Modern Vespa

I just joined the board and I'm not sure if a 24-year-old scooter should be in Modern or not-so-modern forum, so I'll start here I have been working on, and rebuilding my vehicles including engines for decades... everything from B&S lawnmowers to 70's Mercedes diesels to a 600 hp Mopar big-block. But the current project has me puzzled.

This scooter belonged to my brother and he recently gave it to me. It had been parked near the seaside in Crete (think salt air) for at least six years so the corrosion is extensive. I wanted a project so I've spent the last several weeks de-rusting, priming, painting, replacing many parts including most of the fuel lines, and drilling out fasteners that were rusted solid to their clip-nuts. The wiring harness had cracks in the insulation up behind the instrument cluster, and the corrosion had caused EVERY wire to break there as well as the flexible PC board copper traces to basically disappear. To make a long story short, I fixed or replaced it all... except it will not start! It was "running when parked"...

I'm here in Crete with limited hand tools and no scooter mechanics I'd trust that are closer than an hour drive (minimum). There is spark and the plug gets wet. Bottom line is, the compression feels very weak, and there is not much vacuum either. It won't blow my thumb away from the plug hole. I took the carb apart and made sure all the passages were clear with carb cleaner and the float/needle/seat works. Reeds appear to be sealing properly.

The interesting thing is that if I shoot a little carb cleaner into the intake, it fires immediately with a big cloud of blue smoke (oil from previous start attempts burning off?) and also revs quite high despite the throttle being closed. But it won't fire even once on gas/oil...

My current working diagnosis is stuck/broken rings, or maybe even a hole in the piston top. I did originally turn it over with the kick starter before attempting to use the electric starter and nothing felt stuck or binding.

Other than a weed-whacker engine years ago, I have no experience with two-stroke engines so any help greatly appreciated! Thanks.

ps Is it possible to change the piston/cylinder without disassembling the bike?

Sounds like a set of rings, at the minimum, are needed. It just doesn't have enough compression. Compared to the other things you've worked on, this should be easy for you, especially if all you need is rings.
Four bolts on the head, pull it off, pull the jug off, install new rings and a new base gasket, jug back on, new head gasket and head.
As far as doing it without removing the motor, can you get the head off, and is there enough room to pull the cylinder if you do?

If not, you can jack the bike up in front of the engine, remove the bolt holding the engine, remove the shock at the engine, and push the engine back, probably far enough to put on a new top end.

Thanks for the reply. It looks like the head bolts should be accessible from the front (possibly unbolting the coil)?

One more thing I want to try before disassembly - spray some gas into the intake and see if it lights off like the volatile carb cleaner does. If it starts that way then the problem is still in the carb.

If it's just stuck rings, I'm hoping the rings will free up if I can get it running. I know, a long shot, but before I spend the $ and effort on a new top end it's worth a try

Remember the old "Bon Ami down the carb"? No, I'm not going to try that

Update: I put a couple of ounces of gas in a clean spray bottle... cranked while misting it into the air intake opening in the frame - and it fired right up! The RPM also climbed way up again despite the closed throttle, and a huge cloud of whitish/blue smelly oil smoke again, so I shut the key off.

Thinking about stuck rings... but doesn't the carb still have to be the problem if it'll run on gasoline spray but not from gas in the carb?

It's alive!

The problem wasn't actually the carb, but the control cable going to it. The sheath had slipped out of its socket at the splitter end, when I was moving many parts and hoses around, and was holding the throttle 3/4 open at idle (and beyond wide-open at full rotation)...

After fixing that, I reinstalled the carb, refilled the fuel bowl and hit the starter... fired right up although the idle was too slow. I cranked in the adjustment knob and now it idles.

It's still blowing a lot of oil smoke out the exhaust at mid- and high rpm though, even after idling for a minute or two and a couple of revs. I need to get it fully warm and run it up (i.e. an "Italian tuneup") and maybe the smoke clears.

Still suspecting ring, piston or bore corrosion... we shall see

I rode it around town today... sometimes the smoke was very light (normal haze), and sometimes it would return with a vengeance.

I also noted that it was hard to start when hot. And that the plug is black.

What this tells me is that the automatic choke may not be functioning (it defaults to rich, correct?) so it's time to check that out. Otherwise I think it's time for rings...

edit: Choke measures the correct 35 ohms, 12 volts measured at both pins with key on. Tomorrow I'll start it and make sure one of them goes to ground so the choke will heat.