yamaha qt 50 runs actually flies...

hi i'm new. I have a yamaha qt 50 1987. Found it in the garbage, after a few weeks i have gotten it to run well with a new piston and hone job (standard size) my question is this.... The bike requires the rider to start it in the START key position which i have and still can do. It runs at idle smoothly once it is warm and the choke is closed. When i hop on and switch the key to run the bike surges foreward with the throttle all the way closed. Well, i first fiddled with the throttle adjustment slack with the bike running in the RUN position and found it didn't surge foreward any longer. however the throttle response is sluggish and does not respond well when i try to slow it down(pulling the twist grip towards me). Now here's the kicker. When i shut off the bike it refuses to restart in the START position without me readjusting the throttle and the bike surges forward again in the RUN key position,. WHEW! well i thought it may be a carb thing, a cable tightness problem, but the more i think about it i fear it is the dreaded CDI unit. Help! i want to ride this QT without having to always hold the brakes, because with a new piston she pulls my 195 lb butt around like a train. Cheers, greg

Re: yamaha qt 50 runs actually flies...

everything points to a throttle problem.. find it.

Re: yamaha qt 50 runs actually flies...

hi joe do you think it could be related to the choke side of the card or just the throttle slide part? G

Re: yamaha qt 50 runs actually flies...

i would guess something is happening to the slide and the cable_when_ you jump on it.. like a cable is misrouted and when you sit on the bike, it opens the slide.. the throttle (slide, whatever) should be closed but is it? If so, what is causing this surge? And, after you adjust it, all of a sudden throttle response is sluggish.. This says that changes involving the cable or something directly related to it is having a profound effect on this problem.

it's impossible say what the problem is from here.. but i bet that careful observation will eventually make it obvious.

Re: yamaha qt 50 runs actually flies...

Thomas Malcolm /

There's actually a warning sticker on mine that says something to the effect of "The bike may jump if you switch to 'RUN' with the throttle open. Love Yamaha."

This leads me to believe that

A) somebody got hurt like this and Yamaha didn't wanna get sued again and

B) either your throttle cable is sticking somewhere or your carb has funk in it and the throttle slide in the carb is sticking a little.

Try cleaning up the main slide and the hole it's hiding in with spray carb cleaner. And get some cheap solvent-resistant gloves. Gas and carb cleaner can chemical burn your fingers if you get too much of it on you.

You may want to take the slide out to work on it rather than just spraying the carb cleaner down the throat of the carb. I'm not sure if you really want your moped to suck that junk down into the engine, since it breaks down oil.

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