Modern Vespa

I have this 61 Allstate that won't start.
The background:
Rebuilt the motor, new seals, some new bearings, rotary pad looked okay, pads for the clutch.
There was some scoring on the cylinder wall but not too bad.
I reinstalled the motor and set the timing for the first time and the scooter wouldn't start. There is good spark, theres fuel.
I checked the compession and it was low, a bit over 50 psi.
I ordered a new 125 top end from SIP, and installed that yesterday, compression is still low and the scooter still won't start. Even with good strong spark and starting fluid.
I just dont get it, am I losing compression somewhere else?

The hose is too long for checking compression on a small cylinder. Shorten up the hose. Have you checked for air leaks?

Probably leaking air. It's easy to test. Should be done routinely, at least every time the engine is apart or out of the bike.

I bet the flyside seal is whooped

I plan to do a test tomorrow. I sealed the carb box area with thick gasket material and flat piece of wood cut to fit. I will use a 2.25 diameter inner tube (cut with one end sealed) connected to the exhaust stub with a clamp, to introduce 6lbs psi.
Hoping to do the test with the motor still on the scooter.
If im full of beans on this, please let me know. I can pull the motor.

How many times did you kick it over? Try kicking it over with conviction about 10x. If the pressure reading is still 50psi, perform a "wet test"…which is where you squirt about about a small spoonful of oil into the combustion chamber, then kick it over again to see if the compression is any higher. Report back after that.

Low compression is either bad piston rings, severely scored cylinder wall, or a very leaky cylinder head.

what is the compression on the new cylinder? generally, needs to be 90 or above.

The compression on the new cylinder/piston was between 70 and 80 psi.
The air leak test showed a small leak from the outer (metal) portion of the flyside seal and a pretty massive leak from the case to cylinder surfaces.
I torqued the cylinder head nuts to 10 ft pounds each and the air leak improved. Still a bit of air escaping, but not a lot.
I rechecked the compression and now its up to 90 psi.

There is no safe air leak. Get that sealed up or you'll be chasing your tail trying to get the jetting right. The leak will get worse and eventually it'll seize.

Get that sorted first.