Modern Vespa

Im new here from Sweden and new on Vespas. Since mars Ive had three Scooters: 1961 GL with PX200 engine, Polini 210, 28mm Delortho.
A 1998 green PX200 and currently a 1966 sprint with PX200 engine and Polini 221.

I have problems with tuning my engine. It does not seem to like more fuel than the jets in it now, I dont know the english words to explain how it runs but it kind of misses or jerks from idle up to 3/4 throttle. Not mutch but very annoying.
I have tired a lot of jets.

Polini 221 (BGM 60mm crank)
BGM Si26/26E faster flow carburetor
Original airfilter with the two holes
Idle jet 50/140
AC 150
Atomizer BE3
Main jet 138 (four strokes from 140)
BGM cdi
Ignition 19°
NGK B9ES
BGM Pro Electronic 2300g Flywheel
BGM Big box sport
BGM 24T superstrong clutch

AC120 BE4 130MJ 55/100 will be closer to correct.

I have tried 45/120 and ac 140 but it got worse. Maybe the BE4 is the key to make it worke.

All jets are way too lean. 45 pilot and AC140 are stock for a 6 bhp 125cc.
Needs to be much, much richer for a 221 (even a polini).
The jerky stuttery feeling is because it's way too lean.

Is it that ritch jets that I should aim for, even with original airfilter and box and box exhaust?

Yes. When actually rich you will know. For this if you run a filter it is better not drilled. And if you adjust the jets with a filter fitted, you must keep it fitted.

The idle jet fuel orifice sizes you're trying are too small for that bike.
Don't go by the equation.

Get a set of idle jets (if you don't already have them) and try 60-55/160.

Do the same as the MJ. From rich to lean.
If you can't dial-in smooth acceleration up a mild hill from a rolling stop in 2nd gear by adjusting the mixture screw 1-5 turns, try the next one down. You can do this without the filter/cover, then re-adjust the mixture screw with them back on.

I have a 55-160 but isnt that leaner than a 45-120 or a 52-140?

All way too lean

Read the description here:

https://www.scooter-center.com/en/idle-jet-set-bgm-pro-dellorto-type-9892-si-58-60-62-65-68/160-bgm857kt?number=BGM857KT

Mostly recycled old info from the 70s

What is incorrect there?

Nothing, it's all correct, just incomplete. Since then, a stock PX125 uses a 45/140 pilot, a stock T5 a 50/100. Back in the day the curve of the 160 was all that was needed. As cylinders got beyond the big bore kit, with no real performance era, the 160 is too lean. And with today's kits, they need as much grease as possible. Even at maximum, using every richest jet/AC, an SI carb can't keep a very high performance cylinder cool.

I was taking my BGM Si26 faster flow carburetor apart to clean the passages with compressed air. I found that the float bowl passage is only 1,8mm in these carbs. I thought 2,5mm was the size to aim for in this area?

You can easily drill that passage out to 2mm, which would be better for sure

I did, but 2,5mm.

Thanks for that link. All bits of info adding perspective and better understanding of the SI carb.

https://www.scooter-center.com/en/idle-jet-set-bgm-pro-dellorto-type-9892-si-58-60-62-65-68/160-bgm857kt?number=BGM857KT

Interresting, but most dyno results for a Polini 221 I see have the smaller (120-140) AC and idle jets. Can someone show or tell me about dyno results in the 160 range and the exact jets used.
Best would be with box exhaust.

I want a reliable setup, not looking for the last possible horsepowers here.

And I hear you Jack, just curious to see some results from the other way of thinking.

So I have tried AC120 BE4 130MJ 55/100, and a hole bunch of other combinations. I have a lot of mj jets, atmizers, idle jets and ACs.

Have tried drilled filter, undrilled and no filter. Took the carb appart, cleaned and used compressed air in all jet passages.

Still, the engine kind of spits, or jerks, maybe fourstrokes when I ride in town at low throttle. Runns well when I pull the throttle, even just a bit.
WERY annoying.

The setup exept jets is in the first post ⬆️
Im interested in more examples of jetting and tuning Polini 221 with si26 carb and box sport exhaust.
What am I missing here?

You won't accidentally stumble onto the exact correct jetting. It has to be adjusted one part at a time, in a certain sequence.

With higher performance kits it's very difficult to get them rich enough. Your set up is possible but near the limit.

First 2 parts to adjust are the AC and MJ. Holding wot on the stand, keep going richer on both jets until it won't rev. When it's wot with 6000rpm max and smokes like a ship running on crude, you have progress.

Thanks Jack! Will try a bigger main untill it runs like you say.
Running better now though,
120ac, 50/120, 132 main and what made a big difference is BE5. Better than 4 and 3.

Would a 65/160 be better than the 50/120 or should I stick with it?

Great it's getting better. Once decided on the final AC and MJ, then worry about atomisers and pilot jets.

I cant go richer than 50/120, 140ac, BE5 and 125mj or it will jerk/four stroke at low throttle riding i town.

Is it better to go 50/100 and 120ac and an even smaller than 125mj?

Its willing to rew with a way bigger mj than 125 on the stand but it seems impossible to get rid of the jerking/four strokeing with a bigger mj.

Have a lot of jets, even the 68-55/160 ones.

You need to find out what main jet it will go up to on the stand. When you know the number that is too much for wot. We can work out what to do next. The lower throttle can be fixed later.

Your bike's "range*," from Germans with afr's, cht's, egt's:
135-138/BE3/150ac
55-58/160

*There's no way to get an exact jetting recommendation.
Even the channels on Spaco carbs can differ