Modern Vespa

Getting there. According to FMP, the spacer gear can face either way.

The spacer gear I removed is thicker than the others I have. The one I removed from my P200 has little wear and is thinner as is a spare Stella spacer. I think the thinner spare was the one the PO was running with this clutch.

So I put the clutch back together with some gently used pates that were not hammered. Going to use the thinner spacer gear.

From my reading from various sources, it looks like the clearance between the clutch cog and center has to be between .2-.5mm in order to engage/disengage properly and give enough range of motion. The spacer gear thickness becomes more critical when running a bigger clutch because it can foul the cover. It did measure out within this range.

All of this might not have been the cause of the grabbing. Could have been improper seating of springs, preloading the clutch with overly tight cable adjustment or even short, hard rides in freezing cold weather without much warmup. Clutch was fine in fall weather. I'm also going to change the gear oil and get rid of any debris.

A better clutch may also be a good idea. This bike is no screamer, but has a lot more power than my old stock 150.

chamfer towards the crank, otherwise they can crack

Thanks!

Kind of a good day for this kind of thing. Snow day for me and the kids (I work in a public school). Warm enough to work in the garage with the door open.

Reinstalled the clutch. It's great now. Thanks for all the help.

Uploaded a video of WOT in second gear. I shift to 3rd at :36. Still running the 119 MJ.

119 sounds fine. May want to slightly adjust it later but ok for now.

Now to get onto the rough running. Remind me of the full set up of everything. Stella with reeds? Kit? Carb size? Filter? Autolube? Pilot jet size etc.

It's a stock Stella reed block with Boysen reeds, 20/20 autolube carb. It has a Polini 177 with 60mm Mazz crank, 22t clutch cog and SR2 exhaust. Current jets are 30/140 idle, 140/be5/119.

Thanks!

Air filter? Drilled?

That rough running sounds just like mine currently. Interested to see where you end up with it.

Yes drilled stock Piaggio air filter, which was on it when I got it. The plastic cup filter under the seat has also been removed. It also has a slide cutout…not sure which one. Maybe a #4. It's a Spaco autolube carb.

MJ 119 sounds good now, but you will have to go down a little when weather warms up.

Are you certain about that 30/140 idle jet? I've never heard of such a thing and it would be crazy lean if it actually does exist. Certainly would explain the hesitation I'm hearing when you're first getting underway.

I'm not hearing the off-throttle four-stroking you've mentioned. I'm especially interested in that because I get it too. Clears instantly when I open the throttle again, but still annoying.

In that vein, it is interesting that you have a throttle slide with a cutout. My Stella still has the stock flat bottom no. 7 slide. Was thinking of trying a no. 1 (smallest cutout), but your experience suggests it won't make much difference. Also, the SI 20-20 slides available now all have pockets on the top. There is a lot of conflicting information about whether those pockets actually do anything, but I am reluctant to add another variable to the mix.

Oops, 40/130. You might not have heard the 4 stroking because I rarely had it below 1/2 throttle during the video.

That makes more sense. The 40/130 was the stock Stella idle jet. Actually worked pretty well on a stock jug, even with the typical exhaust and air filter mods. I worry it's too lean, though, once you start bumping up the ccs.

It may well be. I tried it out awhile back when I started playing with jetting when my understanding of rich and lean was upside down.

Some other observations:

I never try jets with the air filter off because it runs so much better through the first half of the throttle range without it.

With an undrilled filter, it would run like crap with the current jetting. It ran ok with this filter, but much leaner jetting (160/be3/112-115) and didn't pull 4th very well.

Btw, here is the slide that is currently in it.

The number 7 slide is usually better. That one you have is an LML type and not necessarily better.

Let's try to get the pilot a bit better. You have any others? 48/140 would be nice to see what happens.

I think the richest one I have at the moment is a 55/160. I can also try it with the #7 flat slide that I have on hand.

Great. Fit the #7. Try the 55/160 and we can see what will be needed next. This won't be the jet but will be better.

Does your scooter have a rev counter?

Once the slide and jet are fitted. Open the mixture screw 4 turns. Up the idle screw to near max. Hopefully idling, smoking and rich. Turn the mixture screw in until the rpm picks up. How many turns?

Will do. Which pilot jets should I order? Yes, I have a rev counter and cht gauge.

Thanks.

See where the mixture screw ends up on the 55/160 first. Pilot jets are not cheap.

Right, one step at a time. I'm going to try the pilot first, then the pilot with the stock slide, just to see the effect of each one.

Thanks again for the help. It's like teaching a man to fish. One of those skills you need to know. It will help me set up my bus carbs too.

The clutch is sooo much better now. I definitely had it too tight. The 2 mm gap at the lever is not a great way to adjust. I just left a little slack in the clutch arm and it's great.

These are the pilot jets I have on hand:
40, 42, 40/120, 40/130, 55/160, 42/160.

I changed that slide out to the cut in slide that is stock on the Px series. It originally had the flat slide and still ran with low end 4-stroking. It was a pain to get that slide funny enough.

Try it but I ran that in recommendation from others on here when I originally had the 4-stroking.

Yeah…just taking another crack at it. I'm not that worried about getting rid of the 1/4 throttle 4-stroking. I would be happy to smooth it out from 1/4 to half. The way you had it jetted was the best it ran. I think the seize was a result of the Stella crank reaching its 5k mile life span.

Just throwing this in. This is probably the best description of the difference between idle jets that I have found:

Description
The side jet of the SI carburettors does not have the simple function of a conventional jet, i.e. the pure quantity regulation of the fuel quantity. With the introduction of the PX 1977 the SI carburettor type and its side jets were modified. The old carburettor types, such as those still used in Rally engines, had a separate air duct for the side jets. These were therefore equipped with nozzles which could only regulate the amount of fuel (simply named nozzles, e.g. Rally 200 ND=50)
DOPPLE FUNCTION
The new carburettor types have a closed air hole for the side jet. Piaggio has been using mist nozzles with a double function since then, recognizable by the double designation (e.g. 55/160). For this purpose, the side jets are hollow drilled from above and thus themselves ensure the appropriate premixing of petrol with air.
This means that the mixture preparation in the side jet area works in the same way as in the main jet section via an air jet and a main jet, here only combined in one part.

AIR Nozzle VS FUEL BORING
The air nozzle comes into play as a corrective element. As the engine speed increases, the engine receives more and more fuel from the fuel hole of the air nozzle. The air nozzle then mixes in air proportionally to keep the ratio of air to fuel always linear.
A 120 air nozzle has a bore of only 1.2mm, thus provides very little air and is greasing accordingly. A 160 air nozzle provides a lot of air and is therefore rather lean.

example case br /> A 200cc engine should have a fatter side nozzle. According to the old system a 52/140 was chosen instead of the standard 55/160.
The nozzle with 52/140 (2.69) has a smaller hole for the gasoline (52) compared to a 55/160 (2.91), but at the same time a smaller air nozzle (140). The smaller air nozzle greases massively from the middle speed range, but the smaller petrol hole becomes thinner from idling. Thus one receives a nozzle which behaves more leanly up to the center of revs, but greases clearly afterwards. This means that a ringing in the middle of the rev range can usually not be cured with such a nozzle. A 58/160 would be better here, but such sizes are only covered by Pinasco with nozzles that are only available in expensive sets.

INFLUENCE RANGE
It is important to know that the influence of the secondary nozzle extends over almost the entire speed range. At higher speeds and full slide valve stroke, the proportion of mixture of the secondary nozzle in the total mixture is relatively low but still clearly measurable.
If the proportion of air nozzles is reduced (i.e. instead of 160 a 140) the engine will run richer on top and vice versa. Likewise, with a smaller gasoline hole in the side nozzle, the engine will run significantly leaner when pushing and accelerating out.

What a thread Orwell! Took me a couple of weeks but read it all, a great resource for me and my current project 👍🏼

Glad you enjoyed it.

I tried the 55/160 pilot jet, still with the cutout slide. 4 turns out and tickover turned up to like 2200 rpm.

Turning in the mixture screw, tickover only started to pick up in the last half turn in, so 3 1/2 turns in of no difference.

Is the cutout slide the #7?
With 2200 rpm the idle screw was mostly in but not all the way?

If both these correct then, good. This will mean the 55/160 is way too lean.

Trying the test again with the #7 slide.

Some notes: The #7 slide completely changes what the screws do. I had to adjust the idle speed screw way out to get the idle speed down. The mixture screw now seems to actually have an effect on the idle.

Still sounds like the 55/160 is too lean. How many turns out?

I didn't have time to do the 4 turns out test, but I will tomorrow.

After putting the #7 in, I couldn't get the idle low enough by turning out the idle speed screw, or the mixture screw. I think going back to the flat slide richened it up, but it's still lean.

I tried the test again today. I had a hard time hearing when the idle change speed so I slowed the idle and turned it out from closed.

The idle slows down when I have turned the mixture screw 3-3 1/2 turns out. This is with the 55/160 pilotjet, #7 slide and the fine threaded mixture screw.

Still too lean. Do the same with the 42/120 you said you had. And while waiting buy a 52/120. Once there is a jet that's rich enough, it will be time to start riding it.

I don't have a 42/120, the 55/160 is the richest I have.

I will order some richer jets. What should I order?

Actually, I think I have a 40/120, not a 42/120.

Isn't the 55/160 richer than the 40/120 or is there something I'm missing? I know the 120 will flow less air, but the ratio works out slightly leaner (for the 55/160).

Thanks

55/160 and 42/120 are the same ratio. So what's the difference? How they deliver that mixture.
Do the same test with 40/120. See how many turns it is. Then short ride at lower rpm to see the issues.

Will do.

I have been really surprised with how hard it has been to tell the difference between rich and lean and how I have had a bias toward adjusting things leaner…especially at the lower end of the throttle range.

I was sure that the 4 stroking I heard pointed toward rich. It seems like such an unmistakable sound. I think I was paying way more attention to the sputtering sound than how the bike actually felt at low throttle:

Very little acceleration within the first 1/3 of the throttle range, then suddenly taking right off.

It was only when playing with the mixture screw yesterday that it made sense. It was probably the first time that I found the point where the mixture screw actually slowed down the idle…which was too far off from the starting point for it to be the right jet.

In my limited experience with automotive carbs, the transition off idle can be hard to get right. When taking off, a stumble or a flat spot can feel similar whether too rich or too lean.

You're not alone. It's just not obvious. A bit rich or a bit lean have almost the same sounds. A lot rich or a lot lean is very different.
With a bit lean it's all revs and no power. With a bit rich it's slower to rev and still no power. Listen carefully and a bit rich has a deeper note.
Always easier to go from way too rich down.

Ok, tried the 40/120. Turning the mixture screw out from closed, the idle slowed at 2 1/2 turns out.