Modern Vespa

I think the small hole refers to the main jet. The big hole is the passage you are drilling.

The ID of that hole is 3mm or so. So yes, it will look like that. What size bit did you use?
If you can see the tip if the bit with a flashlight down the main stack bore you're through.

Remove the idle jet and blow out through that bore first!

You can make things much easier going forward if you cut the mixture screw at the red line below, and cut a slot for slotted screwdriver turning.
Will also allow you to fit the rubber plug when you're "done" jetting

Ok got it. I'll finish drilling it through then. I was a little confused with the 10 mm and big hose, small nozzle so was thinking it's not all the way through

It's all good. Better to stop short and ask than drill too far and cry.

Ray, I used the 2.7 mm bit you sent the link to. Just need to finish drilling it through. I'll blow it all out starting with the idle jet hole.

Chandleman, exactly I have a tendency to drill first and ask questions later and I end up breaking things so I'm trying to try questions and patience now

The nozzle is your main jet.
You're drilling the big hose.

You should feel much less bite resistance when you're through, which should be VERY close at 10mm in.

Great and useful topic !
Thanks

Is there any advantage to drilling it larger than 2.3mm?

I think I got that size because it's a common SAE size for drill bits.

No panic. 2.3mm is the ideal size. Bigger has the risk of having thinner wall thickness on a non optimal casting.

So after drilling it out I put a 130-be3-160ac in there and it's still starving at WOT. Should I keep going with an even bigger main or start messing with the air corrector or even the mixer tube? The sequence in this thread seems to say keep going bigger on the main first but 130 is super big right?

First off, drop your air corrector to a 140. And remind us what your setup is. 130 main is a little big for a mild tune, but not out the reason of possibility. Also, did you pressure test to make sure you don't have an air leak?

I just went through this. I didn't drill the hole all the way through. I swore I was through… but wasn't quite. You can back the whole main stack out a turn or remove the main jet to see if it will sputter rich like mad. If that doesn't get you enough fuel, then it's time to look at other possible culprits.

I have a 140 coming in the mail.
I did a pressure test a few months back and it was tight. But worth another test. When the 140 comes in I'll update

I hear you but I could see the end of drill bit in the main jet hole. Pretty sure I'm good there. So you're saying if the main stack is turned out a little it will make it extra rich? Just as a test?

You should just remove the main from the stack entirely and run it with just the AC and atomizer. that'll cause the float channel to effectively *be* the main jet. so if it's all the way through, you'll have a 230 main. If it's not, you'll have a 125.

I tested this by drilling out an old jet with a drill bit to make a stupid big main jet. The scooter barely moved because it was so rich. All it told me was that fuel flow was not restricted, which was all I needed to know at that point. Removing the main jet entirely does the same thing quicker.

When you screw in the stack it bottoms out on the tapered tip of the main jet, doesn't it?

If you leave the main jet out what would it bottom out on?

The threads of and for the air corrector?

Not super big. From the chart I linked to, most ride with jets in the 130-140 range.

Do try a 140 AC if you have one, especially if you're riding with a drilled filter.
The heart above the jets is also an air corrector.

I needed to go to a 138 MJ to get to the point of too rich (on a 187cc). Not surprising at all that a 130 MJ wouldn't get you there.

Pressure tested and it's 150psi.
My setup is a 79 rally with a 210 malossi kit ported with the sip 2.0 pipe. I now have a 132-be3-140ac on there and it feels a little better but sputters at 3/4 throttle and wot

Plug looks like this

That's your compression test. He was talking about crankcase pressure test to check for any air leaks which could come in play here, so more like 5psi. Look here for more info: Pressure test question

Oh great. Guess I'll buy more crap to test more things. This is the life.
Would the fact that the plug seems dark and still starving at 3/4 throttle support a leak? The bike idles and accelerates perfect before 3/4

You're fuel starved. You need to drill out your float bowl passage to at least 2.5mm. Stock, SI carbs can't flow more than a 125 MJ worth of fuel from the float bowl to the jet stack.

I did drill it out. 2.7 mm hole. And I took off the main jet and put just the air corrector and mixer in there and it idles but when I hit the throttle it bogged down hard and would die.

Have you tried a 140 ac?
Do you have an un-drilled filter?

He said a few posts up that he's already on a 140 AC. Probably should try no filter, just bellows-to-frame next.

The plug will color quickly with a rich pilot jet, unless you kill the engine and coast after a fixed throttle position ride.

If you have an air leak, rpm's will dance (hunt for fuel) and not return quickly to idle at operating temperature. CHT won't settle under load.

Was this the same experience with your previous carb?
What ignition are you riding with?

Missed that.
I'd try both. Easy enough method to provide more clues.

To all that have posted in this completely awesome thread, thank you thank you thank you! After 35 or so years some basic stuff now makes more sense.

Thanks!

It was supposed to be the content discussion for the Wiki article, but it's taken on a life of its own since then. I need to go make some updates to the wiki where things were unclear or could be expanded based on questions in this thread, so it's adding value either way.

You can hear what it's like in the first 2 min here.

I didn't mean to hijack the original post. Just had some questions and it kept going.
I have no hesitation returning to idle after throttling. The idle is perfect (55-160).
Just to be clear pilot jet and idle jet are the same? People are using different terms.
My rally is a 1979 so it's the Ducati electronic ignition.
I started going the opposite direction and using leaner jets. I'm now down to 116-be3-160 and it's running lean but the hesitations at wot are less for some reason then when I was 130-be3-140

I actually thinking my fuel line might be clogged a bit.

I'm done hijacking the thread. Just wanted to give an update in case anyone's wondering what the deal is.
Built a leak down test and i definitely have a leak so I'll investigate further on that.
However. The hesitating at wot is fixed. The spade connector going into the cdi was a little loose where the wire crimped into it. When the bike was at wot the connection was too weak to keep it consistent. Now that it's tight theres no issue there

Just our regular reminder that 90% of all carb problems are actually electrical.

Glad to hear you're back up and running. Now get that leak fixed!

And a reminder that XX% of "air leaks" are the tester setup.

An air leak would not improve with leaner jetting.

*Leaner mix is more ignitable, so it makes sense that things would feel better riding with an electrical rev limiter.

Main jet tuning question:

Can anyone clarify a bit on what it means to "Rev out cleanly"?

I'm trying to find my main jet by revving through 3rd. I find that I'm gurgling most of the way there to above 3/4 throttle even on WOT.

However once my RPMs get up there, everything cleans up. And once I'm into the power band the scoot takes off and feels wonderful. Then I can wind up and down and it's got crazy pull.

Would you say that is or is not revving out clean?

If I drop the mj a few points I can rev all the way to WOT in 3rd without a gurgling. Would that be revving out clean instead?

Good question. I am still trying to figure that out. The way I understand it, the whole rev out in 2nd gear means that the too rich jet you are looking for should still splutter/gurgle at maximum revs, even if intermittently and not really clean up when the revs are at maximum. To get the feel for it, I drilled out an old main jet to be ridiculously large. Sometimes it's hard to tell rich vs lean rough running.

I usually take short videos of my WOT tests. If I get one of a bonafide spluttery WOT test, I will post it for the good of the order.