Two stroke moped repair - broken ring?

[cjolson140]

I’m working on getting a Yamaha vino 50cc 2t running that’s been sitting 4+ years. It’s my understanding that it ran when parked. I thoroughly cleaned out the gas tank & carb (also installed an in-line filter), and hoped to take it for a test ride but couldn’t get it to run. It would fire up and high idle for a few seconds on carb cleaner shot down the intake, but couldn’t stay running by itself. I noted that when cranking on the electric starter, I could see fuel atomizing in the venturi of the carb (air filter removed) but the engine wouldn’t fire. The plug would never really get wet with gas even after extended cranking, and it didn’t seem to develop much vacuum.

After further investigation I discovered the rubber manifold/reed box below the carburetor is extremely cracked & rotten, I think giving enough of a vacuum leak to prevent the engine from running. Planning to replace/reseal this and try running it again.

I didn’t have a compression tester at hand, but my thumb-in-plug-hole estimate felt kinda weak in terms of compression. I felt more “suck” than “squish” for sure. Being unsure the state of the engine, I removed the exhaust to get a look at the piston. I think the piston/bore look alright (visible crosshatch, no piston skirt scoring) for having about 5000 miles, but the lower ring looks broken/recessed in the groove? I forgot to take a picture, but the old plug is a good brown color & pretty oily.

I’m hoping this guy will run and ride with the intake leak fixed, but I’m getting worried that a top end job may be necessary. Is there any harm in running it like this, assuming it will run? Could a lean condition from the vacuum leaks have caused the piston/ring issue originally? What’s the life expectancy of a motor like this anyway? Thanks for any advice, I will update when I work on it next

 

[ZeeOSix]

The bottom ring does look weird in your photo. The top ring looks barely broken in. Has the top end been redone by the previous owner? Bottom ring almost looks too small in diameter for the piston. If someone rebuilt the engine, then anything is possible.

A lean conditon on a 2-stroke will either burn a hole in the top of the piston or cause piston skirt scuffing. It wouldn't cause a ring to break.

Sounds like the badly cracked rubber intake manifold is definately causing problems. If that doesn't fix it, I'd suspect the carb after that even if it looks clean. How far did you tear the carb apart?

Did you have the throttle wide open when doing the "thumb compression test"? If not, that would make it seem lower than it would be if wide open.

 

[vw7674]

doesn't look broken, more like lodged in there due to sitting & being gummed up by all the injector oil fed to it.
fixing the intake boot & warming it up along with a decent oil in the tank may unstick the bottom ring.
Sometimes a two stroke fed too much oil or the wrong oil can end up with a ring stuck, clinging to the piston lands instead of
being 'free floating' in the lands & sealing against the cylinder wall. It happens, usually a cheap fix, even it you have to take it apart
and clean the residue formed on the piston ring & piston. You can even re-use the ring if the gap is still within specs.

You can also use a wooden dowel or toothbrush handle to push on the rings to see if they are free in their grooves.

 

[FowVay]

That's a neat little project. I don't know your model year but searching on-line shows that parts are still available and cheap. The intake book was $20 and a standard ring set was $14 (quick search at Partzilla). Clean everything up and make sure the injector pump works and have some fun with that little thing.

Two-stroke engines are very critical when it comes to air leaks so always use new gaskets when dealing with the induction system. I'd use pre-mix fuel until you're satisfied that the injection pump is working correctly.

 

[Motu]

As above, push on the rings to see if they are stuck, if they are you'll need to pull the top end off. Check your exhaust stinger...what we call mason bees like to make nests in 50cc two stroke exhausts, it's the perfect size. Make sure the oil pump is working and all lines clear - run premix until you are sure.

 

[tmorris1]

Fix the intake and I bet it rips. The reed valves might be bad or leaking as well. 2 Strokes hate air leaks anywhere in the intake or exhaust. That ring looks fine to me. It will probably loosen up after it is run with fresh fuel and proper oil

 

[Wrenchturner44]

CJ, One thing I will add to the others who have provided great advice so far. Check the spark plug heat setting per the manufacturer's specs. You want to consider putting in a hotter plug than recommended in the beginning to help make sure the fuel oil mix is burned completely. Once you are satisfied that is running ok (non-load condition), switch back to the recommended heat plug.

Some 2 cycle engines will load up quickly (over rich condition) when they are not under a load, thus the reasoning for hotter plug when you tinkering to get it running correctly. I hope what I am saying makes sense as the engines with injector pumps are often set up to provide enough injected lubricant for an engine under peak load conditions. Good Luck.....as there is nothing finer smelling than the exhaust smell of properly running 2 stroke engine....LOL.

 

[Warlord]

I think that bottom ring will be fine once it gets running and moving. If not, it’s pretty easy to pull the top end apart and free it up. Keep us posted.

 

[cjolson140]

Finally received the new intake boot and got er goin! Had to play with the idle speed & idle air screw to get a decent idle, and I still need to change the oil in the final drive (manual spec is 10w-30 SE?). Also would like to try another cold start to see if the auto choke is working, as it started pretty hard on the first attempt.

I took a chance and didn’t premix like some suggested, since the plug & piston were plenty oily from the last time it ran I figured the oil pump was okay. The exhaust does have a two-stroke smell to it, but it doesn’t visibly smoke which is nice. I put about eight miles on it and it runs great. Top speed is about 44mph indicated, didn’t verify with GPS. Fun fact: this bike has the same battery it came from the factory with in 2001!

Here’s a video of the first startup:

 

[ZeeOSix]

Fun fact: this bike has the same battery it came from the factory with in 2001!

That's a miricle. Hard to believe it was never totally dead along the way especially if the scooter wasn't used for years, unless someone kept it charged up all the time.