Modern Vespa

I've been hunting unsuccessfully for a post that said something along the lines of... if you have leaning issues at WOT with a PHBH (with bell-mouth intake) , then it could be simply related to the sheer volume of AIR coming into it & really there is no fix....

EXCEPT

it then mentioned something like

if you fit BELLOWS (like original ones, but bigger) that make the intake pickup the air from under the seat, it ensures there is a MAX volume of AIR it can suck up (ie. only what is coming down the bellows, rather than at least 180deg of air coming from all directions), and therefore, it should resolve the issue!

Does anyone know about this idea & if there is a chance it will work?

Sorry couldn't find the thread you're talking about.

But lean issues most likely an air leak.

The other issue is usually ignition timing and squish.

Don't solve those and you'll never get it jetted properly.

I've worked on 6 phbh engines, Reed and rotary all running great. The carb is pretty basic and doesn't draw a ton of air because of the slide and the 28 and 30mm size.

I usually install (and recommend) a bellow unless the owner doesn't want it.

Bellow just keeps blowback through the carb consistent. And so you don't stall at traffic lights because of rich conditions.

The phbh is super forgiving when jetting, but all jets affect ALL throttle opening positions.

And don't mix up lean with rich conditions. Losing power at WOT and having the scooter suddenly slow down isn't just a sign of it being lean…

If the engine has no air leaks and generally good. Lean issues at WOT are limited to ; atomiser too small, needle tip too fat, main jet too small.
Remove the main jet and ride with none. Should be normal at 1/2, spluttery at 3/4, get heavier at 7/8 and cut out at WOT.
Once it rides like this you're ready for a main jet.