Modern Vespa

Finished a P200 today (yay!) but....

Doesn't hold an idle and drifts into gear, stalling the scooter.

Mixture screw is at 1.5 turns out, idle is at 2.5 turns out. Stator set where it was when I opened it.

I've been working on the clutch cable, but are there issues obviously apparent given my idle and mix settings? I'm a little confused why it won't hold an idle wihout my active assistance.

drifting into gear by itself is usually worn gear selector parts, check the arm .awl and spring and its probably really wobbly ,you can replace the whole thing or individual parts. Be care full one of the selector boxes available is about 1mm different and requires a spacer.

Failing to idle I would suspect points gap or the dreaded air leak (try spraying carb clean at a running engine and see if revs go up. aim around the carb first as you may not have to split if its just a gasket or loose carb box screw (under the carb box gasket is a hidden flat screw)

Do some p200 still run points?? or are you on CDI..if so replace the CDI

All of this started with me going to a first over cylinder following a nasty soft seize. The piston demonstrated a history of this before I bought her. The selector box was working perfectly when I opened her up to deal with the seized piston and replacing worn seals, so I'd assume it has no issues now, but I'm open to a new one. Threw in a new cross while I had it opened up so that's not likely a problem. The miles were low and the gears looked like new.

It seems the sector box is my first issue...I can't really deal with the idle if it spontaneously jumps into gear while running it. The shifting cables are new and tight at neutral. Perhaps too tight?

As to the idle, again working fine before the rebuild so I doubt it's a CDI issue so I'm focused on an air leak or obstruction.

My process would be first to check exhaust plugging...it had a LOT of crap come out at start up. Admittedly, I did get it wet trying to eliminate a lot of surface rust. Second would be checking a leak at the carb box (done...seems tight), and then the cylinder (air leak at the spot where the cylinder meets the engine and then the cylinder head). I haven't done this yet. Maybe replace the aluminum gasket and make sure the engine gasket between the fly and clutch sides is shaved flush?

On the upside, it's now starting on first kick...wouldn't do that when I got it.

Update - pulled the selector box and reinstalled it. Works great.

Idle fixed once I could blow out the exhaust with some higher RPMs...now just paranoia the engine sounds too 'loose' but it's had no break-in so what do I know?

By any chance did you pressure test it after the rebuild? I've found a few leaks by doing this.

I use a piece of inner tube and bolt the carb box over the top of it by bolting on the carb. Then a bicycle inner tube over the exhaust and clamp it on. Then pump it up with one of those blood pressure cuffs and gauge so I only put in 5 or 6 lbs. Then wait - if the needle is moving, you've got a leak.

We found a lead on the cylinder gasket from a fresh rebuild. And I've also found a leaky fly side seal around the metal.

It's gonna be hard to tune without the case being air tight...But who knows, maybe it is!

I haven't done a full compression test on it (I have a tester) but once I resolved her slipping into gear I was able to deal with how she runs and it seems good to go. have about 90 minutes of runtime on it and it's great. I probably will test the compression at some point soon, but it seems to have all worked out okay.

Thanks for the tip!

Hey PopGun, I meant a leakdown test. Testing the engine seals for leaks. Here is a vid of what I'm talking about.

I just blocked off the carb inlet and crimped the inner tube and only went in the exhaust - that sounds nasty. Ha!

https://www.youtube.com/watch?v=SAEi0M66uXA