Modern Vespa

Hey guys,
Some notes:

Took off the filter and switched the ac from 140 to 120. "Feels" really great.
I'm assuming that means I was rich(?). Cleared the slight hesitation at 1/3 throttle.
No plug chops yet. VMC's mj spec is 125-130, so assuming I'm on the safer side. Idle adjuster turned out kills the engine, so assuming yet even more the idle jet is rich enough.

Turns out that ride I posted was with ignition timing retarded 6 degrees! How it rode so well I have no idea. Set to spec (19 btdc) and the front wheel lifted in 2nd. IMO this kit, plug and play, is not leaving it in want of anything. Sounds and feels so great.

Keep thinking that not port matching and keeping all the original parts on the shelf might leave someone the chance to easily make it stock again. If we lived on an Island in Greece the stock bike would have its own charm. It had a bullet-proof feel to it, if that makes sense.

I ordered two more things.
23 tooth BGM clutch for a 67/68 primary. Step up from the 22.

https://www.scooter-center.com/en/clutch-sc-largeframe-type-cosa2/fl-for-primary-gear-67/68-tooth-vespa-px80-px125-px150-t5-125cc-cosa-sprint150-rally180-gt125/gtr125-ts125-gl150-super125-vnc1-11001-super150-23-tooth-sc6823?number=SC6823

KR Automation's CNC shifter box. Got spoiled having that guy on my Allstate. No going back once you've tried it, IMO. Actually a safety item, since jumping into gear with that level of power can be really bad. That's what I told my wife, anyway.
It's available through SIP and Scooter Center, etc. But here's a link to their website. It has to be the sexiest engineering website out there.
Scroll down to their gear selector:

https://1537118469.jimdofree.com/english/vespa/lf-engine/

If all you did was change from 140 ac to 120 ac and it felt better.. that would mean you were too lean and went richer. But if you also had timing off and changed that, that would affect how it feels when running as well.

I removed the air filter.
Took a long time to find the 120 ac in my cup of brass bits.
Actually felt better with the 140! Tempted to put that guy back and not tell Jack

Factory air filter is a restriction so that helps, then switching to 120 AC probably helped richen it up a bit also. Still have the 130 main jet in?

Yes.
130/Be4/120ac

130MJ seems a bit big. What size main jet made it splutter at WOT in 2nd gear?

Good to know that it works just stabbing it in. In FMP's pnp video he seems to just match the openings of the ports rather than widen them aggressively. Thanks for sharing.

132-134 with the carb filter. 130 with the 120ac no filter this morning.

"Feels" like I'm really close with the 130 and 140ac I just put back. Smooth throughout.

Is there anything like a formula in terms air corrector and main jet changes?

As mentioned in one of my recent posts, one of the White One Racing guys has a video where he says the rule of thumb is 3:1--going down 3 points on the AC is equivalent to going up 1 point on the MJ.

Have you been testing at wide open throttle yet? That AC140/MJ130 combo is probably a bit rich for a 177. Would be surprised if you couldn't get that to splutter.

Good info! So if I understand this correctly:
- going from a 140AC to a 120AC is almost 7 points
- so a 140AC/130MJ to a 120AC would be a 123 or 124 MJ?

Yes, that is what he said. It didn't work for me, but I suspect that was because I was trying to jet in with a BE4 mixer tube, which caused me all kinds of anomolous results. My bet is it would hold true with the usual mixer tube suspects like a BE3 or BE5.

Excellent - thanks for the info Kowalski

That just doesn't seem right to me. If you do the math on a 140ac with 130 MJ the ratio is 130/140=0.928. Switching to 120ac and keeping the same ratio of air to fuel gets you 120*0.928=111.428 or either a 111 or 112mj. So that's 18-19 main jet sizes difference from going 140 to 120 ac. Which makes sense cause 140-120=20. But given they don't have ac in increments that small, you could say a change in one ac size from 140 to 120 is equal to 20 MJ reduction to keep the same fuel to air ratio. Or 20:1 in their way of describing it.

I don't really know. As I said, it didn't hold true when using a BE4. On the other hand, if there is one thing I took away from that experience, it is that there is more to the fluid dynamics involved than just the size of your orifice.

There most likely is more to fluid dynamics then size of hole, but over the past summer I did a fair bit of swapping air correctors and main jets to fix the jetting in my tuned Stella and this methodology seems to be very accurate for me. If I start with a 140ac and 130 MJ and want to go to 120 ac because I don't want to use a MJ size that large, using that math to find the equivalent MJ on the new 120ac and it never fails to line up within 1 or 2 main jet sizes in terms of matching same feel of running and same plug looks when pulled.

Will not splutter with that combination and no air filter. I tried again.
There's a 1/4 mile section of Sunset Blvd where it seems anyone wanting to open it up opens it up there, anything from a moped to a Dodge Charger to a Tesla. Should have recorded that ride.

Pack of plugs to chop arriving in a few days.

If it won't splutter at WOT in 2nd its not safe. As you have splutter with an AC120 and 130MJ. This is now a fact. And guaranteed MJ combo not to seize, or do any accumulative bore damage.

As said by all AC140 will take much more MJ to splutter. Probably about 145MJ maybe not at all.

Back with the AC120, no filter, box top, bellows on, Two sizes down on MJ should be about right. 125MJ for sure, maybe 122MJ.

Running lean is addictive. Nice smooth, zippy, feels great. Rips grooves in the bore, blows up. Waiting for the tow truck.....probably not the first time for some readers

Still the case?

Not yet.
Drilled, upgraded carb ordered from SIP stuck in transit. Now at Charles de Gaulle(?), according to Fedex.

Still confused here about how drilling a 2mm hole for a 1.4mm or a 1.2mm nozzle could be safer.
In my primitive understanding of fluid dynamics a larger hole makes sense, since a garden hose nozzle performs better with larger diameter hose at a given psi.

Just not getting the danger in not doing so.

What happens is that when the motor is not drawing more fuel than the passage can deliver, the jet chamber will stay full of fuel and your mixture is good.

When you run at WOT (or any fuel demand that exceeds the passage's ability to keep up), eventually (which can be anywhere from a minute down to a few seconds) the motor will lean out just when you need it not to and cause a seize.

Maybe a silly question?
When the hole for the Fuel-bowl is drilled out, do you need to reset the carb settings (idle and mixture) and jets?
I would think that the higher flow of fuel to fuel bowl would only fill the fuel-bowl up quicker and the main-jet could stay the same?

Yes and no. Low settings are similar. High totally different. And if not is too near stock to need drilling.

You are correct if passage wasn't a restriction to flow, nothing would change. The main jet should be the restriction. What people find is passage under nothing more than gravity feed can't keep up. As Jack said unless your running a stock motor or very close, then everything works as designed.

I watched a video on YouTube, who I can't remember put a 160 main jet in his SI carburetor and it ran fine. Then drilled the passage and it wouldn't run on a 160 main jet it was to rich.

now I want to find that video

Thanks!
I read on a German wiki that a tuned bike with main jet larger than 135 needs to have the hole drilled.

I have a guess as to why one needs a 2mm+ hole to feed a 1.35mm main jet throughout the rev range:
Larger(1.5mm in stock 24 Spaco carb) hole feeds the 1.35 jet by gravity. The venturi effect pulls fuel via the main jet. At WOT the relationship changes, since the static gravity-fed passage can't match the increased draw from higher revs. Am I nuts?

Will do, as reluctant as it may be to give up what feels like a perfect setup with the 140ac

Btw, by "feels" great, I mean it feels (or better, sounds) nice and rich. I don't like a lean feel/sound. That's just scary

After a WOT uphill ride the plug looks nice and rich, for what it's worth. Not a fresh plug but engine shut off at uphill WOT.

Moving on with the project.

Installed the KR Automation shifter box. I went over that experience on another thread. Gears still not landing as well as my other px. I'll keep at that. It's worth it, guys.

Installed the BGM 23 tooth clutch. Very happy with that result. Feels great, smooth throughout, yet torquey(?).

Three things need to be done to fit the Cosa style clutch in a stock 125:
There's a tab that needs to be broken off ( see pic) in the case. It could be dremeled, but I saved the mess by snapping it off with a wrench.
The clutch cover needs to be dremmeled (see pic) to fit the larger clutch basket.
The stock round push-plate that clips on the clutch won't fit into the semi-circular Cosa style clutch. I could have waited and bought the correct type but I just dremmeled mine. It's easy enough to replace later.

Had to drop the engine, which in this case meant removing the hoses, carb, and disconnecting the shock. Stupid castle nut wouldn't come off without an impact wrench, and that won't fit up in there. Tossing that nut is worth the price of the new clutch alone!

Any corrections/advice welcome.

What's going to happen without the oil trough you cut out on clutch cover? Are you going to get enough oil to plunger and plate to keep them lubricated? When I've seen people grind for clearance they keep the trough.

Not sure. I used the SIP bored-out cover they offer as a guide.

I'll be curious to see if you have clutch lubrication problems after taking off so much of the oil guide. Someone (Christopher?) just went through that after taking off too much of the oil guide.

I recall seeing a vid by FMP where he checks the clutch for lubrication after grinding the oil groove. No issues and he explained why.

I may be mistaken but I don't think it's an issue to grind it down...

I also did a dry fit by resting the clutch upside-down in the cover, with the plate and pin installed. Rotated it by hand and marked the scratches on the cover with a sharpie.

Overkill for sure, but really wanted to avoid that sound of grinding metal on the road.

Wasn't me, I just remember reading it, I think it was Swiss.

It was me. I ground down around as much as him on my p200 and it caused problems. My Stella cover I didn't grind much off the oil guides, just what was necessary and it never had problems.

Fix was to Dremel a new channel feeding into the remaining original oil guide. I did it on the p200 and it doesn't have issues anymore. Haven't pulled the cover to check the new plate and push rod yet but I'm sure the issue is fixed as it doesn't make any squeaking anymore and clutch functioning properly. Did it on the Stella cover as well cause it couldn't hurt, only help.

No issues so far.
Saw a video with a BGM clutch and clear clutch cover. Oil doesn't flow that high. Mostly swirls to the pressure plate area. Jibes with FMP's video regarding this subject.

I'll follow up when I pull the clutch. Going to switch out 4 of the springs for the lighter Piaggio and replace my dremeled-to-fit stock pressure plate with a BGM strengthened kit.

https://www.bgm-tuning.com/product/bgm8007evo-kupplungsandruckplatten-set-bgm-original-largeframe-vespa-px-cosa-rally-sprint-ts125-gt125-gtr125-super-vna-vnb-vba-vbb-gs150-gs160-ss180/

I went all-in and splurged for the original version of the SIP ball bearing pressure plate a few (4? 5?) years ago and that thing has been awesome for me. Ginch had a less-than-awesome experience with his, so YMMV, but I'd say it was money well spent.

I used to go through plungers and pressure plates on a pretty regular basis, partly due to the clutch level constantly getting stiff from road salt, partly from having a really stiff clutch and while I still had the road salt issues, I no longer had to change out the plunger and pressure plate at least twice a year as a result.

Current (final?) setup:

Carb bowl hole drilled 3/32", around 2.4mm
Braided, 8-9mm fuel hose.
No air filter.
Idle jet 45/120*
Main stack 125/BE4/AC120
*Couldn't dial-out a slight flooding at startup with the 50/120. First kick now, two when cold.

Added a BGM electronic conversion kit, and later a Trail Tech 12vac regulator to the headlight after cooking the sealed beam in a few hours.
Replaced headlight with a Wagner H5006, which fits perfectly after demeling a few nubs on the back side.

So far, so good. A delight, actually. Highly recommend the Stelvio.
Thanks all for your advice!