Modern Vespa

I finished rebuilding a 69 GT engine for a friend. New Pinasco 2 port 177 kit and new Spaco 20/20 carb. I couldn't get it to idle without screwing in the idle screw all the way. I knew it wasn't an air leak, because I had tested for air leaks. I had also tested the rotary pad to crank seal, so not that. Replaced the coil, points and condensor, so not that. Checked for air leaks at the carb, not that. I replaced the new carb with an older, known working carb and it idled perfectly. So I started to compare the carbs. I found that the slide on the new carb wasn't open very far when the idle screw was set at 2 1/2 turns out. See my pictures.

Anyone else have this issue with new Spaco carbs? I'm thinking I'll just open up the slide more with a file or Dremel, thoughts?

Are you referring to the Idle Screw (bit one on top that protrudes through the air filter and adjusts how far open the slide is) or the Mixture Screw (the one in the back that allows for fine adjustment of the idle mixture once the correct idle jet is installed)?

In neither case should you need to file or dremel the slide. Just adjust the slide with the Idle Screw. Any guidance on how many "turns" something should be set is just a starting point for dialing in the carb. It's adjustable via a screw for a reason.

The other thing to check is the throttle cable. You can use your fine adjuster (little thing that the cable threads through as it enters the carb box) to make sure the cable has the right amount of tension.

Might be the mixture screw, too. The thread on the spaco one is tighter than the dellorto one. So 2.5 turns isn't the same.

Not sure why they did that. Everything else is identical.

The gap in the slide would be the idle screw. There's a taper it hits that adjusts that part.

Interesting find.
Pictures seem clear.
Slide is simply not open.
There is no magic to "2.5" turns with the idle screw.
One thing you could try: just use the idle screw from the old one.
That is probably the difference maker.
But again - no magic to the number of turns - only to the amount of gap they create.
My $.02

The idle screw pushes down on this taper.

So screwing it in more should open the slide.

Might also check the pilot jet. Those clog easily, bc the opening is so small.

Thanks for all the advise.

Since chandlerman advised against modifying the original slide and I see his posts all the time modifying stuff.. I figured I better not.

Seeing charlieman's advice made me think to switch out the slide...

Bingo! https://www.dropbox.com/s/iuo2ikq7i4ddiry/PXL_20211222_222423420.mp4?dl=0

And people always think that I'm joking when I say I live my life to serve as a Cautionary Tale for others.

I'm glad you got it sorted.

Glad to hear that works now. Could you post the side by side comparison of the idle screw? I really curious

I've got the engine back in the GT and pretty much sorted. Waiting for some warmer weather to go for a ride. I thought I'd post up some pictures.

I just helped a local forum member on a Stella. His slider wouldn't return and it was impossible to set the idle let alone make it run decently good. I disasssembled the carburetor slide and smooth it out with 3000 sand paper, what a difference that made and that is not the first time. don't just limit the sanding to the bottom flat part but put emphasis on the side and even the corners.

Good to know scooterist! I binned the slide on this carb and bought a new one. I'll try sanding it a bit next time. Thanks!

^^ What I found out is that sometimes the slider gets warped from heat cycles and sometimes from just use. the first time I fixed a slider I was impressed on how easy it was to actually flatten it back. I think I used a piece of glass and the 3000 sand paper and it took me less than a minute to fix it. Very worth trying. Also, as soon as you start smoothing it out you quickly find what part of the slider was off, the slider would get Matt gray in some areas and still "shiny" in other areas.