Yamaha QT50 Carb Question - No idle & will only run when choked

I just got my first moped recently, a 1980 QT50 in pretty good shape. She has been running well for the past few months until recently. The other day, I stalled at a stoplight and immediately pulled over and noticed gas freely flowing out of the small hose at the bottom of the carburetor bowl. I shut off the fuel petcock to stop the flow and walked her home.

After some searching online (I'm new to this stuff) I concluded that it would be a good idea to just pop the carb off and give it a good clean. After cleaning, I put the carb back on and the gas leaking appeared to be fixed. WHen I went to start her up, though, I couldn't get her going unless I had the choke on all the way and then it would actually be revving instead of idling. If I shut off the choke, she would die instantly and wouldn't start back up unless choked.

I tried just about every adjustment I could with the idle adjuster screw, cable adjuster for the throttle valve, and different settings on the jet needle inside the throttle valve, but nothing changed and she would only start choked and never idle.

Finally, I decided just for the hell of it (and also because I don't really know exactly what I'm doing) to take the jet needle (#18 in the attached diagram) completely out and try starting her. And she started right up and idled beautifully. I rode her around for a couple of minutes and she seemed to run great.

Can you guys help me figure out what is happening and how I can get her running with the jet needle still in? I'm pretty baffled and not sure if driving around without the needle will hurt anything.

Attached is the diagram of the carb out of the manual if you aren't familiar with this particular carb. I used it as a guide to make sure everything was pit back together correctly.

Thanks guys!

carb.jpg

Re: Yamaha QT50 Carb Question - No idle & will only run when choked

Jimbo Grand Daddy /

You have an air leak. Between the carb and intake, or the intake and engine.

Re: Yamaha QT50 Carb Question - No idle & will only run when choked

Would that explain why removing the needle makes it run and idle normally?

I'll inspect the connection between the carb and intake more closely tomorrow and see if I can find a leak. Do you have any tips or tricks to identify an air leak if there are no obvious signs?

Re: Yamaha QT50 Carb Question - No idle & will only run when choked

Air leak for sure

Re: Yamaha QT50 Carb Question - No idle & will only run when choked

I just went out and took the carb off again and inspected the connection with the intake and everything appears nice and clean and seems to be sealing well. I've attached a few pictures, one of the intake and one of the carb.

Can anyone see anything that might be causing a leak here? I don't think there should be any issue between the intake and the engine because I haven't gone anywhere near that and she was running like a charm a few days ago. If this connection between the carb and intake isn't the issue, where else might I look?

Thanks again for our help guys! I'm still learning a lot about this stuff.

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intake.jpg

Re: Yamaha QT50 Carb Question - No idle & will only run when choked

Gruff "(OFMC)" - /

thats all weird it sound like its fuel starved at certain points, mainly at starting. If its not an airleak anywhere, Id go back to the carb. lets back up

you said it was fine and stalled at a light = could be the idle jet clogged, flooded ,any number of things .

Did you mess with the carb before all this happened? if so what did you do? these carbs have to be level otherwise your float wont work properly.

"then it would actually be revving instead of idling" maybe you put the needle sleeve in backwards? that would cause the high rev at start.

[i"]take the jet needle (#18 in the attached diagram) completely out and try starting her. And she started right up and idled beautifully. I rode her around for a couple of minutes and she seemed to run great."[/i] I never heard of this troubleshooting technique before what are you doing? this is weird so you are having difficultly starting but not when the needle jet is gone? but with needle in it wont start unless you choke it ( means it needs gas) normally.

If you take the needle out then you have no secondary jet no 1/4-3/4 gas range, just air movement and the main working fully open (again this points to needing gas)

tear the carb back down (all the way) and clean it 2x, make sure you put it back together correctly this includes the needle and sleeve, take a look at your throttle/choke cap to make sure the seal is in good condition and sealing. pay special attention to the idle circuit.

When you got it back on the bike look down the throat of the carb and work your throttle back and forth too see if your needle is working correctly, make sure at no throttle the jet is in the main and at full throttle it all the way up out of sight. with it not running of course. and check your compression

Re: Yamaha QT50 Carb Question - No idle & will only run when choked

U need to clean your gas tank out, remove & clean petcock, replace fuel line, add inline fuel filter, clean your carb properly and then reassemble. I bet your carb is dirty from gunk builtup on the petcock filter in the gas tank. Qt50 gas tanks are very easy to remove and clean so do it!!!

Re: Yamaha QT50 Carb Question - No idle & will only run when choked

Hi Jayson,

Actually before I even touched the carb, I popped off the petcock and gave it a good cleaning. It was pretty nasty on the fuel tank side, but I carefully cleaned it all out and made sure everything was flowing through well. I'm certain that the fuel line is delivering plenty of gas to the carb.

When I get some time this weekend, give the carb another thorough cleaning, particularly at the fuel intake to make sure it is flowing in properly. If that passageway was clogged, would it make sense that it will start up with it choked?

Re: Yamaha QT50 Carb Question - No idle & will only run when choked

Hi Gruff,

I think I have figured out the original problem that caused the stall at a light and gas leaking out. I'm thinking this was all caused by my float and needle valve being gunged up. After cleaning the carb up the first time, the leaking stopped, but somewhere in the process, I think I caused this new problem of either fuel starvation or an air leak.

Before any of this happened, I hadn't touched the carb or anything else at all. I got her about 6 months ago and she just ran well so I hadn't messed with much of anything mechanical.

My strange troubleshooting technique was mostly out of frustration after trying every single adjustment and nothing working. I finally just removed the needle altogether and she started up just fine and idled with no choke. Obviously, my solution to the problem isn't to just leave the needle out, but I thought that might be useful information for figuring out what is going wrong.

When I have time this weekend, I'll tear the carb down again and scrub it out with carb cleaner and compressed air. By 'sleeve', are you talking about the piston-like piece that the needle sits inside (#16 on the diagram attached to my original message)? Those pieces are very clean (were never dirty to start with) and I'm pretty sure I have them in correctly because the sloped notch on the side of the throttle valve (sleeve?) lines up with the throttle stop screw. I can look down the throat while adjusting the screw and see the parts moving up and down accordingly.

How does one check the compression (like I said, I'm new to this stuff)? Do I need a special tool for that?

Thanks for all the tips and explanations!

Re: Yamaha QT50 Carb Question - No idle & will only run when choked

Gruff "(OFMC)" - /

compression guage (auto parts store) but if it was running ok before your all good and probaably dont need it but its nice to have to see where your at with the psi's. a good carb cleanin is your solution air compressor is great for blowing out all the passages. if your needle jet is working right then yeah you got it in right. dint mean to knock you down over the needle jet removal just never heard of anyone running without it before hahaha

Re: Yamaha QT50 Carb Question - No idle & will only run when choked

Excellent! I really appreciate all the tips and I will give the carb a good cleaning and double check the needle and everything tomorrow or this weekend. I have a job interview in the morning so I'm trying to keep from smelling like gasoline until after that.

Re: Yamaha QT50 Carb Question - No idle & will only run when choked

Well I feel pretty silly. You guys were right about the carb needing another cleaning. I took it apart and went over everything one more time and noticed the pilot jet had gotten clogged since I cleaned it the first time (I'm certain I cleaned it the first time because I passed a wire through it). After putting everything together again the first time, something must have gotten passed down from the petcock or fuel line and plugged it up causing the fuel starvation.

I have it all cleaned out, cleaned the petcock again, replaced the fuel line and added an inline filter and she's running like a champ again! Even more responsive and a better idle than before.

I appreciate everyone's help and now have a much better understanding of the carb and how to troubleshoot it.

Thanks guys!

Re: Yamaha QT50 Carb Question - No idle & will only run when choked

Knew It. Awesome man glad to hear she's back on the road :)

Re: Yamaha QT50 Start/Run switch

My QT 50 will start easy and idle ok in the START key position, but as I put it in the Run key position it dies. (warm or cold engine temp) Has no spark in the RUN position. Any help would be awesome !!

Re: Yamaha QT50 Carb Question - No idle & will only run when choked

Jack Rutherford /

You really only need to post once.

Re: Yamaha QT50 Carb Question - No idle & will only run when choked

bruce Hamillton /

Sounds like my QT50 only runs with choke open so I bought a new carb got it put on and still will only run with the choke on also put a new top end on with no difference, only thing left is crank seals or reeds.

Re: Yamaha QT50 Carb Question - No idle & will only run when choked

bruce Hamillton /

it was an air leak carb wasn't pushed on all the way..runs good now.

Re: Yamaha Ca50 Carb Question -

Heath Shamburg /

So I just picked up a Yamaha Riva Ca50

And bought it knowing it had carb issues.

I figured bad gas and need the carb cleaned but so far haven’t found myself to be so lucky. The bike would start and idle fine. But as soon as I went to accelerate it would die. I took the carb apart and cleaned it thoroughly. The plunger assembly seemed a bit floppy and it was hard to line up correctly almost like it had been taken apart too many times before.

After putting it together and messing with it more I can’t even get it to idle now.

I’ve tried to find s plunger assembly online but they’re all unavailable for this bike.

I see these other 19 mm 50cc 2 stroke carbs available online cheep but I’m not sure of the fit. I’ve even found one for a newer riva with an electric choke but almost the same but not quite. So not sure if I’d take the chance trying it .

Any ideas guys?, where can I get a carb for this thing or even a rebuilt kit?

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