SI24/24 VS Dellorto 28mm

Hello !

I have read that the si carbs can cause heat problems.
Iam bulding an engine with this setup:
Polini 177cc, racing crank, Leovince exhaust,ported engine.
But i havent bought the carb yet.
So i wonder if i go with the 28mm(J40112000)or
30mm(J40113000) Dellorto kit, what more do i have to buy ?
eg. choke kit ?? Jets ?? Electric fuel pump ??
obviusly i need a new airfilter.
Thanks for any respond.

\Patrik Landell


Hello !

Ok so now i have fitted all the goddies… [:O])
But the PHBH28 carb is rubbing the cylinder cowl,
should there be any mods on the cowl ? Or how much
can you turn the carb (in angles) so it will be free from
the probably very vibrating cowl ?



happy you are happy!
As far as your carb set up: in wich range you feel is rich?
I think is more on the transition idle - 1/8. If is there open 1/8, 1/4 your air screw since you have a fast response trottle.
For the future you can consider a PK flywheel modified for Px, you will gain another hp and faster response. After you feel you have the right main jet or sligthly rich you can set up you ignition to 19, the engine will be fuller at mid range. and be sure you have the right spark grade (BOSH W3AC or equivalent)…and you know about the exhaust…
good luck with your new engine.[:roll:]

…remember that main jet size I give you it is a guide you have to try it and be sure not to be lean…

…sorry for fragmented answer.
conversion choke art. numb. is the right one.

for Adrian,

could you tell me please if this tuning manual you are talking (Norrie kerr) about is interesting, new ideas ecc, good photos, and where I can get one.
Sorry about my ignorance but it has been not even one year that I discovered internet and I don’t know big names and books.
I will be glad to anybody that could reccomend me some litterature.
Thank you in advance


…yes Adrian I also have cut of a fin from the 1/4 part of the cylinder using a pwk but in any case is a good idea to angle the carb if not when you are riding in two the top of the carb can touch the frame.


…thank you adrian [H]

hi curare! what you mean about the leovinci exhaust?

…and check the timing well with strobo[H]

Hello !!

No problems, your`e the man Curare !!
Many thanxs for the help !!

\Regards Patrik L


Oh man !!!

The engine started after the fifth attempt…
And is running like a god ! and what a nice sound too !!!
The stock engine is a piece of sh*t compared to what
it is now… I must try to hold my „horses“ until it is proparly
braked in Grrrrrr… I satt the ignition to 18 deg and i have a 109 main jet in, it runs a little rich but i think this is ok in the brake in period. WHERE IS THE SPRING !!! Muuuaaaaa…
This is what i have done with my engine.If someone is interested…

01)Mazzucelli race crank
02)Polini 177cc kit
03)welded and ported enginge halfs and intake.
04)28mm Dellorto carb(PX kit)/choke converter kit
05)Leovince exhaust (i know Curare…)
06)All bearings and sealings exchanged
07)Exchanged Cruciform/first outgoing gear(damaged by a VERY bad cruciform)
08 )New standard clutch/reinforced mallossi springs
09)Repair kit primary/clutch bussing/washer disc clutch
10)And so on… Thanx so very much for all help !!! \PL

[:bounce:] [:bounce:]


yes with polini much better using a 28 for both power and reliability.
You would’t need a pump. I needed one with this kit only after going longstroke and reed.
I don’t like Leovinci with Polini 177. Leovinci better with Malossi 166.
Get a Simonini if you want to stay in this price range.
If you buy a Polini dell’orto 28 carb, you need just few main jet
2bigger and 2 smaller then the one it is standard so you can set up.
Then you need a choke conversion, and would be useful a cable tube 90 degree for the carb and don’t forget a new complete trottle cable.



if you buy the polini dell’orto 28 I think you have a 105 main jet, but … I can’t say 100%. If somebody here remember better then me please confirm or correct.
Slow jet I am sure is a 55.

To start try this:

slow jet 55 (it is standard) and adjust air screw.
main 108 (have a couple of bigger ones since it is starting getting cold and need bigger main then when warm)
needle clip position 1

as far as the choke matter I didn’t understand but:
if you want to keep the lever choke in the original position you have to buy a choke conversion, if not you can stay with the lever on the carb.


Hello !!

I found the choke kit for a PHBH carb (40320000)
I think this is the right one if i want to leave the
choke lever att original position (chassie)

Ok so i will buy main jet like this 102 (std 105 in carb)108, 109, 110.

I didnt understand what you mean with clip at position 1 ??
Is it the first position from upp or down position.

I hope i dont ask to many „obvius“ questions…
Its better to be safe than sorry ;D


…in all the cases that I know of course using this carb.[H]

Norries manual is a great guide and a fantastic read .  you can find it in pdf format on the web.  Any and all Vespa owners whether they want to tune or not should read this.  His book has been around for along time but no better out there.



these people want you to spend a lot on fuel. si carbs dont cause overheating, thats caused by idiots that dont know how to build an engine and set up a carb. use si24 or si26 for good result. delorto phbh28 is a shit carb.need to know more ring the doctor at revival scooters


you can angle it up to 40 degrees.