Hi curare, what about the jetting of my 30mm carb, I think the main jet is 115, isn’t it too low? What do you think will be ok for my setup?

Setup :

  • PX150E
  • Cylinder Malossi 166
  • Exhaust SIP Performance
  • Upgear kit Malossi 23/64
  • Carb Dell’Orto PHBH 30 with standard manifold
  • Cylinder and manifold ported.

Friendly, Mikkke[:D]

Thanks a lot Curare, your help is more then usefull, I know it’s not a air filter problem as I’ve just fit the carb and that the air come with the Malossi hose in the frame just like the original, I’ve to buy the air filter, maybe an advice for that?

I have another question, wher do you come from? Here now in Belgium we are obliged for the custom office to have the original papers to have a number plate for an old Vespa, former solution was a invoice in italian and it was ok but now I can tell you that it’s difficult for us to go to Italy and buy Vespas, just like we use to do as it’s difficult to have the original papers. Is it the same in your country?;D

Once again thanks for your answer, always useful, I hope that you go back to our homecountry and don’t you think that nostalgy will give you more love and happiness when you gonna go back to Italy? Keep on going, friend!![:bounce:] [:bounce:] [:bounce:] [:bounce:] [:bounce:] [:bounce:] [:bounce:] [:bounce:]


I have 5 engines, 2 small frames, 3 big frames.
The first time I tuned a vespa engine I was 14 and I remember well because while porting the cranckcases I made a hole!
Maybe up to now I tuned for myself, friends and customers, about
50 vespa engines. But I also love Ducatis and Guzzis.

For the race crankshaft:
a race crankshaft is a cranckshaft that has the inlet timing altered so the inlet port stays open longer. In an engine as yours that develop the full power at high rev, and with a 30 carb, it is necessary this kind of crankshaft if you want to get the most power out.
Or you can go reed.
In this case you can keep your standard crankshaft.
I know right there there are full circle crank and special lip for reed intake, but in dyno test and road test I made I can assure from experience and not theory, that globally is better you keep your standard one.



if your main jet is 115 I think it is too small with a SIP exhaust.
Last time I did a set up similar I used 125. If I remember well. But consider that was not with a SIP exhaust but with a PM. And it had a race crankshaft 57mm.

Unfortunately I can’t tell you which will be the right main jet.
But if you buy 118 120 122 125 128 130 132 135 you will not spend too much and you will find the right one. (It is the way we all do, we try different jets, so you must have a few)

Start with the bigger so you won’t risk since when you have to find the right main jet it means you have to open the trottle for few seconds to let the engine reach its max rpm and when you do it is better you are not too lean…
And check the air filter. It must be clean.
And spark good.



I experienced everynow and then a similar problem.
Twice it was a spark too cold ( 2 AC ). In this case the engine started for about 10 to 20 mn. and then stopped and no way of restart, but as soon as fitted a 3AC spark was resolved.

Another time was a cable problem so I had to check out the wiring and connection.

Another time was a air filter problem. The air filter was very oily and the engine started but after a while… I took the filter out and magic…

so as you can see the causes can be differents.
I will start taking the filter out and putting a 3AC spark. It doesn’t
take a long time. Then let me know.
Don’t be surprized if your engine will be more powerful…


Of course it helps, how many engine do you have? or how many engine did you make? Do you thing I have to order only this one or some more? Probably 2 before 132 and 2 after 132. Please ref of the racing crank? Have a good night Mikkke

All right!! Let’s see!

…another thing for the future

using similar carb you often experience a sudden lack of fuel when
reving high on low fuel tank level. It means you are going fast and then your engine stops like there is no fuel.
So if you experience also a problem like this don’t worry about, it is preatty standard.
For this reason there are fuel pumps.
To avoid this now, run with your tank not less then
half full or
when your tank level is low don’t go more then 1/2 trottle.



no, it was not an advise to get an air filter. Any air filter, even the best, will reduce air intake and consequently power.
But they protect your engine.
In your case protection is ok since you won’t get dirty because of the hose connected to the frame.

As far as regard fuel pumps, they are pressure devices fairly easy to install, not electricity needed, that will keep fuel pressure constant even at low tank level. Mikuni ones are small and work well. But dell’Orto are good too.

I am Italian but not long ago I moved to Switzerland since my wife
was not ready to move to Italy…she doesn’t speak any italian, but she is learning it and maybe in the future I could go back…it is not that Switzerland is a bad place to live, the standard of life is high, but the quality of life is low, human relationships is a virtual world… (and no motorcycle races and no vespa races; I remember with nostalgy my first races with small frame vespa with Kawasaki cylinder, there were killers… but it was 24 years ago!?..oh man I am getting old !)

for paper problem I don’t know anything here in Switzerland…


Thanks Curare, but you know I told you about a spark problem, on friday I finished to mount my carb and refit my engine after porting. I run the engine for about 30minutes and thn it stops. The day after it starts on the first kick then after 10 seconds it stops and didn’t want to start, I checked the spark plug but no spark, it seems that some friends of mine have the same problem and we try different degrees and different sparkplug, Bosch, Beru, NGK. Do you know what it could be?

hello Mikke,

I made a mistake in a previous answer.

Just found a paper where I wrote the set up I did for Malossi 166, Malossi reed, racing crank, enginecases ported, carb PHBH 30, and PM exhaust.

The main jet I used was not 125 but 132.

I hope this help.


…forgot about the jet,

buy all the jet I wrote you in a previous answer, they are not that expensiveand you will find the right one. You don’t have a racing crank so I don’t know if two after 132 and before it will be enough. I don’t think you will need bigger than 135 so I will say 135 the biggest main jet.
In your set up I will guess 128 - 130 but it is a guess it could be 122 - 125! For this reason I would buy starting from 118.
Then just for curiosity let me know.


Another thing too, if you have time to lose, can you give some information about the fuel pumps?;D [:D] [:D] [:D] [:D]