Modern Vespa

I have recently upgraded my 1957 VN2 using a 160cc Pinasco kit with a Pinasco PHVB22 carb. I have fitted the 105 main jet, and Gickspeed recommended an air screw adjustment of 1.5 to 2.5 turns out.

Timing is set at 25° as per Pinasco instructions. Gickspeed uses 24° and in SIP's tuning video they use 23°.

It definitely runs better with the air screw 2.5 turns out. However, I do not get a smooth engine sound, but a stuttering sound.

What must I change or modify?

Constant stutter or is it only when you add throttle?

While riding. It runs smoothly, then stutters with a small loss of power, and then it carries on again.

Spitball:
Maybe a smaller main jet? 102, 103?

I am not familiar with that carb, but it *sounds* like your main might be a touch rich.

If its running rich, try dialing down that mix screw from 2 1/2 to 2 or maybe eventually 1 turn out as you maybe able to avoid reducing your jetting.
Mix screw under 1 turn out maybe an indication for a lower jet and it seems you have some room to play with that screw.

Thanks guys. Today it is pouring with rain, so I will give it a try tomorrow. I have fitted a 102 main jet and will see how it goes. I will also go for 2 turns out. Will keep you posted

Ok, I'm 90% there. I went up the mountain pass next to our town, and I am quite pleased with its performance. It stutters when I accelerate, but at full throttle the stutter vanishes. What is the reason for that?

A note: I am still using the original exhaust, although slightly modified. I cut it open, cleaned it and fitted an outlet stub the same size as used on a P150 exhaust.

I have a 100 main jet available, and I can play around with the air screw, otherwise I don't have much options left.