Modern Vespa

Hi Friends. I am new to vintage Vespas, so asking for a bit of grace here.

I recently got a 1968 VBC with a Scooterworks LML Engine (this scoot was actually bought from Scooterworks in the early 2000s). It has a Spaco SI 20/20 Carb.

The scooter idles and revs cleanly on the stand, but bogs (and quickly dies) when riding, even when I'm just sitting on it and opening the throttle past idle in neutral.

The bog happens around ¼ throttle. If I push through to full throttle, it usually recovers and pulls if I keep the RPM up but will die when the revs go down.

This all started after I changed out the float needle and float because of a small leak. After noticing this issue, I quickly switched back to the original needle/float and same issue.

Additional details:
- New spark plug installed
- New air filter installed
- Mixture screw currently around 2 turns out (I've been adjusting to try and fix this)
- Starts easily and idles steadily
- Issue only appears under load, not on the stand. The minute I get it off the stand and sit on it, the bog begins and it will die unless I crank the throttle.
- I've cleaned the jets and fuel tap filter.
- I've reseated the float/pin multiple times.
- Pulling out the choke makes a negligible difference to avoid the cut off.
- I've done my best to follow instructions on adjusting mixture screw and idle. I see small differences with the mixture screw, but nothing that fixes it.

Help (please and thanks)!

I had that same issue when I replaced the damaged original Dellorto 20/20 on my PX 150 with the Spaco 20/20. My suspicion was that the Spaco jets were not sized correctly to match the numbers stamped on them.
So, I swapped the jets from the Dellorto, and the problem improved considerably. What jet sizes do you have in your Spaco?
In your case, if sitting on the scooter creates the bog, I would first check the fuel line routing, as there is a chance it may get pinched when the weight is applied to the suspension. Also, the correct length of the fuel line is important; needs to be 24 inches. Too short, and the line may stretch and become disconnected; too long, and it gets pinched as the suspension travels up and down. As the line ages, it will shrink in size, because of the ethanol in the fuel. I have actually seen up to 2" shrinkage in the length.

Main thing to remember is that you're way too lean, so don't try to ride it until you have some progress on a solution.

I'll have more thoughts when I'm at a keyboard and not holding a wine glass.

Also, I forgot to mention that Spaco carbs have their mixture screws with a different thread pitch, which may require an extra turn out: 2.5 turns out instead of the original "baseline" 1.5 turns out from fully seated in.
The ideal setting of the screw affects the first 1/4 turn of the throttle.

Take a look at this from the SIP web site:

file:///G:/Documents%20-%20Copy/SI%20Carburettor%20technology%20(1).pdf

My Stella did this exact thing when one of the two Boyesen reeds broke off, leaving 50% of the reed block open. Idled fine but bogged and died with any throttle. The stock LML reed is steel, was it replaced at some point? Look down the carb throat and open the throttle. Have a look. I'd say that it's getting too much air from somewhere.

Do a pressure test to find the air leak.

Thanks all. Some good ideas here. I'll get back at it when Chicago is less of a frozen hellscape.

June is a long time away!

Don't be so dramatic. Bare fingers usually quit freezing to exposed metal in Chicago by mid-May.

Pshh - June?! I'll be in shorts by then. Give me any day above freezing and I'll be out there.

Side note: I made some decent progress. Reseated float/needle and did (another) cleaning of the jets. To get it running halfway decent, I had to back the air/fuel screw really far out. Probably closer 4 turns, but I stopped counting after a few alley beers. Tested fuel line and flow seams good with and without load. Spark plug is looking ok, but its a new one so hard to tell without riding more.

Still needs some additional tweaking, but I can get through all the gears at least without losing power. Still riding a little rough in first gear, but not horrible.