Modern Vespa

I can't find any leak on my lml. I check everything but can't find the leak that causing the lean condition.

Symptoms:
-hard to get engine rev from idle
-back fire while going downhill
-noticeable lost on engine breaking


I test every possible leak between carburettor and the engine, float level, clean carburettor, valve gap adjust, every thing.

I read on a forum that the leak is possible to come from crankshaft seal. It's possible on a 4 stroke?

I need help...

Being a four stroke the crank seal has nothing to do with how the engine runs. All the air/fuel mixture business is in the top end unlike a rotary valve two stroke like you are thinking of.

The rev from idle issue makes me think vacuum leak. Take some carb cleaner and spray all over your intake and vacuum lines. If your idle changes you have found your leak.

Most motorcycles I have riden have a normal amount of back fire under decel especially if under hard engine braking. If it where back firing under power I would think CDI or ignition timing.

Engine braking is all about compression. Have you checked that ?
Burned exhaust valve ?