Modern Vespa

Thanks Jack i'll have a read

Hiya. Just entering this chat for first time and also trying to jet in a similar build.
VMC super G, 57mm mazzi crank, 24/24 si carb and bgm big box tourer, vape ignition on a sprint voloce engine.

Had no end of issues with jetting.

Currently on 140ac, e5 atom, 120 main 55/100 pilot.

Been thinking the dirty plug is down to the pilot or possibly the air corrector. Everything else feels crisp and good but also think after reading this mj is too lean. If it's relevant I live on coast, more or less at sea level.


Can I confirm you are using a 52/100 pilot jet? Cant see them
Listed anywhere.

Any help greatly appreciated.

Andyx has a sidecar. More weight needs more fuel.

I'm convinced most of the frustration while jetting a bike are two.
The printed numbers on jets are not their actual size. They need to be at least compared to each other by using a tapered needle and a marker.
Fuel to both circuits goes through the main jet. Until the mj is sorted you're chasing your tail everywhere else.

Take your 140ac/?/120mj to a hill. WOT uphill pull the choke. If more power you're lean. Less power you're good or already a little rich.
Whichever mj provides the most power up a decent hill is a rich jet, since while at WOT (pretty much all main stack) you don't have the rpm (negative pressure in the bore) to pull fuel as you would screaming down a flat road.

You should be in the range with that main stack. At full power you're close to a 160ac/125-128.

I'm imagining that plug gets cleaned up on the road.
You can verify this by riding a km or so, then pull the clutch and kill the engine.
After rolling to a stop take another look at it.

Hi all. Have been following this thread and Jack seems to be the man when it comes to carbs. I've fitted a VMC kit to my 1986 Vespa px125 along with clutch and 24/24 carb (drilled fuel passage and drilled filter) but having problems with idle jet. The bike will not tick over at all unless a 58/160 idle is fitted. When it does idle with that it's very Smokey and seems very rich. Any help would be appreciated

My main jet stack is
120 AC
Be4 atomiser
125 main jet

I've not drilled mine, but your idle looks very high. Try dropping it.

If I put a different idle jet in it will not idle at all

I've never heard of a 58/160 idle jet, but that wouldn't be crazy rich. Only a little more than a 55/160, which is the minimum i would expect with that kit. Where are you ending up on the mixture screw?

Check to make sure your choke is closing all the way.