GN125 first build, fu
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GN125 first build, fu

This is a discussion on GN125 first build, fu within the Technical forums, part of the Caferacer.net Forums category; Bought a 1997 Suzuki GN125 for my girlfriend to learn on. Decided it was too embarrassing to ride alongside, so I've used it to do ...

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Thread: GN125 first build, fu

  1. #1
    Junior Member TABA's Avatar
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    GN125 first build, stalls after hard riding

    Bought a 1997 Suzuki GN125 for my girlfriend to learn on. Decided it was too embarrassing to ride alongside, so I've used it to do a bit of learning myself. It has become my first build. I mostly stuck to simple mods, decreased weight, custom paint (all ceramic actually), and a bit of engine work.

    I'm looking forward to dropping the front a bit more, using some clipons and maybe finding a smaller front wheel or larger rear to improve the stance and balance. Hope someone can appreciate what I accomplished so far...

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    My issue is this, ONLY after hard riding or pushing top speed (not too thrilling mind you), if I close the throttle and pull the clutch, like coming up to a stop light, the engine with stall. It'll fire up again instantly.

    However, if I keep the clutch engaged and let the engine rev down more slowly, or give a couple blips of the throttle before stopping, that will keep the stall from happening. During light riding, 45mph stop and go stuff, the engine runs like a top.

    Mods include:
    Drilled exhaust, four smallish holes (hey, sounds so much better)
    Pod filter
    Jet kit to accomodate the above
    Carb is a Chinese knockoff, quality appears fine, running great in MOST regards

    Notes:
    Valves are spot on with new lifters
    Timing chain is new and tensioner set
    Clutch is new and transmission shifts perfect, slight drag in clutch until warmed up well
    Starts instantly with choke and idles perfect after short warmup, set to idle at 1000rpm
    Sparkplug is tan at full, half and closed/idle throttle (cut engine, pull over, check plug method)
    Float level is good but tested allowing more fuel in anyway without change (I thought surely hard riding was drawing the fuel down too low for idle)
    Rode with air filter partially blocked and ended up with no HP whatsoever
    Also blocked the extra exhaust holes with no change
    Rode with higher idle with no change
    Overall power and pull seam good, it his some pep

    As a small CC engine that runs great otherwise, it just seems a bit fickle with the mods and simply doesn't react well to going from hard riding, open throttle to closed so abruptly.
    I can't stress enough how well it starts, idles and runs under light riding conditions.

    I could live with this bug, but if anyone has a thought on the problem, I'd love to hear it.
    And yes, I know, my first problem is... I started with a GN125.
    Thanks for any assistance in advance.

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    Last edited by TABA; 06-26-2015 at 10:59 AM.

  2. #2
    Banned Witworth's Avatar
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    Forget the "stance and balance" garbage, you have been fed a line of pure BS from someawheres. Do you want the motorcycle to be safe and stable for the SO, or for it to look good on instagram?

    A smaller front, and a bigger rear wheel: no, nup, nyet, never. A total waste of cash, and time, and it would make the bike unstable, weird in steering response and potentially dangerous. Forget bigger tires than stock, too. You want fatter tires?, get a fatter bike.

    I say all your tuning issues are due to removing the airbox. Put it back on and go back to the OEM carb and carb settings. Problem solved.

    Forget clipons on such a slow bike. Furthermore, it restricts the scope of good vision, and makes it unsafe for noobs and is painful and fatiguing. Clipons used with stock pegs are dysfunctional and lame.

    Slam the forks?, no thanks, please.

    Danger, is my business."

  3. #3
    Senior Member brad black's Avatar
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    from memory, i think idle mixture too rich is a point to check.

    will your girlfriend find it comfy to ride with the mods you want to make?

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  5. #4
    Junior Member TABA's Avatar
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    Quote Originally Posted by brad black View Post
    from memory, i think idle mixture too rich is a point to check.

    will your girlfriend find it comfy to ride with the mods you want to make?
    My idle mix appears perfect. I can turn my idle screw in 1/4 turn and start to lose my smooth idle, or turn it out 1/4 or so and and idle speed begins to drop. I'm right in the middle, and my plug looks tan after running idle for extended periods of time.

    Even though I have a good mix, perhaps my pilot jet is not a good size, not allowing the fuel to flow easy enough or letting it flow too easy... despite obtaining a good mix with the idle mixture screw. Guess I'll try another pilot jet and see what happens.
    Or, I'm wondering if some other circuit in the carb is not doing it's thing as a result of the decrease in vacuum, resulting from the enhanced airflow.

    Thanks for getting my mind back focused on the idle mix!

    As for the girlfriend's comfort, the bike is setup as is for her comfort. I won't be making it any "sporty" changes until she grows out of this little bike, which is fast approaching. At that point I'll nap a couple pics, call it finished, and sell it.

  6. #5
    Junior Member TABA's Avatar
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    Quote Originally Posted by Witworth View Post
    Forget the "stance and balance" garbage, you have been fed a line of pure BS from someawheres. Do you want the motorcycle to be safe and stable for the SO, or for it to look good on instagram?

    A smaller front, and a bigger rear wheel: no, nup, nyet, never. A total waste of cash, and time, and it would make the bike unstable, weird in steering response and potentially dangerous. Forget bigger tires than stock, too. You want fatter tires?, get a fatter bike.

    I say all your tuning issues are due to removing the airbox. Put it back on and go back to the OEM carb and carb settings. Problem solved.

    Forget clipons on such a slow bike. Furthermore, it restricts the scope of good vision, and makes it unsafe for noobs and is painful and fatiguing. Clipons used with stock pegs are dysfunctional and lame.

    Slam the forks?, no thanks, please.

    Danger, is my business."
    I agree with the clipons and stock pegs comment. Would put the rider in a folded up position, especially since this bike is already very small. I'd have to move the pegs... or just not bother.
    Regarding wheel size, I'd never go smaller on front AND bigger on rear. Only one or the other. Smaller front? No different from any sport bike in the world with decreased trail resulting in agility. The engine is nearly perfect, around town riding is great. No one would ever even know there's a problem unless they really get on it hard, so I'm pretty certain there's a solution. Just need advice from someone who's familiar with tuning for increased HP versus keeping things stock.

  7. #6
    Senior Member TrialsRider's Avatar
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    Re: your stalling problem.
    Your carburetor is vacuum controlled. To me, that sounds like the source of your problem.
    It might even be related to the pod filter modification.



    btw: The bottom end of your engine needs to be vented and that is normally done through rubber tubes that vent into the air filter box. Where does your engine vent to now?

  8. #7
    Junior Member TABA's Avatar
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    Quote Originally Posted by TrialsRider View Post
    Re: your stalling problem.
    Your carburetor is vacuum controlled. To me, that sounds like the source of your problem.
    It might even be related to the pod filter modification.



    btw: The bottom end of your engine needs to be vented and that is normally done through rubber tubes that vent into the air filter box. Where does your engine vent to now?
    Maybe the slide isn't dropping down quick enough when the throttle is closed quickly, causing a momentary rich issue. If the pilot jet test doesn't get me anywhere, I'll take a look at the the circuit/s that allow air back in above the slide diaphragm. I should let it stall and check the plug right then, see if it it's wet.

    The engine is venting via a filter attached right at the vent outlet. You can see it in the top photo, above the 3 slots of the front sprocket cover. It does vent, but the gasses are not being pulled/sucked to the intake. Not more EGR on this bike.
    Last edited by TABA; 06-27-2015 at 06:23 PM.

  9. #8
    Banned Witworth's Avatar
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    Quote: "Smaller front? No different from any sport bike in the world with decreased trail resulting in agility. ".

    No not really. You are dealing with a little tiddler commuter bike with flexy forks, and a flexy frame. You won't gain any more "agility" with a smaller front tire, just instability. It's not a 600cc sportbike, and never will be.

    You can convert a VW Beetle's suspension and steering geometry to NASCAR or F1 spec, but it's still a VW Beetle. Capisce?

    Danger, is my business."

  10. #9
    Senior Member TrialsRider's Avatar
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    Quote Originally Posted by TABA View Post
    Maybe the slide isn't dropping down quick enough when the throttle is closed quickly, causing a momentary rich issue...

    I was thinking lean, not rich. I don't think that will show on a plug that quickly either way.

    Vacuum carburetors operate by leaking some vacuum pressure off their own intake. This effectively means they operate by stealing away some of their own performance. Vacuum carbs are the epitome of carburetor complexity and were fitted to motorcycles that would have otherwise failed the pollution control requirements of the time. Compared to fuel injection or earlier, more simple carburetor design performance, CV carbs suck.

    Diesel engines are fitted with mechanical vacuum pumps, because they need all the intake vacuum they can muster just to operate. Same concept applies to gas engines, but it's too cheap and easy for manufacturers to simply steal some vacuum off their own intake. The CV carburetor is a counter-perfomance design.

    Considering yours is already giving you trouble and you want more horsepower, this would be a great place to start imho.


    … your bike would be far more practical to ride in wet weather if it was fitted with an inner fender and a chain guard.
    … and your unprotected pod filter has Less filter area then the original filter, not more.
    Last edited by TrialsRider; 06-28-2015 at 07:54 AM.

  11. #10
    Banned Witworth's Avatar
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    All carbs suck, thats what carbs do. CV carbs never hindered the performance of CBX 1000's, Slingshot GSXR's or V-max's any. They were the horsepower, top speed and benchmarks of their eras.

    Danger, is my business."

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