OzVMX Forum
Marque Remarks => Yamaha => Topic started by: kristan on April 07, 2009, 06:31:16 pm
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For god sake will somone put me out of my missery!
I have spent the best part of a day and a half trying to stop my gt50 from fouling its plug after 10 min of riding!
things i have tried so far:
new plug
hoter plug
decreasing oil pump
proper injector oil
increasing oil pump
fidling with air screw
fidling with the needle
diferent ignition
running with no airbox to lean the mixture off
only thing i have not been able to try is changing the main jet due to un helpfull bike shops. i didnt think this would be the cure as if you thrash the nuts off it you can keep it running for a fair while.
the plug colour is soot black when it comes out so i know it is running rich some how.
does any body have the standard jetting specs for GT50 (not GT80)?
if any body has a clue or had a similar problem please feel free!
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Hi ,
My first thought after reading what you had checked was to check float level.
or it could be weak ignition system, coil etc
cheers
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Kristan,
Is the plug gap carboned up or does it just stop sparking?
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the whole end of the plug is carboned up, which stops it sparking
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Std main jet size for GT50 is a #84 P/N 288-14343-42, pilot jet is a #34 P/N 367-14342-17 info is straight out of the parts manual for 1973 GT50/GT80
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How do thy compare to the gt80?
got a sneeky susspition it has a gt 80 carb on it!
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GT80 runs a #90 main jet and #38 pilot jet as std
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what air filter is it running Kristan if its in the box that is usally hidden behind your air filter it may be dodgy try a uni filter that screw straight to carb but never had a problem with mine not hard to get ignition for one of them the 80 should be the same how is it going did you find a front fender plus what fuel are you putting in and does it have an inline filter the bottom of the tank you can bet is full of crap when you say it runs for a while flat out does it bog down low as the crap in the tank always blocks gets are the plugs crook do the still work get a good spark through them i would check that fuel tank i even had the same problem on a 74 mx100 inline fuel filter is the fix or clean the tank right out sorry i asked some of the same questions my guess from experience is fuel or air never heard anything like that before even if your float is wrong they will go and shouldnt foul 1 check that tank hydrochloric acid will rip the rust out of it and then obviously put a some fuel through that before you reindtall it dont wnat any hydrochloric aci floating around if you say it runs i reckon its the tank tip it upside down get a container and tip fuel in see what comes out dirty brown rust fuel mix most likely thanks hope this helps because even cleaning the tank will help it go better let me know if its ignition or something silly like that thanks Michael. :)
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drop needle so clip is on the top notch, go one step hotter with plug, lean off mixture screw.
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Have fidled around with the carb to try and tweak it in but no luck. Tank is clean as a whistle. the jetting however could be the problem
My carbs do have the 90 and 38, but it seems to run fine untill it carbons up that is, IE no bogging or lack of fuel.
do you know if the bike shops will still stock these types of jets?
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The nozzle main is different for GT50 P/N288-14341-03 (the nozzle is the part that the main jet screws into) all other parts for the carb share the same part numbers
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The way I see it is, if it was running too rich the plug would be wet and not carbon fouled. To me it sounds like too much oil in the fuel creating excessive cabon deposits fouling the plug.
Or if its feathering the plug (bridging the plug gap only with carbon) it has a air leak, seals, manifold or gaskets.
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stupid question but hows your head and basegasket no air leaks any where?
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The bas e gasket and head gasket are good. had another fidle with it today. tried a diferent ignition and reed block but still no joy.
seriosly! this bike carbons up a plug in minutes!!!! i wouldnt be too bothered if it was once in a while. surly if the jetting was this far out the bike wouldn't run properly?
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Can you disconect the auto lube and mix up some pre mix and see what that does, I still think its too much oil in the fuel mix.
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mabey the carbon is cumming from other places , perhaps it needs a decoke , (can bits come off excessive carbon build up in the port or exhaust header ? any one have a thought ?
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using much gear oil?? could be crank seals , how much smoke is it blowing oh and another thing just check your coil lead is in the coil , give it a light tug and make sure its secure in the coil. cheers BONEHEAD.
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thanks for all the input guys, think i might be heading in the right direction now. after scouring my parts bins i came accross a spare carb with slightly diferent jets in. For the moment i have swaped the pilot jet which seems to be working, i also swaped out the worn slide and played around with the needle. Seems to be ok at the mo, still a little dark when putting round the garden, but good when getting it in to the mid and top end.
Had a convesation with the mikuni oz fellas and managed to come to some conclusions based on the fact that the further i turned the air/fuel screw out the higher the revs got. I was told this would indicate a too big pilot, the more i thought about it the more it made sense being that the plug indicated a too rich mixture. I dont know about you but a normal setting for the screw would be around 1 and a half turns to start then fine tune when engine is hot. Normally you would not be able to tweek more than half a turn in either direction before the engine starts to bog then you set it in between. whell you could wind it out a long way and the thing kept going!
You learn something new every day i guess?
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gt's are a strong little bike once the engine is done they dont stop great first bikepretty much bulletproof once you have it running right