Modern Vespa

I am back to tuning my Stella again to get her ready for the season.

Set-up:
Polini 177
SIP road exhaust
Ducati CDI
20/20 carb w/ cut out slide
Drilled metal mesh filter
BE2/160/112 Stack

I was tuning the idle screw since I have choppy 4-stroke sound off idle. Found that I could screw it all the way in and ride like normal without an issue. Is this due to an air leak or my throttle slide open to much?

Runs good with fine temps, although I did get a puff kind of back-fire after a WOT run. A little weird.

You mean the mix screw at the back of the carb, don't you? Not the idle screw on top of the carb?

Yes the mixture on the back of the carb.

Choppy 4-stroke idle means too rich.
Idling with mix screw all the way in means too rich.
Too rich can mean too much fuel or too little air.

Also, if the slide is open too far at idle, it will run okay above idle but the mix screw won't seem to do much.

Does that give you any ideas?

Makes sense, I'm having the same isssue you used to have and even changing the throttle slide didn't help. So kinda frustrated

What jets are you running?

You can do the following to get dialing it in back to a decent starting place.

Unhook the throttle cable from the slide.

Back out the idle screw until the screw is no longer touching it. The slide should now be completely closed.

Slowly turn the idle screw back in until it just touches the slide. Index the top of the screw with a permanent marker. This is the fully closed or zero position.

Now turn the screw in between 2 1/2 and 2 3/4 turns and memorize this position. This is the point where the transition port is starting to open and will begin to bypas the idle port that's controlled by the mix screw. Past this point and the mix screw will stop having much effect, i.e, it's past idle. Therefore, you're going to want to work on achieving a satisfactory idle before it opens this far.

Turn the idle screw back out a bit to somewhere between 2 and 2 1/2 turns open. Around 2 1/4 gives me a good starting place.

Reconnect the throttle cable and make sure there's sufficient slack to not pull the slide open in spite of the screw. You can readjust the cable slack later once it's dialed in.

Set the mix screw to somewhere between 1 1/2 turns to 3 turns and start it up.

It's important to learn the feel of when the mix screw is too far in or too far out. When it's just right, the engine will idle evenly (or nearly) at around 1100-1300 rpms.

As you turn the mix screw slowly in, the engine will increasingly feel choppy and start chattering until it begins speeding up and abruptly dies from lack of fuel at the idle position.

As you turn it slowly out, it will feel like it's idling "heavier" and much slower. Here it is actually 4-stroking and the spark plug will foul and kill the engine before long.

The right place is in the middle of both of these effects.

Find the right balance of mix screw and idle screw until it idles evenly, being careful not to turn the idle screw beyond idle range.

Once you find a steady idle, now you can test for the other operational parameters like cold starting or acceleration off-idle. Any change to the pilot jet will require readjusting the mix screw and idle screw to get back to a steady idle.

Just remember that for a stable tickover at idle, the idle screw can't be opened too far, and even though the mix screw affects the entire throttle range, it's main purpose is to fine tune the mixture coming from the pilot jet at idle position.

Nice write-up, pdx.