Modern Vespa

EDIT: For anyone looking to solve similar issues-- teeth from gear cluster ("Christmas tree") had broken off into the engine casing, also causing damage to the opposing gears in the gear box. Gear selector rod/cruciform were also fused together. Not a seizure in the cylinder.


I have a 2005 PX150 that I recently (~80 miles) kitted with a VMC177, 24mm carb with bigger jets, and big box exhaust. I have been riding it at less than 50% throttle and adding ~1% oil to the gas alongside the autolube.

It has been running well since the install, to the point where I let a buddy try it out, with the caveat that he keep it under 50% throttle. While he was riding it he gave it a "hard shift" and it stalled out. Since then, it has had a random clanking sound and brief seizure of the rear wheel. It will keep running despite the noise, though I have run it only briefly to help with diagnostics.

I first thought that it might be popping out of gear, so checked the shifter cables and the plate (the toothed plate that the shifter cable moved to select a gear) and they are operational. Noticing that the noise happened in neutral or when the clutch was engaged as well, I turned the focus to the engine.

Turning the flywheel by hand or using the kickstarter, I found that the engine will "catch" briefly, coming to a stop. Continuing to turn the flywheel or press the kickstarter will free it and it will continue to turn. There does not appear to be any cadence to when the engine catches (e.g., sometimes it will be after 3 turns of the kickstarter, sometimes it will be after 30). The seize seems strong enough to cause the wheel to skip, but not enough to stall the engine. It does not seem to be coming from the piston catching, but seems internal to the flywheel.

Obviously I do not want to run the scoot in its current state, but do not know where to begin troubleshooting this. Any advice would be appreciated.

Pull the head, turn the engine until the piston bottoms out, take a look at the cylinder wall and put up a pic here.. If it seized, you should be able to see it. This should take about 30 minutes tops to check out. Eventually, you will probably have to remove the cylinder as well to be able to look at the piston sides and the rings.

Thanks for your response. I pulled the head already and didn't see any deep scoring — there was a very light marring from me installing one of the piston rings backwards but that has been there for the roughly 80 mile break in. I'll take a picture tomorrow and post it.

Possibly the cruciform has come unscrewed. Remove the gear selector box and tighten the rod. Reverse threaded.

Thank you both for your responses. Checked out the cylinder today— there was additional scoring to what I observed from the piston rings, but I don't know if it is deep enough to cause a seize. (attached).

In terms of the cruciform rod, I rotated it counterclockwise approximately 15 turns and it didn't seem to tighten at all. I couldn't find any torque spec for the rod. Is it supposed to "bottom out" or will it just continue to turn?

I tried hand turning the engine again until it "caught" to get a video. It will lock up and cannot continue the stroke, but will free itself if moved backwards slightly and then forward again. My guess is that the clanking I was hearing during operation was it catching, but that the ignition is strong enough to push through the catch/there is enough play forwards backwards that it comes free and continues running. It won't stall, it's just the clanking noise and temporary seizing of the drive shaft. Could a loose cruciform prevent piston travel?

I cannot get the video to post, but I have it and can upload it to imgur or something similar if that would help

What does the piston look like?

It is currently in my friend's side yard, but I asked him to snag some pictures of the piston when he gets a second. I'll post them asap!

In the meantime, here is a video of the catch:
https://imgur.com/a/D0Q7Vct

It should eventually bottom out when tightened counterclockwise. It sounds like the threads might be stripped where the selector shaft screws into the cruciform. A misaligned cruciform could definitely cause noise and or binding. You could unscrew the shaft to check but you would probably have to split the cases to put it back in. This might not be THE problem, but it's definitely A problem that has to be solved.


Your friend's missed shift could either be the cause or the result of the cruciform problem. My guess is that it came loose and caused the missed shift.

You may want to pull the clutch lever while someone else spins the engine by hand and see if the catching is still going on so you can eliminate the possibility of something messed in the clutch area.

Sounds like stripped cruciform/selector rod threads to Me

Thanks, all! I'll see if I can get the selector rod tightened and, if not, then start the process of splitting the case.

Appreciate all of the input.

Wanted to throw out a quick update before I go to order parts, in case someone else happens along the same issue.

Gear selector rod definitely appears to be freely spinning (but stuck) in the cruciform.

Splitting the case, I found several teeth and metal chunks at the bottom. Upon inspection, they came from the gear cluster on the primary shaft (pic). Yikes. The drive shaft gears are in better condition, but there is some damage to the corresponding gear (3rd?), so I'll likely need to replace: gear selector rod, cruciform, (at least) 3rd gear on the driveshaft, and the primary gear cluster. Sound about right?

While I wait for delivery, any bright ideas for separating the gear selector rod and the cruciform?

Ouch! This will be expensive. Might as well get something out of it. There are clusters with better gear spacing, straight cut ring gear options and stronger cruciforms to consider before buying OEM parts.
When the drive shaft is on the bench, you'll get the cruciform out.

Any recommendations? Fighting the urge to replace it with a Stella kit that runs ~$150 for the entire gearbox.

That would be going backwards

I don't know if the LML gearboxes are bad, at least the EFL versions that came with the Stellas seem pretty decent and have been used by a number of forum members for high performance builds. Maybe LML aftermarket parts are not as good?

The breakage you experienced suggests that the cruciform or selector shaft stripped when assembled. The takeaway for me is that you need to get this right next time, not you need a high performance gear set.

It's one of those infuriating things about Vespas that one small thing can be wrong and end up causing major damage.