Modern Vespa

New to the group and learned a bunch from search some of the threads but I think I have a unique enough situation to post a new thread on the subject. I have a 1980 P200e with 1400 miles.
I bought it 3 years ago at an estate sale from an old timer who had only put 250 miles on it back in the late (or possibly early) 80s. I had to swap out the gas tank and fuel/oil lines (fuel had turned to taffy over the years, and oil line rotted out) and replace tires and tubes. And maybe importantly, i swapped the carb out from an old PX150 thinking the original one was a mess from the old fuel. I didnt even think to put the P200 jets into the 150 carb body but assume I was if anything under-fueling and would do no harm. Nothing else done other than an oil change. While it has started reliably and run reliably for the last 1200 miles, it has soft seized 3-4 times during the first 4 months of use and very recently has started to occur at least once during each outing. And just to make sure I am talking soft seize correctly, at any speed over 25ish, I will be moving along and then very suddenly the motor seizes up and dies; unless I clutch in, I will quickly come to a stop. The motor cuts out. Now that I am familiar with it I usually coast along as far as I can and 30 seconds later when I pop the clutch it starts back up, after which I gingerly make my way home. I have not checked the piston for scoring but assume the evidence is there. There's no pleasure in every ride being a bit of russian roulette with the seize potential.

Looking for clues given the new/old nature of the engine and wondering if I am not getting enough oil to the chamber and if there is a good way to test it. It's been years since I had the guts of an old Vespa on a workbench but nothing is coming to mind other than it was caused by decades of sitting unused. Any insights, common culprits for this model/symptom, or diagnostic steps would be greatly appreciated.

You need to list what carb size you have and what jets are currently installed.

Unfortunately low miles doesn't necessarily equal engine health.
It is possible that a seal went bad, you have an air leak and it is running lean.
A pressure test will sort that out.

For a motor of that age & neglect I think you need to rebuild it. That means pulling the motor, splitting the cases, replacing the principal bearings and seals. Not hard, but def a learning experience.

On the other hand, In my personal experience soft seizes have always been either from (a) oil injector not working or (b) air leak around carb base gasket or airbox gasket. You may want to make sure that the oil line is sucking oil (put a small bubble in the line and see if it advances) or pull the carb and airbox, smear a very thin layer of RTv gasket make on the relevant surfaces, and torque down to spec. (Warning: use of RTV will result in PITA when you disassemble and need to clean the surface). You may also want to check to make sure the carb is flat, lapping it on sandpaper taped to a pane of glass.

What carb & jets etc.?

Sounds like they've got a stock 20.20 installed in their P200 which comes stock with a 24.24 and richer jetting.

OP- can you clean the old carb you took out and put that back in? Or swap the jets.

Some of the P200s came stock with a 20/20 carb installed from the factory for sale in the USA. I had 2 of them like that. I'm guessing it was a poor attempt at lowering the pollution emissions.

Totally missed the "new" part of their New England location! Thanks for correcting me.

This happened to me when I didn't realize my 2T oil line for the auto injection was cracked and not supplying oil to the carb anymore. I'd double check that you are for sure getting oil sucked into your carb. As others have mentioned maybe you have an air leak, but we can't tell without a video to hear how it's running. A photo of the spark plug would also help, can get a better idea if you're running lean or not. Also possibly a fuel delivery issue as well. Either way, if you're consistently soft seizing and have done so multiple times, a new top end is in your near future anyway. But obviously would be best to figure out why so you can fix it at the same time.

First off, jetting from a P150 is way too lean for a P200. That could be causing seizes right there.

While original seals won't last long if you're using pump gas (i.e. ethanol gas), I don't think you're there (yet). I agree with JohnDon that an autolube failure is the next thing I'd check.

The autolube system is pretty robust (I've only had one fail once in over twenty years of vintage riding), but it can still happen.

The easiest way to test is to get a section of clear tubing and fit it onto the autolube, then fill that tube like a very tall, narrow oil tank. when you rev it, if the pump is working, you'll see the level go down.

You can see where I was doing this test running a newly built motor.

That's not Russian roulette, that's making absolutely sure it will seize.

Thanks for all the suggestions. This is a great resource! I am looking forward to eventually splitting the cases and doing a thorough re-build but for the immediate, when I have a chance this weekend I plan to put the new old P200 carb back in with new gaskets and will confirm the oil is flowing while I am at it. And of course will post a followup to the group for hoped for closure! Thanks all!