Hi all! I don't really post much here but I have found lots of helpful info during my Vespa P200E rebuild. I thought I should pay it forward and contribute with a quick vid I made showing my freshly rebuilt engine failing a leak-down test.
After meticulously rebuilding it from the bottom up with new bearings, seals, gaskets, etc. Here's a quick vid showing how it sounded when I finally started her up: (This is literally first kick after rebuild- I hadn't tuned anything yet).

https://www.youtube.com/watch?v=uaoMngjPbEU

Now, just a few days after this I wanted to set the timing, tune the carb etc. and went to start her up again. She wouldn't start! I kicked and kicked and kicked. Many pops and sputters and (quite large) backfires later I finally decided to re-check my timing. Doh! Flywheel key is sheared!
I replaced the key and cleaned the flywheel and crank real good and got the timing set just right to fire 2mm BTDC (@18 degrees BTDC) - I use a machinist dial to mark my timing and occasionally use a degree wheel with piston stop if I'm in a hurry. Well, as you guessed by the title of this post, it hasn't run right ever since. I spent days adjusting timing with a light etc. to no avail. I feared the dreaded "clutch side blown seal" and sure enough that's what happened...
Now I'm back to square one splitting the cases again. Just wanted to share my experience and warn against being hard headed like myself and if your bike isn't starting right or even shows signs of wanting to backfire CHECK YOUR TIMING!! Torque those flywheel nuts down good and beware the dreaded sheared flywheel key! Thanks again for all the great info I've found here and hopefully this post and vid will help out someone in the future. Scoot on!

Here's my vid I made showing my scoot failing a leak down test.

https://www.youtube.com/watch?v=LjJdE1GrVps&feature=youtu.be