Modern Vespa

Because the higher octane is more resistant to pre ignition events which likely is what happened in your case.

Remember, 95 is specified as the miminum. Think of that number as a plan B when you cannot get the good stuff.

Also, higher grade fuels typically have more ingredients to keep engine internals clean.

no i've done lots of conversions with the 492cc it will become a 487cc you onlt have to make the piston fit so it wont hit the rod by shaving of 1 or 2 mm of the bottom where the pen sits

Yesterday I put everything back together with new cylinder kit, replaced the oil, filter and coolant, I also bleeded the cooling circuit and .... it works

I needed to crank it for like 2 seconds for the first start (probably the fuel lines needed to fill...) and then it started. For like 10 minutes it worked a bit erraticaly - from 1200 to 1600 rpm and also stalled 2 times. But now it start instantly and idles steadily at 1500 rpm and does not stall. I went for a 20 km ride (not riding it hard - max 80 km/h with minimum throttle). Rides very well. and doesn't stall when coming to a stop. I checked the tube on the front of the airbox and it had one small drop of oil in it - probably from before... I also double checked for any other oil and coolant leakages and found none.

The only thing I don't like is the decel "pop pop pop". It's still doing that. Maybe it's due to a new piston and cylinder and/or maybe the ECU needs to readjust itself, because the battery was disconnected for like two weeks.
So far so good - I hope it stays like that.

One interesting thing I noticed when checking the timing - the intake valves fully close only after the piston already starts going back up a few milimeters (maybe 5 or 10). Didn't know that's normal - it's called valve lag

And also one other weird thing - when I had the head on the desk I set the valve clearance to exactly 0.15 mm. But when I installed it on the cylinder and tightened the nuts the clearance went to like 0.08 mm on two valves and to 0.12 mm on other two. Seems like the whole head gets warped when you tighten the nuts. So I readjusted them back to 0.15 mm.

Today I went for a 100 km ride and it worked well. Not a single drop of oil in the in the tube on the front of the airbox. I hope it stays that way for a while

Today I replaced the graphite gasket between exhaust manifold and muffler in order to resolve decel popping, but to no avail. It still pops. I noticed it starts to pop when decelerating and the rpm fall below 2500 rpm. It's popping until the engine drops below 2000 rpm. I also tried something - I was driving down the hill with zero throttle and the road was just so inclined that the engine held approx. 2200-2300 rpm for like 40 seconds. It was popping all the way until the rpm felt below 2000. Meaning it's not some kind of fuel residue but rather the fuel is obviously being added while it is not being burned (plug not sparking). I checked my throttle cable and it's properly set - I left just a bit of play for the colder days when the cables get shorter.

Other than that, the scoot drives very well with the newly installed cylinder kit. I made almost 500 km and I have zero complaints. No oil in the airbox, no stalling, no hesitating.

I mean, it's not a big deal, it's just that I would like to understand what's happening.

Any ideas?

My 500 HPE do it as new (cero mods) so I think it is something normal?

I change the exhaust and now I don't have popping, but not sure if is by the new exhaust or is more the new variator or something else