Modern Vespa

Or a timing light?

Yeah it's solid, working fine.

Stoked - all my lean slides and leaner idle jets are on the way. They made it from Germany to scooter-speed and to Scooter Mercato. One got the SIP parts and the other the Scooter Center stuff.

Let's get this puppy jetted in and roadworthy.

I also replaced the stator I stole from the Stella engine.

Lean slide in and 42/160 idle in place of the 48/160. Runs way better but still fails the 5 minute restart unless you turn off the tap for 15s before turning off the bike. Will restart 5 min later if you do that.

Have to wait again to try cold start. I tried that with 45/160 but it flooded big time. Maybe less/no choke when I try with the new setup.

Oh snap - today's cold start was the clean out the rotary pad test oil ingested cold start .

I bet the next one is better.

James Bond smoke screen option could be a plus when you go to sell!

Well that's the smallest idle jet that can be found, so I hope it works. After that I'll be cutting the main if need be. At 100 with a sito plus it seems reasonable but may be bleeding in too much. Perhaps a 98 or 96 might work. I'm going to try not to though.

That's the same idle jet I was running in my Stella when it seized. It was mentioned as a possible cause of seizing. Still, it ran a lot better with that jet. Just wondering if there was something I was missing that would make it safe to run a leaner idle jet.

Different animals, with the reed valve thing in the Stella. It's clearly still a bit fat at idle, but the only way to lean it out more is to steal from the main. I already closed the plug gap to .020" but maybe I should go to a b6 plug.

Is there any chance the float level is too high? If the float is not so floaty, needle too short or seat worn/damaged etc. Fuel will dribble into the Venturi.

wrong needle? There's the "big" and "little" needles for the SI. For some reason, I've had issues ordering the big one and getting the little one.

Using the little one produced similar symptoms for me.

How do you know if you need a big or small needle?

I've had the same issue with my other carb on this bike so I'm doubting it's related to one carb body but I could exchange float bowls as an experiment. I'm game to buy both needles and a new float or even a new float bowl.

Another bit of carburation arcana. Didn't know there were different sized float needles…

Lemme see if I can get some comparative pictures. The "large" needle is significantly larger, but they are both the same height. The smaller needle just doesn't seal as well and produces dripping.

Someone else will chime in with better detail, I hope, because I may have to pull the float chamber off a carb to show the size difference otherwise.

Well - I made it to work on it.

Cold start was OK, didn't need much choke but did start without a fight with all the filter and top on.

Drives decently - it is torquey with this top end. 2nd wants to lift the front end often. No speed range on the thing, tops out so early. Gutless up hills, has to stay in 3rd. But about what I expected.

Refuel was an adventure. Wouldn't restart. Figured it was flooded and started with the whole flood routine. But in the end it wouldn't restart until I gave it some choke and turned the tap on. It seems I may have found the lean end of the jetting, which is very useful. This means we can make it work....

And now I just tried the 5 minute restart at work - flooded all to hell again.

I think a float bowl, float, and needle are probably a good place to begin.

Okay...here's what I meant about the needles...

Per SIP, there's a 4.5mm needle which is 3.7x3.7mm and a 5.2mm needle which is 4x4mm.

The easy way to tell them apart is that the rubber extends onto the shaft of the larger needle.

The 5.2mm float is the only one that works without leaking similar to your situation in my SI carbs.

Anyways, that's what I got. Like I said, I don't know why or the origins, just that's what's worked for me.


4.5mm / 3.7x3.7mm

5.2 / 4x4mm

new float bowl and float on the way from mercato

new 5.2mm float from scooter-speed, as it is unclear what is in the mercato basket of floats...

Just went through this.

Why that scary-lean slide for starting issues?

What can happen with a needle seat leak (even with a working tap off) is there's still enough fuel to get a small leak into the venturi. 5 minutes isn't enough time for it to evaporate. In this case it isn't a hot/cold issue. Starts right away cold, since the leak has evaporated.

I'd suggest just letting it run 15 or so seconds with the fuel tap off before killing it. But as you're building this to sell, try a spring-loaded needle with a viton tip and see if if you have luck. Should be 4.5mm(?).

You could also test what you have in this manner:


Bench test.

The 5.2mm needle is for the "Cosa" float top. All others are the "normal" 4.5mm. I only use the spring loaded needles...

well good thing I have a spare cosa top in my stash

I like the float check test there. I'm wondering if it has to be gasoline for the test. I'm thinking water would work fine. I'm also wondering when that hose goes upwards, how much "tank" they used to create the pressure and how high up it was.

I chose the lean slide to make it run right. It would just bog and run like crap. The slide is making a difference on pickup off idle, which had been kind of a mess. All of which might be because the float needle is leaking. I have another lean slide that has no cutout on the back but has the indents on the front. In the end, if the float needle is leaking all the choices will need revisiting, but now I have the right parts to play with.

5 hr soak and restart = flooded. Did not turn tap back on, started after ~10 kicks with WOT hold to clear the flood. I think we are onto it with the float bowl / seat / needle / float idea.

Had the same problem with a brand new Spaco SI26 (actual 25) on my blue P. It was peeing itself, emptying the whole float through the main jet. Same with tap to OFF. Never managed to fix it whatever I tried (new float, needle, etc.). Replaced it with a Spaco - Pinasco SI26 (actual 26) and all my woes were over...

I used water. Just poured it into a 10" hose with a funnel.

If you use a blood pressure gauge with your leak-down kit, you can turn the float/needle/cover assembly upside-down, lay it flat, and see if it holds 200mmHg.
Didn't end up trying this.

Testing a spare float chamber. Marked the water line with scotch tape and waited for 5 minutes. No change in level.

Next up this one on the bike and that one on the tester….

I got you you bastard.

You can see the drips from the float.

Yes! I love the fact you did this so much.

the cinematography leaves much to be desired but it's hard to hold the mustard bottle and film.

(insert that's what she said joke here)


all kidding aside, that's awesome John. it must feel great after all the aggravation it's put you thru!

I will try to fix it by cleaning up the seat. It's a Dellorto float bowl, worth a try. Toothpaste, q-tip, and a drill coming up.

After Voodoo turned me on to that truck, it's become standard maintenance when a carb is apart being cleaned.

It's really cool you nailed this down though. I've never taken it that far...

I really want to sell it

Nah even more than that I want it to work right. I can't leave it like it's been running.

First try around the block and 5 minute restart with different float bowl flooded.

Found I hadn't secured it well, and the top circle part wasn't tight either.

Fixed that and was able to do the 5 minute restart.

Upped the pilot from 42/160 to 45/160, closer to stock. 1.5 turns out (Dellorto). Restarts fine and feels better. Short drive confirms. 5 minutes later, still restarts. 5 minutes more and still restarts ok. All with tap on, never turned off.

Fingers crossed, we might have it.

I'll wager that whatever this shit is is the problem with the float needle seating. I cleaned this carb before using it. Gunk from sitting??? I don't know.

Fatberg? Don't dump grease in your gas tank?

Seriously, WTF is that?!?!

Maybe that's what happens when a scooter and a small pickup truck get drunk and make bad decisions.

That could have been your big shot at owning an Ape right there.

oh wow! that's gnarly.

ethanol fuel with an additive package reacting with water or oil? possibly a cleaning solution?

what does the tank look like?

I had ethanol fuel gun up a carb. It was like it had been filled with moisture absorbing pellets, only made of Vaseline.

That looks like lithium grease.

You said you swapped it, so it was known-not-gnarly at some point?