Modern Vespa

Apologies if this has been covered before. Need fasteners for an oopsclunkthud bellmouth. Threads in carb are M5. All the locally available M5 fasteners have heads that are too large to fit in the space available. O.E. front screw is a cheesehead with a 7mm diameter head. That head will fit in the front but not in the rear. McMaster Carr has these with 6.5mm diameter heads. Anyone try them? Not seeing anything smaller.

No takers?

Anyway, I went ahead and ordered the McMaster Carr cheesehead screws with the 6.5mm heads. Delivery was promised for Tuesday, but they actually showed up today. The fit is spot on.

Can't tell how much is the bellmouth and how much is just the airfilter delete, but the change in jetting is pretty dramatic: from AC160/MJ115 to AC160/MJ128. Doesn't seem to have done much for max horsepower, though. Top speed in 4th gear is pretty much the same.

I will say this bellows-to-frame stuff is like crack cocaine. It's pretty amazing that after all these years no one has come up with a decent airfilter for these scoots. I live a mile from the beach, so something has to be done. I know it means I will be sucking hot air, but those Uni Pod things are cheap, so might give one a try.

Actually Piaggio did with the unfortunately ugly Cosa. Not that you could easily fit it to older scoots...

Are you using the Stella airbox lid with the bellows? I've always gone with the Uni/ Marchald/Polini filters with bellmouths and the PX Stella airbox lids. I'm interested to see if there's any difference?

https://www.sip-scootershop.com/en/product/carburettor-casing-cap-with-oil-pump-sip-airbox_249114G0?usrc=Air%20box%20tall

I dohave this still in the packaging next time I try a bellmouth.

Yes, Stella/LML airbox lid and bellows to frame. Feels so good, but I know it can't continue. Local conditions demand some kind of air filtration.

Right on! Did you up the idle jet at all? Or just do it with the mixture screw?

None of the above. Idle jet remains the Malossi supplied 52/140. Haven't touched the mixture screw. Seems fine. Jetting will change when I slap on an air filter anyway. Will wait until then to do the really fine tuning.

The 20mm bell was designed to use the stock air filter screw with 7.0mm head diameter on the rear. the one you found is likely even better.

I've been running this filter on my Rally for many years now.

In my case, the 7mm was too thick. Would have required some modification. The 6.5mm fit like a glove.

Uni Pod filter supposed to arrive tomorrow morning. Will be interesting to see how that impacts jetting.

Uni Pod filter is oiled and installed. It must breathe pretty well because there was no impact on the main jet compared to bellows-to-frame.

Closing this chapter without having seen much of anything in the way of increased performance. Engine does sound and feel better though.

Have you ever found anything in that filter that made you think, "glad I caught that?"

Trying to wrap my head around how anything could make it there (bellows only, tray/rag under the tank on the P series/Stella)

Nothing more than dust after running on dirt roads. That use to be all over the inside of the carb box, now only under the filter.

A bigger airbox cover will help here.
I only got a day with the cheapo SIP airbox and Oops' 24mm lean-to version before it cracked.

Polini arriving hopefully this week. The idle screw can be adjusted vs the others.

It's possible to open the 20/20 carb the match the 24mm lean-to.
No need for a screw/loctite. Much easier to jet.

This is what I did on a friend's engine; angled UNI filter. I tried stuffing the bellow with filter foam chunks but it was too restrictive. The angled filter works really well even though it pains me that it doesn't look stock.

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I just picked up this bellmouth (thanks qascooter) and am wondering if I can expect any noticeable difference in my top speed or if it will be "pretty much the same ". What experience have you guys had?

I'm currently running a 122 jet (with a SIP Road 2.0) and am going to switch to probably a 130. Any thoughts ?

My situation did not need the angled filter. Straight one fit fine. Model No. was UP4229. A 13 point upjet suggests you are flowing a lot more mixture, which would seem to promise a lot more performance. That has not proved to be the case. Might have picked up 100 to 200 more rpm and 1-2 more mph in 2nd and 3rd. No discernable gains in 4th. This mod seems to be more about torque. Butt dyno is sensing slightly quicker acceleration and slightly better ability to carry speed up a hill, but I have no scientific way of measuring that.

Might a larger or smaller jet than 130 improve my top speed?

It depends on the overall air/fuel mixture at wide open throttle. If you are too rich, such that you splutter and can't rev all the way out, a leaner main jet may be faster. If you are too lean, a richer main jet may be faster. The best way to tell if you are too lean is to start out too rich and work your way down.

I say "may," however, because at some point the limitations of your set up take over. I feel like I am at or very near that point now, having squeezed about as much juice as you can out of a kitted smallblock running an SI carb and an LML reed block. To take it to the next level, I think I would need a side-draft carb and an RD350 style reed block.

Thanks for all the good information Kowalski… good stuff

I'm going to be out of town for a few weeks but will do some experimenting when I get back to Fresno