    Any ideas ? as i'm running out ?

    COuld it be a weak sparkplug ? ignition coil or other ?

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    GN250?

    Known to have problems with rectifiers.
    Life seems more precious when there's less of it to waste.

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    .. it's the battery...
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    Why did you change the CDI?
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    is it getting full throttle when you twist the grip..? will it rev out in lower gears..?
    Have you replaced gaskets and seal in the carb? Set float level--checked air breather vents not blocked?
    if you used carb clean with the diaphram in they will be damaged slightly.. Check alignment of cap- spring -slide diaphragm for creases splits etc. Is needle free to lift fully up? Use ya finger in the back of the carb...
    Checked for vac leaks.. ( check the fuel tap? Ride with tap on 'prime.'. note differences.)I guess it is a vac type tap
    Full throttle test is it..? You need ..max air flow--max fuel flow-around about 12.5v to 14.5v ( at the battery/power feed) -max timing advance.(cdi takes car of that) or should
    .good spark..91 ulp should be fine..8.5:1 comp?
    It's a tiny gutless engine..so you need everything to be perfect before they run -on song-
    Check battery frame earth return circuits..add wiring as reqd. mi2c is carby..
    " Imagination is the seed of life..."

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    I assume by power, you mean that it doesn't feel like it is accelerating as quickly as it used to once you get higher in the rpm? Does it occur in every gear? or just the higher gears? Is the clutch slipping?
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    I have fully rebuild the carburetor, and fully cleaned as well. No carb cleaner on the diaphragm, and have checked them for holes.
    I bought a complete rebuild kit, all new springs and jets. Compression is fine, and I have done a leak test where the carbie goes to the engine.
    Just today I notised that it pops some in the exhaust on deceleration. not much but some. I was wondering if it could be the camchain that has slipped a notch ?
    I replaced the CDI because it feels like ignition runs retarded.... or at least fires too late. or it could be flow into cylinder?
    The throttle opens fully, and the same behavior in all gears. it seems to loose all willingness above 6k.

    - - - Updated - - -

    Yeah I know that "power" is somewhat an overstatement but less than it should have. And no clutch is not slipping.
    And engine have only run around 12K kilometer. and 185 psi compression says that it's not broken

    - - - Updated - - -

    Quote Originally Posted by GsxInShed View Post
    .. it's the battery...
    Battery is brand new GEL type 12,3 Volts (14,6 running)

    - - - Updated - - -

    Quote Originally Posted by Scrap View Post
    GN250?

    Known to have problems with rectifiers.
    Can rectifier do any of the above ? besides cooking or undercharging the battery ?

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    mmmm... blocked exhaust? Valve timing checked?

    Carb jets in the right places..? Might be nothing ..yet there are 2 that look alike yet make a huge difference in running. ( not too sure tho'....above 6000 rpm should be all jet needle and needle jet..Main.)
    " Imagination is the seed of life..."

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    Sounds frustrating.

    If it's all factory settings and you're 100% that the carb isn't blocked and has fresh fuel, then I'd be concentrating on ignition.

    Pull the plug, earth it to the bike and hit the starter. Is it a strong spark? A weak spark would be enough to get the bike running but it'd break down at higher RPM.

    If that's fine though I'd consider calling out someone like Derek Ball to check it out. A fresh set of eyes and decades of experience is worth the cash and saves you from going postal.
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    He might be in Denmark ( not the one near Esperence )
    " Imagination is the seed of life..."

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    I am located in Denmark This forum just seems to have a huge technical knowledge base. So I thought i'd try it out.
    Will have a look at the ignition, and the valve and cam timing.
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    I'm guessing it's cam timing as well, where abouts in Dk are you living? My wife's a Dane and my oldest boy is currently living there and going to university

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    I see by your posts on KiwiBiker the problem has resolved itself..

    After "fiddling with the cables"..? you mean hoses eh? kinked breather hose..?

    Nice to see the weather is warm 0-5 deg C mmm luvly...

    Any hoo .. all the best .. elverdane ;-) Enjoy the ride...

    The whole story was at wot.. it would lose power then downchange made it worse.. pull over and rest the bike for 5 mins get on ride off and all is good for 15min then goes to shit again..

    Waddayaknow....?
    " Imagination is the seed of life..."

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    No unfortunately it has not been better. Not sure what on Kiwibiker I have typed Had a carb problem at start, and thought it fixed the issue. But alas, still same issue.
    2day i have checked timing chain, and it is as it should be. Valve adjustment are redone and set to ,05 mm in and ,10 on ex. and both pairs are very closely set.
    the engine starts to clatter and sound weird at 4500-6000 and then starts to sound really weird like firing in the exhaust.
    Think i will try another ignition coil and lead/plug cap. and see if that helps any.

    - - - Updated - - -

    cam timing was good, and chain was tight as i should be.
    You are wellcome to set me in touch with Your kid. I'll buy a beer or a coffee if he will have a lissten to it

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    Quote Originally Posted by GsxInShed View Post
    He might be in Denmark ( not the one near Esperence )
    With a user name of Elverdane, I initially thought he's from the very far north, not down south.
    Trying to think of a wise and sincere signature quote, but the only words that leap to mind are, "TITTY SPRINKLES"

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    Ok So It wasn't you on KiwiBiker..There is an identical issue on a GN250 fix is in . That one was a blocked /melted breather hose on the carby.

    When you have 180 + compression, all seems ok note that cranking speed does not cover engine running OR loaded.

    Is there a broken valve spring? ( stickinh valve would do the same but prolly hit the piston.)

    yeh it would clatter from 2500 rpm on up the range of course...I only mention that 'cause there is an unusual amount of replacement springs on eibai and OEM..
    " Imagination is the seed of life..."

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    Took a look under the top lid. Seems no broken springs.
    Gonna have to check with a new coil/lead/cap in the weekend.

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    Is your bike original or a custom job? Looks like a good bike to customiz3. Original airbox, exhaust, carbs?

    Photos?
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    Quote Originally Posted by Elverdane View Post
    Took a look under the top lid. Seems no broken springs.
    Gonna have to check with a new coil/lead/cap in the weekend.
    You won't see a broken coil in a valve spring although you might see misalignment..it will defo be easy to push down with a socket extension don't use a thumb ...

    you said it seems to push back out the carby..? right?

    I think you still have a vacuum leak at the inlet tubes....what colour are the spark plug tips...the porcelain bit - at the tip near electrode.

    When you say you checked the cam timing-- what exactly did you check..? and what clearances did you set the valves to.? ( yeh I can read... I just wanna hear it again.)
    " Imagination is the seed of life..."

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