Modern Vespa

So I have been struggling with my PX125 for some time trying to figure out why I keep having low compression and piston seize.
It all started when I kept having seizes after driving a couple of miles, it would usually start after and I could get it home. I tested the compression and it was low, around 50-60. I took off the cylinder and had small marks on the piston and barrel, so bored out barrel pretty well and eventually just got a new piston.
-My first thought was bad seals so I split the engine and replaced both seals and replaced all other seals gaskets needed, still low compression.
Did a leak down test, which passed several times and still low compression.

-Replaced carburetor and muffler, still low compression.

-Replaced cylinder with a new one and did leak down test, which passed, no help.

-Did wet leak test on rotary pads, passed and held oil for 10 minutes.

-Thought maybe carb box was leaking so replaced gaskets with the blue SI premium gaskets, still low compression.

My next thought is maybe squish or just rebuild engine again, but the leak down test holds air for 10 minutes and I've sprayed every inch of the motor and can't find any leaks.

Becoming a bit frustrated at this point and hoping someone might have a solution or advice I haven't thought of?

After you replaced the piston and cylinder, did you test for compression? Was it also low, or better?

What jets did you have in the carburetor? Did you mess with jetting while it was seizing?

It's still autolube, right? Have you tested to make sure that the oil pump is working correctly?

Yes, I did compression test after replacing the cylinder and even put a little 2T oil in spark plug hole that didn't improve compression any.
As far as jets, I just put the jets in my old carburetor in the new one. I think main jet is 102 or so.

Do you have any pictures of the seize marks in the cylinder? Scuffs only on one side? All 4 corners? Top and bottom?

Are you riding the motor at max rpm on the freeway? Or just putting down to the coffee shop?

And yes, it is an auto lube, which seems to work, though I haven't tested outside of noticing that the gear turns when I crank the engine and the oil line feeds oil correctly.

Don't have the piston right in front of me at the moment, but I believe I recall most of the scratches being on the top side and mostly top right side.

I don't take it on the freeway and my commute to work is about 7 miles, top speed about 50ish and mostly neighborhoods.

Keeping apples to apples, are you always using the same compression tester?

Are you breaking in the top end correctly? Or just going for it?

And the new carb is definitely an autolube one

I just got another compression tester to try and will probably do that this weekend.
I generally take it pretty easy to break in. I haven't been taking it out any after new parts, especially cylinder stuff since I can't get the compression past 60 and don't want to f up another piston.

Yep, new carb is auto lube

I'm surprised that it would start with compression in the 50-60 range.
Maybe the compression tester isn't accurate.

^^^ this.

Have to ask: You sure you put the piston in right side up?

Leak down is different from a pressure test. Which are you doing and how?

The piston was put in with the arrow marked ex pointing down to the exhaust port.
The test I'm using is by pumping air into the spark plug hole with a blood pressure gauge I rigged up. I put the engine set to top dead center and plug up the carburetor hole and exhaust hole. Then pump I believe 2-3 psi or 190ish mmhg in and let it sit for about 10 minutes while spraying soapy water around possible air leak locations. I only lose a couple mmhg's during the minutes if that much and can't see any bubbles from spray.

Normally, you'll get a little bump in compression by adding oil.

Did you remember to install both rings?

Agree with the others, test it again with a different compression tester.

When you tested the compression did you kick it repeatedly until it reached a maximum? Typically, I might see about 50 on the first kick. With around 90 being a general minimum I also can't see it even starting on 50-60.

When you say that you " bored out barrel pretty well and eventually just got a new piston" did you over-bore to proper size for an OS piston?

OP: How long is the hose on your pressure tester. It needs to be as short as possible, or it will always read low.

(That doesn't mean it's useless, just that it will only be useful for measuring change, not absolute accuracy)

-Both rings were put on.

- As for compression reading. I generally kick over several times pretty hard until the gauge stops and reaches it highest reading. Also have checked the connection to the spark plug hole to make sure it's not leaking.

- For boring out barrel, I used one of the dingle berry bore rods you put on a drill and went through it until the scratches were smoothed out.
I put a whole new cylinder on it now, so brand new and no scratches.

- The hose is maybe 10 inches long. Got a new compression tester I'm going to try this weekend so hopefully that makes a difference.

Just for clarity. A 2 stroke compression tester has the release valve in the part that screws in the head.

I didn't know that. Do you know why?

A complete top end and you're still just getting 50-60? My gauge has a 20" hose and it just takes a few more kicks to compress the greater volume.

How much squish does it have?