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.

[sc00ter]

When we (myself and 3 other friends who play with Zuma's) have had a lean seize, the scooter is rolling along, stalls suddenly, the smell of burning rubber fills the air and the scooter will usually start back up-with slightly less power. The rubber smell is from the motor locking up for a second, allowing the belt to slip over the now stationary variator because the scooter was still going forward and the rear wheel is still turning. I run a rpm tach and temp gauge with the appropriate alarms set to notify me of pending doom. We initially set the motors using a much more sophisticated equipment (I work in engineering at a small engine company.) but stuff does fail and weather changes can affect out tunes. I now prefer nothing more than a Stage 1 set-up.

When my Zuma 125 blew a belt in the middle of nowhere, I called a friend who owns a tow truck company to rescue me. Wife was with me so no pick-u truck. The tow truck had option to haul a motorcycle in a one piece track. I should have taken a picture, it was really nice. Loaded on street level (my rear wheel was locked TIGHT) then strapped down and raised in the air. So any movement with yours is still movement. I would get it home if possible. I still think its a lean seize.....

[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?

[sc00ter]

Ok, heres my take on it from the other side of the country. If I was in your area I'd be more than willing to take a look at it. If you are 100% sure the piston looks ok, and it was not a lean seize, then something went in the transmission is my guess.

The rear wheel should turn freely with the scooter OFF and at idle. The belt will always rotate when the scooter is running, but the rear clutch assembly disconnects it during idle (and in the OFF position). BUT, if the variator is not returning back to its normal position, weird things happen as well.

So you either have something wrong with the clutch if its still "engaged" while OFF or idling (broken clutch spring(s)?), bell came loose-backed off, torque (contra) spring may have jumped the retainer cup. If you do burnouts, they kill a clutch assembly FAST! I know all to well.....

If the variator failed, it might be a v-slider cracked/broke, roller shattered and/or the variator nut came loose. Impact wrenches are no substitute for a proper holding tool and torque wrench.

Finally, if the reduction box final gears failed from improper spacing, the either ate the end bearing(s) and/or shattered, locking metal chunks INSIDE the reduction/gear box.

I have seen, and fixed, all of these at one point. I love Zuma's and we keep enough parts on hand to do full rebuilds of anything that can/will fail. Let us know what you find.

[KDS4444]

Got your message. I guess I am going to be opening up first the crank case and having a look at the variator (which I will be taking apart) and second the gear box and seeing what is going on inside there. It sounds like I have to find SOMEthing wrong with ONE of them, given the symptoms I have here (still don't understand why rotating my rear tire is causing my piston to move, though, 'cause it seems like it shouldn't and the fact that it does seems like it aught to be telling me something important, but I dunno what that is).



Will be reporting back once I have done these things. Fingers crossed! Thank you for staying with me here! I ain't done just yet!!

[KDS4444]

Crank case opened. Variator looks fine. Turns counterclockwise with no difficulty, rear wheel turns accordingly... V-belt is in perfect condition, nuts on both the variator and clutch are holding fast...