Modern Vespa

My trusty and rusty P started acting up last night and I'm puzzled. Stock P200 with the aluminum SIP 200 cylinder, road 2.0, 24/24, jetting for conditions. Ran fine so far and did maybe a 10 mile afternoon ride with a stop for dinner. Parked it and during the 5 mile ride back, it bogged out a few blocks from the restaurant. Cleaned the carb and still happening. Cleaned it again and still happened. Limped it home, and today I:

Swapped spark plugs
Swapped float assemblies
Swapped carbs
Swapped jetting
Cleaned fuel tank
New fuel tap and hose.
New gas
Pulled the head and no cylinder damage
Compression at high 120s.

With lots of testing in between, it bogs out like it's flooding itself. Plugs are definitely wet after a test loop. Distance wise, I only go a few blocks from my house and it's already showing itself. I can sometimes save it by turning the fuel off and keeping it pinned with the clutch in. Then once it clears itself, I can open the tap and the process repeats itself. Can't pin it to any one gear or shift.

All that's left is electrical, but spark is consistent and strong. Stator was re-wired and ohmed out ok. Flywheel was replaced a few years ago.

Anything come to mind?

Have a spare CDI to swap out?

Had a stock stator that ohmed out ok… was still at fault.

Engine would start, but fouled plugs like there was no tomorrow.

What jetting is in there?

^^^
I had the same thought.

+2

"90% of all carb problems are electrical."

I'd make sure you don't have a frayed or pinched wire from the stator to the CDI, too. if it's rubbing intermittently, you can get similar behavior.

Yep! I'll get to it in a few hours. Wife was giving me the look before I was able to tackle electrical. The fuel system needed to be addressed anyway (dirty tank, old fuel line, US Spec plastic tap selector that was dodgy) so I started there.

I swapped to simple 160,be4,120 during troubleshooting. Wanted to see if going close to stock would make a difference (it didn't).

No harm trying if you have a working spare, at least eliminate one more thing.

Something I usually try on the center stand using an independent rev meter or strobe with rev counter, and see if the revs rise and drop steady…

Had a CDI not able to rev past 4800rpm, ended up see-sawing up and down. That time, bike didn't even hit the road.

Definitely inspected those and no damage, or no broken/loose connections (tested with multimeter while wiggling wires)

My experiences were no starts, no starts while hot, surging at high revs or backfire popping. Never something like this.

Never had "no starts whilst hot" yet… but I know it's pretty well documented.

Let us know how you get on…

And what pilot jet?

Just stock 55-160

So whoever's got bets placed, it's not CDI #2 , HT lead #2, spark plug cap or spark plug #3.

Thinking it might be time to pressure test the motor. New SIP aluminum is the only thing that's been changed in the last 4 years.

Pressure test showed the tiniest leak on my testing rig but nothing screaming aha! Stator last…

And no to stator swap. 2 blocks and it bogs down. Turned the gas off which clutch squeezed in and it barely stays alive. Turn the fuel back on right as it dies and I can refire.

While I was swapping gaskets, I tried measuring rotary clearance and it's less than .01mm with no observable spit back.

This is just a stab in the dark, have you tried riding it with the petrol (gas) cap off?
Reasoning? carb runs dry during riding, operating the tap somehow relieves the suction just enough to allow some petrol in then cycle starts again?

At some point in this evolution, yes I have. Or loosely on.

SIP 200 cylinder is still unknown for jetting but I'll bet it needs much richer than that.
Change the AC to120 first. BE4 until there's reason to change it. Main jet increasing until it splutters.

Pilot jet will need BGM sizes but if you have a 55/100, that will do for now.
To test if the pilot jet is big enough. Turn the mix screw out 4 turns. Idle screw to max. Should be 1600rpm or slightly less. Any faster the jet is too small.

My "tests" are literally 1000 feet, max! By then it's already bogging so bad it will kill the scoot. Spark plugs are soaked. I can't even rev out enough to build up heat.

Scoot ran fine for the week before Friday. One week of riding reliably, and then boom!

Whoah, that's most of it covered… I don't believe it's jetting… can't be that far off.

Upside down piston? Clutching at straws…

Spark plug wire and/or connector?

Hmmmm...

Swapped today with CDI #2. Threw it on CDI #1 for giggles just cause.

Tested a BGM stator and a Mitsubhi generic that were in my electrical Box of Doom. Both of those were duds so I tossed them.

When I pulled the head last night to look, I snapped this photo. Zoom in on the piston dripping wet. This was however long it took me to rev into 4th and push the scoot home.

Hmmm, in my head, since the plug is soaked and it won't rev properly, I'm thinking the stator (I know, it's new) is not passing along.the juice properly under load, or the regulator/rectifier.

Ok, Im like Schultz "I know nothing!!!".

I'd swap out the stator and reg/rectifier next.

Maybe something shorting out when it starts reving and/or vibrating?

Again, hmmmm....

So strange… yeah that's pretty wet.

Beats the hell outta me!?!?!

For giggles, I google mapped my test route. It's .3 miles with 32 feet elevation change. That's the furthest I can ride this thing before it bogs out. Thankfully it's downhill to my place or just a 6 minute push per google maps.

I can imagine fuel is pooling at the bottom of the crank to make things worse.

There's only one thing left I'd try for your own sanity, is to slap the old cylinder back on and see if that answers the problem.

If it doesn't solve it, it's back to reassembling from fuel tap to spark plug…

How about poking the pilot jet with a wire. I've had the pilot jet clog and makes it a bitch to stay running. Or the pilot jet circuit?

I reread this thread and that's all I'm coming up with, besides a green wire short somewhere...

And we all know I KNOW NOTHINK!!!

It's kinda going the wrong direction, but can you temporarily reinstall the cast iron cylinder to see if it runs normally?

Was the head leaking?

That's all that's left to do!

Was! Addressed that last night and pressure test today was positive that it was solid!

Could the stator have slipped? Timing way off?

Ok - long shot here - but the crowd's identified all the usual suspects.
You mentioned you could "save it" with gas supply off.
What you're describing sounds like the choke is on.
Soon as it warms - it yells at you.
The SI carbs have that weird brass thingy on them.
I've had one catch before - in the on position.
Was so hard to see - I was certain it was going al the way back in when I operated the choke using the cable.
Finally - I took the cable off - and even then it remained partially on.
Had to manipulate it on and off with some needle nose a few times - and then it just flipped back into position.

Look at where that brass thing is sitting.
Compare to another carb.
Make sure it looks like it is 100% back in place.
Maybe it just got hung up some how?

Stator #1 was on the marks where I had strobed it.

Stator #2 I went with the factory mark, no strobe. Quick and dirty.

Worth a try! Think that would happen with two different carbs? Kit was installed 24 Feb and worked fine for a week. What could have set it off?

I was going to say choke. Had them stick a few times. One on the carb valve, others with the cable outer catching on the outside of the box.
It's definitely running way rich. Only other thing that does this is a carb fuel leak. Ironically once you fix whatever this is, I expect it will go lean.

Choke is not the culprit. Disconnected it and pushed the plunger in as far as I could get it. Still bogging. Will try carb #3 and then cylinder swap tonight.

Bad gas???