starting a Yamaha QT50

Thomas Malcolm /

I have just acquired a 1980 Yamaha QT50 that has apparently been sitting unused for some time. I was told that the last time it was started, it ran, but there's no telling how long ago that was.

The gas tank has been sitting empty and shows minor corrosion inside, the petcock feels as though its full of grit, and I can rest easy knowing the motor oil is spoiled (its pretty dark and stinks). Probably needs new tranny fluid, as well, and I don't know how to change either of these.

Anyway, I was wondering what suggestions anybody might have about the "safest" way to attempt starting this thing would be without ruining the the engine, catching myself, my house, my moped on fire, etc.

Re: starting a Yamaha QT50

Replace the oils in both tranny and oil injection tank with new.

Flush gas thru system and probably better remove and clean the carb and all passages it has,iincluding the main jet and emulsion tube or diffuser. New spark plug and bleed injector pump lines until no air bubbles left in new 2-stroke injector oil. Clean and check operation of ignition points. Check for spark before cranking too much. It's also a good idea to squirt a lil' 2-stroke oil into the cyl and turn the engine over to make sure rings and piston are lubricated.

Go to Fred's Guide on this site and readi t and use it. don-ohio

Re: starting a Yamaha QT50

Thomas Malcolm /

I got spark on the thing once I got a new spark plug and shorted the ignition circuit (the previous owner lost the key... :\ ) That means that this little thing is probably runnable.

Now I can get going with the rest of it.

Thanks Don!

Re: starting a Yamaha QT50

Busy here at work. This is a weird little bike that i happen to know pretty well. Don-o's the real expert but I'll read the thread from home and see if I can offer anything.

I have two Qt's and had an express (QT-clone)

Re: starting a Yamaha QT50

You're welcome, Thomas! How's Murfreesboro treating you? My daughter lives in Columbia,TN. don-ohio

Re: starting a Yamaha QT50

I was hoping you'd see Tom's post,Wayne.......I know you have 2 of them,right? I bow to your `hands on' knowledge, Wayne. don-ohio

Re: starting a Yamaha QT50

These things are the mutants of the moped world--- totally bizarre in the combinations of features and parts, hard to modify but so far very easy to work on.

I'm the kind of guy who has a natural feel for electronics-- and the QT 50 electrical scares the heck out of me-- it is a horrid little snake pit. I cannot understand how or why they would stuff so many relays and wires and lights under that control box.

"shorted the ignition circuit....?" uh oh. no good. How's the battery? not so bad on the QT but the jappeds can blow out headlamps etc with no battery--- it acts as an um... rectifier or something. chances are good that the battery is total junk, no sense in trying to salvage. Get a replacement.

These mopeds are 'power assist' starting. You have a much better chance of them starting with a good battery. You see how you have the "start" switch and then the "run" switch?

Mopeds tend to have rusty tanks and old ones, debris. Most people simply scrub the surface rust, oil the tank and keep a spare filter on hand. Running w/rust isn't terrible if you filter everything.

Don't use starting fluid, but put a bit of pre-mix into the plug hole. See if it fires. Make sure your ignition (is it cdi? hmmm) is fried for sure before doing other stuff.

This was just a quick FYI I banged out... a moped with a driveshaft?!? leave it to the Japanese.

Also, from what I've read other posters often mention the can tell their qt could "breathe" a little better. Make sure it's getting air and thhe exhaust isn't clogged. i thought mine was good until I put a mapp torch to it. yuck!

good luck, I'll keep adding what I can-- I love those QT's. There is a shop manual scanned as well-- that's awesome. i'm sure I could e-mail you the link. I'm the one who scanned the thing but I never know where it lives at any given time... LOL.

Re: starting a Yamaha QT50

Michael Miehle /

What do you do with the mapp torch for cleaning the exhaust?

Re: starting a Yamaha QT50

hey, i have an 84 qt50, and love it.... if you have anyquestions, feel free to e mail me at

As for the exhaust, take a torch, and torch the exhaust until it is red hot. Be careful not to get it too hot. Then when it is cooled, band all of the crud out of it.

You can use a camp fire, or a gas grill as well


Re: starting a Yamaha QT50

yeah, and you can even put the nozzle pointing inside the muffler, not all the way, just the flame. That's when it really started smoking.. the outside was already super hot and when I shot flame into the tailpipe the crud really started blowing through.

if you do actually use a mapp torch you probably want to either be careful (heat evenly) or use a propane tank for this job. The mapp was so hot a few times I had to make sure I was spreading the heat-- I think a careless user could burn right through a muffler with a mapp on high.

Re: starting a Yamaha QT50

swap your fluid before you do much--- there's instructions here I can link--- that tranny fluid + whatever's in the injector has to go, new lines wouldn't be a bad idea. if you get it running don't ride on gunky tranny fluid


Re: starting a Yamaha QT50

Thomas Malcolm /

Oh, the 'Boro's great. Gotta love it.

I got everything cleaned out (I think) changed the tranny oil, 2 stroke oil, cleaned out the fuel system and carburator. I have honestly never seen a fuel system with so much grit and corrosion in it.

I've gotten the beast to start and run for a few seconds (complete with white smoke), but only if I dump fuel/oil mix directly into the intake. I'm not going to fool with it again until I can get a new petcock tho. That sucker leaks like its going out of style. Well, and I get some gas-proof gloves. The gas is eating up my fingers. ;)

Oh, and I've taken all the running lights off because the battery's not just dead, its dried up and won't even conduct anymore. I pretty much had to remove the headlight to connect up the ignition circuit anyway.

It just seems to me that the carb is delivering an inadequate amount of gas...

Re: starting a Yamaha QT50

my advice is to replace the fuel line that goes from the petcock, to the carb

since you cleaned the carb, that should be fine- but all that crap can stay in the fuel line

install an inline fuel filter.... you can buy one for $2 at ur local auto shop


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