QT50 Problems

Hi all,

My 1981 QT50 runs fine from a cold start. After 10 - 15 minutes, however, it sputters out and the engine dies. After a minute or two of rest, it seems to run fine for another minute or two. Then dies again. Rinse and repeat.

Anyone have any suggestions as to what the problem might be?



Re: QT50 Problems

In an absence of any indications of overheating and if it runs perfectly normally after a couple minutes rest, it sounds like it's just running out of fuel.

Disconnect the fuel line at the carb and check for a steady, thick stream of fuel flow in all petcock positions except OFF.. fuel must enter the carb's bowl faster than it's used up. The minute or two of rest provides extra time to refill the bowl. A confirmation would be that it dies more quickly at high speeds than at low.

There are other odd-ball things that can cause it, like a choke that vibrates closed after a while.. you're playing with the throttle trying to restart and inadvertantly open the choke.. and it runs again all of a sudden.. Can't hurt to give all the cables and stuff a good looking over.

another possibility is a rusty tank which also means a rusty gas tank cap interior. (i've broken one apart and seen it packed solid with rusty flakes) If the tiny vent in the cap is clogged a vacuum can develop in the tank and slow the flow of fuel.

Re: QT50 Problems

I don't think its running out of fuel or rust in the tank - the tank is new and everything has been cleaned out since the tank was replaced, and a filter is on the gas line. There is lots of gas getting to the carb, and no indications of fuel line issues in the first 15 minutes of running.

As well, choke seems to have no issues - very solid and choke line was replaced at some recent point.

In my entirely non-mechanical view, overheating would seem the most logical explanation, but I have no idea what the symptoms of overheating are.

When it runs 'well', it spews smoke quite well out of the exhaust. When dying, it seems like the smoke stops.

Thanks for your help so far.

Re: QT50 Problems

i was having those problems but i completelytook apart the carb. and cleaned it. than i replaced the main jet. Now mine runs great


Re: QT50 Problems

if it spews smoke you're either running very rich fuel:air mix or you're using too much oil in the mixture... or the fuel is polluted. Any of these can cause the plug to foul and engine to die. The smoke indicates that overheating is very unlikely... you've got too much fuel or oil (or both), not too little.

Without some explanation of why this bike is smoking a lot, i can do no better than make wild guesses.


Do whatever it takes to make the engine die. Remove the plug and look at it. Is its tip is wet or dry?

If you do absolutely nothing but wait 2 minutes, how easily does the engine then restart? One attempt or several?

Are you relying on the oil pump or are you pre-mixing fuel? If premixing, how much oil are you adding?

Are there any other problems or weird symptoms like this smoke-spewing thing?

When did this problem start and does it have any relationship to the time you put the new fuel tank on or when you did any other work on the bike?

Some history of this bike and it's past problems may be helpful.. so far there're very few clues to go on.

Re: QT50 Problems

JoeW, thanks for your help - its very much appreciated.

I don't have much history on this bike, other than to know that it has been in and out of a motorcycle repair shop for various fuel line issues, mostly relating to corrosion inside the tank. At that point everything was taken apart, cleaned, and afuel line put on. Issues arose again, and the same thing was redone, this time with the entire tank replaced. So the fuel line should be very very clean at this point.

It was recently taken to a motorcycle mechanic (I have no idea how good he is, but its a pretty simple carb from what I'm told). The previous owner told me he apparently cleaned out the carb and found it full of gunk, and told him to release the far right bottom screw to release the build up crap in there. When I do this, it looks like fuel drains out (I'm assuming its running through the line).

The fuel that was in the tank was pretty old, but likely most of it has been burned - there was very little left when I refilled it a couple of days ago. I saw it running a couple of years ago and there wasn't any smoke then. Its been in a basement since.

I can't pull the plug out right now, but I'll get back when I get home.

Regarding how easily it restarts, I'd say pretty easy. A couple of kicks with the throttle on a bit and it will go.

I'm relying on the oil pump, not premixing.

Thanks again,


Re: QT50 Problems

ok.. well.. i would've completely drained the tank first thing .. Gasoline goes bad fast, faster in hot weather.. The lower boiling-point fractions can evaporate in as little as a month in a vented tank. This changes octane rating and the gasoline is well on it's way to producing varnish and gumming things up.

The recurring problem makes me think this thing could still be a fuel delivery problem of some sort. The bike restarts easily.. overheating would more likely make it very difficult to restart. So would plug fouling. It's possible that the oil pump is acting up but i'd put that down near the bottom of the list for now.

I'd spend some time on the entire fuel pathway, examining, cleaning and/or repairing or replacing all screens and filters and hoses.

I'm not familiar with that carb so can't be specific but it would also be removed and examined .. carefully reassembled and installed with no air or fuel or vacuum leaks anywhere.. QT50 Shop Manuals are here break>

At worst you'll end up with a pristine fuel system and at best you'll accidentally cure the whole problem.

I gather this bike has never run right for you .. if so take a look at Freds Guide

It might suggest other things to try.

Re: QT50 Problems

Well, I just had a very annoying ride home. Lots of running for a minute, dying for 5, running for a minute, dying for 5. I tried various things, leaving the fuel cap slightly ajar (read elsewhere it could be this), not running full throttle, playing with the idle and air screws. The only thing that seemed to keep it working near the end was completely opening up the choke - basically making the mix richer. Who knows whether this was a random thing or not.

Anyways, I'll do everything you suggested as best as I can tonight - I'm not the most mechanically inclined - I'm great at taking things apart. Its the putting back together stuff that I have issues with :).

Thanks for all your help, its appreciated.


Re: QT50 Problems


Download the manual as per previous threads, and find the exploded view of the carb. Pull the screw on top of the carb between the two cables and remove the carb. Leave the guts (plunger valves) hanging from the bike, I highly doubt that they're the problem. You can spray them with carb cleaner. Take the carb apart, EVERYTHING! Be careful of the little float valve, it's SMALL as is the pin that holds it in place. The float will also become brittle like an egg so don't crunch it. Take everything apart, but don't bother with the plunger valves aside from a good carb cleaner douche. I can't stress the carb cleaner enough, gotta have it! Get an abrasive dish scrubber (scotchbrite) and clean the inside of the bowl and really let it soak in carb cleaner to get the crap out. Pull all the valves, pull the needle on the bottom of the bowl, yank it all and blast it with cleaner and let it soak. Poke the bristle of a wire brush through the main jet. If you have an air compressor(better) or some canned air, blow the entire thing out. Also blow out all the passages and whatever you can. When you put it back together, set the air bleed and idle screws EXACTLY as the manual tells you.

In the grand scheme of things, this carb is retardedly simple... in fact the entire bike is. There's not much to go wrong. Mine behaved pretty much the same way when I first started riding it. I also had ignition problems too, which make it a lot worse. I cleaned the living hell out of the carb and put a fuel filter in, and it starts EVERY time on the very first kick, and I NEVER use the choke. It's a 79 too, and my fat ass puts 10 miles or more on it every single day. Fix your carb and it will treat you well.

Good luck and let us know how you make out.

Andy Baker

Re: QT50 Problems

Okay. Tonight I changed the oil, replaced the spark plug, cleaned out the heat synk type thing that the spark plug screws into (cylinder head?), cleaned and reoiled the air filter, drained the carb, checked the fuel lines, drained them, added some fuel line cleaner to the gas.

Started first kick. Ran it for about 15 minutes and had no problems at all (had to get back to watch this Spurs Pistons game). Things seem to be running fine now, but I worry that if it was truly gunk in the carb, I'm going to deal with some of this again soon. What are the odds that the stuff I did had any affect on the actual problem?

I've got to be honest - I'm a tad scared of taking this carb apart. I took a look at the pictures and there are a lot of parts my big stupid fingers are going to lose and or screw up when putting it back together. How easy is this for someone who has never done anything like this before? And I mean never. My mechanical skills have been limited to bicycle maintenance.

Again guys, you're a great help - thank you.


Re: QT50 Problems

im not gonna tell you that a carb's disassembly and cleaning can be treated lightly, but it's not like working on a camera either.. Anyone who can work on bicycles can work on one of these simplest of carburetors.

lay out a large, clean work space, label parts and/or take notes as things are disconnected and removed. Pause and memorize how things are .. Although a carb can only go together in one way (except for a few minor things) the mental pictures can come in handy.

Cleaning a carb is a learned skill like anything else.. You got cans of solvent and brushes and rags and a battle plan. And even experienced people often screw up and have to go back, which indicates that care and patience is always required.

Hey.. you're the one that wanted a moped.. this kinda work is part of the package. Dive in and learn like the rest of us had to do and are still doing.

Re: QT50 Problems

Glad to hear you made some useful progress. There is probably an even chance that you will or will not have further issues down the road.

Yes, even small carbs have a lot of parts, and the blowup diagrams confirm this. But in practical terms, you normally only need to deal with several of the parts - bowl, main jet, atomizer, pilot jet (if equipped), float and needle. The rest of the parts can stay where they are.

The last several moped carbs I worked with came around nicely with just a good cleaning of the bowl & main jet/atomizer. And thing is, you don't actually have to get them perfect on the bench. As long as you get fuel flowing in all of the circuits, the more gas you run through it, the cleaner the parts get.

HIgh-quality modern fuels all have detergent additives ("with Injector Guard" etc) and will continue to clean as you drive. A couple ounces of SeaFoam in a fresh, full tank of fuel will help speed the process along.

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