Modern Vespa

So I had a soft seize. Again. Second time since October.
Both fresh rebuilds. 2nd oversize on the cylinder the first time. Rebooted to 3rd oversize the second time. Rings were gapped appropriately. (0.4mm I believe).
The first time I thought it was due to poor jetting.

Second time—-first ride out. Not going hard. Just a nice break in ride. Maybe 1/2 throttle. Booom. Soft seize again.

Piston is free. A mark on the cylinder wall but no deep gouges I can catch w my nail. Haven't taken the cylinder off to assess the piston or rings.

Jetting was on point. brand new plug.

So what is it? Could have been the timing bc I never verified it. But it's a p2 and I trusted the timing marks.

I made a homemade leak tester and gave it a shot. I pump it up. No bubbles anywhere. However the cases won't hold air. No bubbles to be found when spraying soapy water. Can't find the leak anywhere. Cant feel the air escaping either. Not at cylinder base or head. Not at carb or exhaust. Not at clutch breather. Not at rear hub. Not along casings where bolts attach.

So what do I do now? Thoughts?

I sprayed mine down with dish soap and water. Your missing a spot somewhere. Is there something oily you can dunk it in that won't hurt it? Smoke test?

Not to be critical - but exactly how are you setting up the pressure test? Some on one here (probably not me) might find an error in your set up.

They cannot seize with over rich jetting. Next time out start here.

Well I used the good old fashioned bike tube and blood pressure cuff method.

I could smoke test it. I have a friend. Lol.

My trials and tribulations with leak testing. Hope it helps.
Smallstate slow to return to idle

Potential leak points to check:
Carb>carb box
Carb box>case
Carb box screw>case (mine went all the way through)
Spark plug>cyl head
Cyl head>cyl
Cyl>case
Cyl stud holes
Crank side seal
Clutch side seal
Case gasket

It's got to have an air leak somewhere.

Thanks for this. I'll be taking the engine out and put it on the bench to then assess more thoroughly.

Stock P200? What's your jetting? And 23 degrees of timing is a little hot for modern fuels.

Stock p2 with sip road 2 sport pipe
160
Be3
122

55/160 main.
Spaco carb.

Stock setting (A) on stator.

without a doubt i'd put a strobe in it to confirm the timing! And i'm not sure how jetting for a Sito+ compares to the SIP 2, but for years i had a timing in my stock P2 with a 128 main (stock as in drilled air filter, flowed carb to air box to cases, 22 degree timing). I'm now currently at a 125 main since i don't ride flat out on the highway every day for 30 miles at a time.

Yah I hear you. I bogged down in fourth gear with a 125. Hence dropping it to 122.

Technically a sip road 2 should go up two points from stock. So approx 120.

Anyways, it's probably jetting that is the cause of my woes. It's a dark art.

Not holding pressure = air leak. Plain and simple. It must hold air for a reasonable amount of time. Opinion varies on how long that should be Tinker with your tester and your engine until you can make it so.

Make your own timing marks on the case using the piston stop method. Recheck until you're sure. Then set it with the timing light. FMP videos on this are helpful.

Good luck.