Evaluate my qt50

Send me pics of your qt50 and I bet I can offer suggestions that will make your life much better.

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Question: Hello! I recently bought a 1981 QT50 from a friend of mine. He had it for a number of years but never really did anything with it. He thought it had low compression because when he first got it, he was only able to get it running after kicking it over for more than an hour. We worked on it last weekend. We installed new .25 over size rings, new fuel lines, new fuel filter, new spark plug, new crankcase oil, and did our best to clean the carb. It is running, but not that great. It won’t idle at all and doesn’t like to start cold without starting fluid. I have ordered a new carb and a new battery, which I hope fill fix some of this. When he got it, it didn’t have an ignition or key. He ordered the ignition that is in the pictures off of EBay with no key. I am curious if that is the correct ignition and whether keys are available? The switch that is wired in only seems to turn the gauge light on and off. It was there when my friend got it. Thank you very much for putting up this site and thank you for any help you can give me. Cheers

Answer: Since you are getting a new carb, I would install that once you receive it. Your problem with the old carb is likely the idle jet being clogged with dirt and crud. If that’s the case, you will have difficulty starting, idling and running. I would also get an air filter. You can get a cone mesh filter on ebay – just search for “28mm air filter”. 29mm will work as well. There’s also a K&N filter that will work. Just use about half the package of oil that comes with it. If you over-oil it, you will have problems. Any of these air filters will work. I like the oval one but it lets in so much air that you will probably need a slightly bigger main jet. I’m working on uploading a video concerning the idle jet in the carb. I’ll provide a link later.

Cleaning the idle jet

As far as the ignition, I would visit a Yamaha dealer. There’s a 4 digit number on your ignition cylinder. Take that number and maybe the dealer can make you a key. That looks like the correct ignition cylinder for the Yamahopper.

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1. My thought is it is leaking gas from right here, where the fuel line connects to the fuel inlet. Replace fuel line with 3/16” inner diameter line and use a zip tie or fuel clamp to secure line to inlet.

2. K&N Air Filter has too much oil on it. It comes with a small package of oil. Do not use entire package. Use maybe 1/2 the package or less. Take filter off and set it upright and let oil drain out of it for several days. Too much oil will restrict air flow and your qt50 will not run as well as it should.

3. I like to tuck this drain line back behind and into the center of the bike. You can use zip ties to secure it down so it doesn’t get caught up in the rear wheel. Just run
it down towards the center stand through the center of the qt50.

4. If you use the oil injection system, it’s not a bad idea to replace this black line with clear line. That way you can just look down at it and know that you are
getting oil to the carb. I use 3/16” inner diameter weed wacker line. You’ll need to take oil pump cover off and detach line from there and at carb. There’s usually a little ring at both ends to secure the line. If you are missing the rings, just use small zip ties to secure line in place. This original oil line hardens over time as well so might as well replace it. The oil injection system is known to pump too much oil into the carb at full throttle and this will result in decreased performance. I usually just switch over to premixed gas/oil and don’t use the oil injection system.

5. See my carburetor adjustment tips for these two screws. Link included in text below.

6. I like to replace this screw with an allen bolt and nylon locking nut. That prevents it from vibrating loose over time.

The yellow staining is the gas going bad as well. It may have gone bad after it leaked out and was exposed to more oxygen. You can check by taking the gas cap off the tank and then taking a little whiff of the gas. If it immediately backs you up then the gas is still good. If you don’t jerk back a little after whiffing some, the gas has lost its punch or you have sniffed too much gas in your life. When winterizing your noped, add fuel stabilizer to the tank and then run the bike for 5-10 minutes to let the stabilizer get into the carb.


I have a 83QT50 that I bought new in 84. It has 3200 miles and used it to go to work while stationed in Guam. Returning in 85 in used it sparingly and attached it to the rear of our motorhome in case of emergency. Since retiring I decided to get it on the road, but . stored inside I put gas treatment in the fuel and kicked it over on occasion. But with a dead battery and ew fuel I was surprised that it started with the choke on but without the choke it died. With the engine running full choke I put the key to run and the engine rpm increased but only on full choke. Did a impression check@ 125ps, using carb cleaner spray checked around the carb and intake manifold. Pulled off the intake and checked the reedvalve and it looked good. What could me this full choke problem? Any help appreciated

Thanks for the question. I had a blue one with the same problem once. I bet you have some good stories about riding that thing in Guam. If they only run on full choke then it’s not getting enough gas. Gas treatment will help but won’t keep gas good forever. You really have to drain the tank and the carb if you put them in storage or don’t run them for a long time. Anyhow, I bet the gas in the carb varnished and is partially blocking your idle jet.

You have several options from easy to more involved:

1. I’ve successfully fixed this problem with Seafoam. Added about an ounce to the tank, start it up, let it run for 5 minutes and then let it sit overnight. Start it up the next morning and see if any better. It will improve over time and won’t be an immediate fix. If you still have old gas in it, drain it out and start with some fresh gas as well.

2. Take the carb off and take it apart and you’ll need to clean it. to get the idle jet clean, I usually just soak it in Chem Dip even though I prefer to use the less potent stuff. If you use Chem Dip follow the directions and don’t soak any non-metal parts in it. I’ve also had success cleaning the idle jet with a small guitar string. Idle jet is 0.375mm in diameter so use a similar size string.