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Thread: suzuki GP125 ignition issues

  1. #1
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    23rd October 2016 - 21:07
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    suzuki GP125 ignition issues

    morning chaps, i know this is a real open ended question that is super hard to diagnose via the internet, but thought i would try. actually thought i should post on the Bucket thread as there seems to be a few GPs used for racing.
    Anyway my issue is i have purchased an ex riding school GP125, removed the throttle governor that was screwed into the throttle tube on the bars to limit it to half throttle.

    the bikes seems to go fine in 1st - 2nd gear but occasionally in these gears or more commonly in 4-5 gear it feels like the spark breaks down between 6000-7000rpm, if you go to WOT it just does nothing and dies completley until you roll right off.

    My first thought was air filter, so i replaced with a new one, then carb, so i completely disassembled the carb and had it ultrasonic cleaned, this made no difference. i have tried a new plug (currently running a B6 ngk plug as opposed to the B8 heatrange that is factory (but i have tried both). this all made no difference. i cant quite figure out if its fuel or spark, it sure sounds more like spark as it sort of jolts and misses, not your usual rich stutter or lean bog. i put a new coil / lead / cap on that seemed better but still bad. the pipe has been cleaned out and decarbonised, the exhaust port looks nice and clean, it has nice compression and starts first kick when cold.

    the problem doesnt become evident until 6000rpm, if you can very quickly "get though" and past 7000rpm it runs fine and pulls like a schoolboy all the way to 10,000rpm, but if you get stuck in that 6-7 range it just wont pull, and opening the throttle wide open it dies completely.

    any help would be greatly appreciated, im not new to bikes but not an electrical expert, and this has be rather frustrated.

    https://i.imgur.com/6z2KlU6.jpg

  2. #2
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    The pipe. The carb. The points. The coil. The plug. Seized ring.

    Couple places to look.

  3. #3
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    13th June 2010 - 17:47
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    You say pipe and ex port are clean. Given it's history of low speed work, those are where i'd have looked first.
    Typically carbon builds up on the edges of where the port meets the bore.

    Head off - pipe off - torch down the bore and look at the port edges.
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  4. #4
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    25th March 2004 - 17:22
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    Quote Originally Posted by Blizzo View Post
    . .

    the bikes seems to go fine in 1st - 2nd gear but occasionally in these gears or more commonly in 4-5 gear it feels like the spark breaks down between 6000-7000rpm, if you go to WOT it just does nothing and dies completley until you roll right off.

    . . . . the pipe has been cleaned out and decarbonised, the exhaust port looks nice and clean, it has nice compression and starts first kick when cold.

    the problem doesnt become evident until 6000rpm, if you can very quickly "get though" and past 7000rpm it runs fine and pulls like a schoolboy all the way to 10,000rpm, but if you get stuck in that 6-7 range it just wont pull, and opening the throttle wide open it dies completely.
    . . ..

    https://i.imgur.com/6z2KlU6.jpg
    Again first thing I thought was the pipe blocked up. I'd almost suggest running it without muffler section but that will make a hellish racket if you live near people. I had an RD50 that was so choked up it just would not rev even in neutral. . Took long time to burn oil out with little propane torch.

    Don't leave that hot plug in.

    Ok my next choice is the carb. Take the needle jet (emulsion tube) out and look at with some sort of magnifying glass. If it is worn oval, like just a bit it will never run right. A bike ridden around car parks might have been there. Check mainjet size on internet. My forgettery says 145, nah don't rely on that but maybe someone has been meddling.

    The ignitions are sometimes troublesome but I wouldn't expect it to run ok sometimes revving out.

    TZ350 in bucket area will likely have spare cdi and carbs to try.
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  5. #5
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    Probably not ignition

    As an ex riding school instructor where we had a pair of Kawasaki KC100's the coking in the exhaust system was HUGE. Not so much the exhaust port or the header pipe but from under the foot pegs all the way to the pipe exit would fill with carbon and black sludge. The only way that I found to sort the problem was to take the exhaust pipe off the header pipe, hang it vertically for a while [gets the liquid crap out] and then seriously heat the pipe [with all removable baffles removed] to a temperature that stuffs the chrome completely and burns the shit inside out. I have heard of someone using a gas torch up the inside to get things really hot and then feeding oxygen up the now hot inside to the point that the pipe glowed bright red. I was never game enough to try this.

    Agree completely that the hot plug is a no no. One of the KC's was taken for a quick [?] thrash down 1km of motorway with the hot plug in it. Melted plug and hole in the piston.

    Also agree on excessive localised wear in the needle/emulsion tube at around where the old full throttle was with the throttle blocker in.

    Enjoy the fun of the decoke!

  6. #6
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    That brings back memories, LOL. I burned out an exhaust system as described - get it hot and feed in oxy to keep it going. Worked very well indeed.

    But the visit from the local cricket team who were playing about 1/4 mile away - downwind - didn't go so well.

    They objected - vigorously - to playing in a rain of hot black soot.
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  7. #7
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    Quote Originally Posted by Grumph View Post
    That brings back memories, LOL. I burned out an exhaust system as described - get it hot and feed in oxy to keep it going. Worked very well indeed.

    But the visit from the local cricket team who were playing about 1/4 mile away - downwind - didn't go so well.

    They objected - vigorously - to playing in a rain of hot black soot.
    I did that with a TF125 pipe as an apprentice & accidentaly turned off the oxy instead of the acetylene. The hearing in my left ear returned around nine months later.

  8. #8
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    Quote Originally Posted by Grumph View Post
    That brings back memories, LOL. I burned out an exhaust system as described - get it hot and feed in oxy to keep it going. Worked very well indeed.

    But the visit from the local cricket team who were playing about 1/4 mile away - downwind - didn't go so well.

    They objected - vigorously - to playing in a rain of hot black soot.
    Hahaha, in white? Hilarious!
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  9. #9
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    25th March 2004 - 17:22
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    Quote Originally Posted by MarkW View Post
    As an ex riding school instructor where we had a pair of Kawasaki KC100's the coking in the exhaust system was HUGE. Not so much the exhaust port or the header pipe but from under the foot pegs all the way to the pipe exit would fill with carbon and black sludge. The only way that I found to sort the problem was to take the exhaust pipe off the header pipe, hang it vertically for a while [gets the liquid crap out] and then seriously heat the pipe [with all removable baffles removed] to a temperature that stuffs the chrome completely and burns the shit inside out. I have heard of someone using a gas torch up the inside to get things really hot and then feeding oxygen up the now hot inside to the point that the pipe glowed bright red. I was never game enough to try this.

    Agree completely that the hot plug is a no no. One of the KC's was taken for a quick [?] thrash down 1km of motorway with the hot plug in it. Melted plug and hole in the piston.

    Also agree on excessive localised wear in the needle/emulsion tube at around where the old full throttle was with the throttle blocker in.

    Enjoy the fun of the decoke!
    I agree with the person agreeing with me. And if he was agreeable he might agree again.
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  10. #10
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    23rd October 2016 - 21:07
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    this is all bloody good info boys, cheers. i did a resistance test of the stator and values are all as per the service manual so shouldnt be that. i picked up a couple of B8HS plugs to use, they only had -10 ones but i just regapped them to factory spec (0.7mm).

    primary and secondary coils both test good.

    cant be points as this bike is a later model CDI version.

    The pipe wasnt (Cakey) and more full of sludge, i cleaned it by spraying a whole can of oven cleaner in there and then steam cleaning it out, is there a such thing as an acid bath that wont eat the chrome off?

    i didnt think about emulsion tube / needle wear at where the old full throttle is, this would make sense as its about that spot it decides to not love life. ive pulled the carb apart again and the E tube out, all seems good. i might try hit the old owner up as he had a bunch of spare parts, might be able to get another carb to try.

    Im pretty keen to not burn all the chrome off the pipe haha, its pretty tidy for the most part.
    Cheers
    Blake

  11. #11
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    Caustic soda is the traditional bath solvent. Tergosol was also good but was always expensive.

    Remember - if it'll dissolve this crud, it'll take skin off too.
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  12. #12
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    23rd October 2016 - 21:07
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    well exhaust spent 2 days filled with a 4 litre water 1.5kg caustic soda mix and is now clean as a whistle, i put a correct plug and gapped correctly in it and the bike still does it. pulled carb apart again and pulled the E-tube out this time and cleaned that out although it was clean, still no difference. i reset the float height to factory specs although it was only 1mm out. i have tested the resistances on the stator and primary and secondary coil of the ignition coil and everything is within spec. i inspected the slide and needle and inner carb body for wear at the old throttle governed position and it all looks near new.

    father in law was up for the weekend and he jumped on it and reckons it felt like a fuel starvation issue, that is what it feels like but then why would it go well if you can get "past" the point where there is the bogging issue and it pulls so well, but if you get it in the bogging range you cant accel more to get past the point, you need to rev very quickly and high to keep it past the boggy point and if you can get it up in the revs in top gear it will hold it and do 100-110 kmh.

    my head is getting severely wrecked over this thing. i pulled the fuel hose off the carb and there is certainly no restriction there, shes coming out at full force. i have asked the guy in wellington i got the bike off (as he had like 12x GP125s) if he can send me a spare carb just to try and eliminate that as a problem, he did sell me the bike as "running well" when in fact it never was it only "idled well" and i was silly enough only to ride to the end of his driveway and back when i test rode as i had my 2 year old in the car.

    anyone else got any more logical things i can try?

  13. #13
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    Probably find he hadn't ridden further either.

    So further experiment; is it affected by throttle position?
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  14. #14
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    Quote Originally Posted by F5 Dave View Post
    Probably find he hadn't ridden further either.

    So further experiment; is it affected by throttle position?

    at first i thought it was revs as the problem would come on around 6000 rpm but i just went on a good test ride and no it does seem throttle position driven, if i used a gentle down hill and just used about 1/4 throttle i could get it to climb right up in the revs in most gears, first i can generally nang right though but any other it gets the issue when more than 1/4 throttle is applied and it gets to around that 6000rpm mark, but if under 1/4 throttle is used i can get "past it"

    here is a shitty night time video of holding my phone in one had, i slowly got the bike up in the revs using 1/4 throttle, you can hear it wake up on the pipe when it gets right up in the revs and will take WOT when its right up in the revs, but i back off here then give it more throttle and it goes the cutting out bog thing. link -
    https://www.youtube.com/watch?v=-56Ix4QHrdU

    one suspicion i had was maybe someone jet drilled out the main jet so i put one size bigger in some mikuni jets i had lying around, no difference though.

  15. #15
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    Waaaaay too rich on the pilot circuit?

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