Zuma rear wheel locked after overhaul

[KDS4444]

I recently did an overhaul on my 2002 Yamana Zuma YW50 (49cc 2-stroke), a thing I have never done before. I replaced the v-belt, cleaned up the variator, and replaced some missing springs from my clutch shoes. I also opened up the gear box, removed and then replaced the main drive gear, and replaced the transmission oil with some fresh ATF. I noticed when I was done that my engine had/ still has a small leak between where the exhaust attaches to the cylinder, but it was driving just fine, had much better pick-up, and no more smoke coming from the exhaust. That was about a week ago, and I haven't driven it much more than an few miles since then until today.


Today I took it for its first long drive, about 10 miles round trip. I got to about mile-4 when as I was accelerating through an intersection, the engine died on me. Also, the scooter didn't just coast to a stop, it was like the rear wheel had somehow locked up on me. I could smell hot rubber, but figured this was just the new v-belt getting situated. I opened up the crank case, found nothing wrong. Closed it up again and restarted the machine. Seemed to be fine. Got back on and kept on driving. Around mile 6 it happened again: opening up the throttle began to make the scooter move slower, not faster, and I dropped from around 35mph to less than 5. The engine didn't sound happy. I was able to keep it moving a little while longer (maybe 200 feet), but I knew I was doomed. When I finally came to halt, the rear wheel would not move at all— not like it was stuck by the rear brake shoe, but as though the gears in the gear box were not allowing it to turn. I put it up on the kick stand, and the rear wheel will move just a few degrees in each direction easily. If I put it back on the ground and push it hard, it resists me but it WILL turn. Also, I can hear what sounds like the engine starting up again, though there is no key in the ignition and it is turned off! It's as though the rear wheel is turning the variator and trying to start the machine up.


I ended up parking it along the side of the road and catching an Über home. Tomorrow I will need to go back out there and try to fix it, but I have no idea what could be wrong. I was not able to identify anything actually burned or overly hot around the crank case, but the smell of hot rubber persisted. My v-belt looked just fine when I opened the crank case up at mile 4. But the scooter is stuck now!


Am eager to know if anyone has any ideas what might be wrong! I sure don't!

[sc00ter]

It sounds like the scooter had a soft seize, from the stall, hot rubber, then started back up description. Soft seize = lean motor. Not good. That can easily be checked by removing the exhaust and looking at the piston bottom. Scrapes on the exhaust side of the piston mean the top end is done. If the piston looks correct, youre ok. You said you did an up-gear? Who pressed the gears? Did they space them correct, with the proper space? Did all the spacer washers go back in the correct location? A friend had a spacer washer on the wrong side and it let a gear shift just enough to lock them up. He was lucky the case didnt get a hole punched in it. Finally, fix the exhaust leak when you check the piston. The exhaust pulls excess heat off the motor and controls back pressure. Running a 2-stroke without one is even worse.

[sc00ter]

Almost forgot. Who did the belt and related tune-up parts come from? Running a wrong sized belt can really cause serious problems. Why were springs missing from the clutch? I've seen those springs break and cause major damage. It almost sounds like a variator roller shattered, or a v-slide from the back plate on the variator jammed. My friend had a v-slide crack on his Metro and it locked the transmission kinda like youre describing. I had to rescue him with my wifes little pick-up truck. Took us forever to remove the back plate of the variator! Let us know what you find.

[KDS4444]

Starting with your last question first, I did the repairs to the overhaul myself— I have tried to be my own mechanic for my scooter, and have so far been successful with this: I rebuilt the engine once, have replaced my own front hydraulic break pads, and have replaced both tires myself in the past. The crank case and its contents are new ground for me, though. I had also been trying to adjust the rich/ lean mix of the fuel in an attempt to reduce the smoke from the exhaust— I had been trying to lean it out, since it seemed to be always running too rich. I may very well have ended up running it too lean. Damn. Damn, damn, damn. You are saying that I will be able to see any scrapes on the piston if I remove the exhaust and look up? And that if I find any, I am gonna have to get it towed home to work on it? And will need to replace the cylinder head? Ow. Lets hope this is not it, though according to what you have said, it almost certainly is. It's a haul to get back out there, so may not be able to check until tomorrow, but will certainly be updating you soon.


I am certain the belt is the correct size: I was careful to select a belt that was a direct replacement for the one I had, and I measured it before installing it, I also watched it operate, watched the variator close while the clutch opened, and it seemed to work fine at the time (3 days ago). I don't recall any spacer washers inside the gear box, but myself pressed the drive gear into its casing: I had to freeze it to do this, but once it was frozen good and the gear box cover was hot I used a rubber mallet to smack the drive gear and its bearing into the box cover before replacing the gasket (one I cut myself, and which was not leaking fluid) and closing up the box. The scooter sat for several days following this, with no loss of fluid from around the gear box anywhere, but I am considering draining it and opening it up again anyway to check this.


I won't have my new gasket for the exhaust/ cylinder connection for a few more days, and it sounds like I shouldn't attempt to even start it until that gasket is in place, but if, as you said, there are scratch marks on the piston, will a new gasket here matter much? I will need to replace the piston (I assume) and the cylinder head? Yes? These are nightmare things to have to do, but if they are necessary and if the alternative is to sell it for parts and have no more scooter, than I guess I will have to do them.


But first, I need to get back out there with some tools and have a look. Ugh. Will let you know how I progress, even if it is only misery that lies ahead of me!

[sc00ter]

Its actually normal to see a bit of smoke on an older/pre-catalytic 2-stroke. Leaning it (carb) out to make the exhaust smoke go away will slowly kill the top end. Thats why I asked you if you had a base setting for the carb.

I always run OEM belts from Yamaha. Only $25 dollars/+ or - $3. There is no accurate way to measure a v-belt, as multiple factors come into play. Trust me, we have experimented enough to know what works and what does major damage when it fails.

Reduction box. There is actually a required "space" when you press the new gear on. I borrow the press at my local machine shop and set it with a feeler gauge. Its a tolerance fit, and if its off (the "space") it grinds the inside of the case, or stresses the big gear-causing bearing failure.

Scooter Swap Shop sells the best exhaust gaskets. We buy them in bulk (I have 10 on hand) because things come loose when youre running 12,000 rpm's at 65mph.

If you do find the piston scraped up on the exhaust side and you decide to replace the top end, find the base settings for the carb before starting it-or the same thing will happen again. I gave away a low miles/mint OEM top end and carb not long ago. Actually, a bunch of Zuma bug-eye stuff. I still have my mint 98 Zuma but its still in pieces. I may also be buying back an old 03 Zuma from my boss if he upgrades to something bigger. If I do, I'll regret giving away all my bug-eye/2-stroke parts.

Good luck!

[KDS4444]

Regarding the v-belt: when I say I "measured" it, I mean I verified that the width of the belt was correct both for my scooter and for the packaging, and when I lined it up against the original one I could detect next to no difference in circumference (the older belt was very slightly larger in diameter, which I expected). I only replaced it anyway as part of the overhaul, not because it appeared overly worn. I just suspect it when I smelled the hot rubber.


Which, by the way, do you think is being caused by what, exactly? My own mortified guess is that it might have been the smell of the O-rings around the piston overheating/ decomposing, and if so, I am obviously screwed. But thoughts on that? Are there any other rubber parts involved here that might have been getting much too hot somehow?


Since I am able to force it to roll forward, I am guessing my best move now is just to try to get it home (since there's no way I am performing a repair on my cylinder along the side of the street in Los Angeles). Will it be damaging to the piston/ cylinder for me to even force-roll the scooter up a ramp and into some kind of moving truck, do you think? I obviously can't tow it by raising the front tire since the back one is the one that won't rotate.

[KDS4444]

Another concern: When I stopped the first time to open up the crank case, I did not replace the gasket seal: it had a tear in it anyway at that point. This means my crank case was not sealed when I drove the scooter the second leg. I've heard that a faulty or absent seal on a crank case can cause oil to get sucked into the crank case from the gear box.... Which would mean I am REALLY screwed. But is this true??

[sc00ter]

What case gasket are you referring to? The gasket that goes between the case and the cover to the transmission/vairator and clutch assembly? If its that gasket, we never use them. Heck, I've ran with no transmission cover at all for a week while I was trying to figure out a clutch issue. If your referring to the gasket that goes between the reduction/gear box cover to keep the ATF fluid in, we've re-used ripped ones many times. As long as there is no leak youre fine.

Now, what I forgot last time. If you had a soft seize and the belt slipped on the variator-there is a chance the belt either got jammed (usually it turns sideways and wedges someplace horrible) or it put excess stress on the V-slides/rollers/clutch assembly and broke something. Again, keep us posted. I have faith it can be fixed on the cheap with either OEM used parts or pattern replacements.

[sc00ter]

So, any updates?

[kz1000st]

Gearboxes need 90W oil not ATF

[KDS4444]

The gasket I meant was the crankcase gasket, the one that goes all around the crank case, not the one by the gear train. I read somewhere that the crank case without the gasket on it can create suction and end up drawing transmission fluid into the case, drying out the gear train. But maybe that is not so.


The last time I checked the v-belt (along the roadside when the scooter died the first time) it looked just fine. I am doubtful the problem is there, but I haven't opened it up a second time to verify. I finally got the exhaust removed and had a look at the piston. From what I can see, it looks fine. I put some 2-stroke oil into the spark plug hole to lubricate it, and turned the rear wheel to move the piston up and down. It seems lubricated....


...But! Should turning that rear wheel be moving the piston at all? I don't think it should. The piston is supposed to drive the wheel, not the other way around. What does this tell me? Anything? Also, the wheel seems wiling (reluctantly) to turn forward, but it is the reverse direction that it fights me (I can roll it in reverse, but it really doesn't want to go that way).


I have not tried starting it up at all. That seems foolish for now, yes?


Also, regarding gear oil: I can get some 90W on Amazon, and will do so. Given that I have already used ATF, what should I do now?

[bandito2]

If the wheel turns when you turn the motor by hand (or by turning the wheel it tries to turn the belt/variator&motor) then the clutch is definitely holding/stuck in some way keeping it engaged. Open up the clutch to see if any of the shoes (or any broken pieces causing it to bind) are stuck against the bell. Betchya if you free the clutch, the wheel should turn easier because it will not be engaged trying to turn the belt which is connected directly to the variator which is directly connected to the engine crankshaft. With the bike NOT running and up on the center stand, turning the rear wheel will turn the bell through the final drive transmission as normal but should not be trying to move the belt because the clutch should be disengaged.

Fix your clutch!
(but you may have possibly done some engine damage too... which is a separate issue apart from the clutch)

Oh, and it should be easier to push the scooter (if the clutch is still stuck) by removing the spark plug... then you won't be fighting the engine compression of the piston stroke.
(and I don't even own any 2 stroke stuff, but did once upon a time... ultralight aircraft engine. It was a thing of beauty; Rotax 503 electric start+ pull start, 2 cylinder, dual carb, dual ignition)

[KDS4444]

So when I reassembled the gear box and re-attached the clutch, v-belt, and variator, the problem continued. But I figured out what was causing it: for some reason, when I screw down the nut on the clutch bell, it causes the rotation of the rear wheel to rotate the entire clutch (as though I had the engine on and was driving). When I loosened this nut, the bell stopped rotating and the rear wheel could rotate freely (and didn't cause the piston to move). It seems something about my clutch and its bell is (right now) causing it to be continually engaged, but whatever it is, it isn't the tightness of that nut alone: I know that the nut has to be screwed down SUPER tight to ensure the bell doesn't come loose inside the crank case! Maybe I am missing a washer somewhere (don't remember there being one to lose, though).


But whatever that problem is, I am considering it minor (for now) compared to the weirdness I experienced while driving a few days ago and the way my engine died in a lean seize on the way home. Haven't opened up the cylinder or tried to remove the piston yet, but am thinking maybe that is going to be next. Any advice on that is welcome!

[sc00ter]

Nothing in that transmission is higher than 40nM. We set both cluch bell nut and the variator at 40nM. I dont have a service manual to see is there if there is a washer, but you could always go to Cheap Cycle Parts and look at their exploded parts diagram. See if everything looks correct. Just looking inside the exhaust port with the piston forward, so you can see the side of the piston, will tell you if you had a lean seize. Spark plug is also a good indicator.

Sometimes, when you have a lean seize at speed, it will reveal everything thats wrong in the transmission.

Off the top of my head, I have no idea why tightening the bell down would cause the symptoms you described. Will think about it in the morning.

I have an ebike and my front wheel caught a transfer on the road (from milling and paving) and spit me off the front at 22mph. I hit the ground HARD, and as a bonus, got a chunk of my hair torn out! I need a bicycle helmet! Road rashed my knee and elbow, have a huge rash on my head and a chunk of hair missing. Ouch! Anyways, will give it some thought in the morning.

[KDS4444]

DUDE! YOU NEED A BIKE HELMET! People don't realize just how dangerous a "simple bike" can be! At 22mph you are almost asking to be killed! And I still NEED YOU!


Tell me if this is what a lean seize looks like...


https://photos.app.goo.gl/SywYSJ2Z9EYyYVKG6


Aw, hell, I know what a lean seize looks like by now, and I know this is it. BUT! I have now polished that piston up, and though some of the scratch marks are still visible, I'd like to know if you think it might be at all worth my time to go ahead and polish up the cylinder as well, add new gaskets, put it all together again, and hope it will run. Here is a photo of the polish job on the piston:


https://photos.app.goo.gl/6Bm4fwrTvpCojPYMA


Stay off that bike until you have headgear!!!