Hi there, I have had a search for the answer to this but not found one that really applies to my bike, unless I have ,missed it...

I have a sputtering at high revs, as soon as i reach just over 40kph.

Here is my set up...

Vespa 50n
DR75cc kit
Polini Banana Exhaust
Polini 16/16 carburettor and manifold kit
New 24/72 primary gear

I may have my jetting wrong so can you suggest a main jet I should go with?
Also, what kind of speed should I be expecting if I get the jetting correct? It feels like it has more power to give.

Thanks 

I forgot to mention that I have an 80 main jet in there right now, it was originally 74 and the spark plug looked quite black. 

Thanks

Hi,

please use a sport air cleaner and check the breaker if you have a breaker ignition... 0,35mm is the ideal breaker adjustment!

regards

the problem is the main jet! 80 is enormous for a 16 carb, also 74 is probably too much, try with a 70! the plug must be brown, not black (confirmation that the carburation is very "fat").

Hi Oscar, yeah it is totally flooding due to the massive jet. 74 has also proved too large, hence me conking out in the middle of nowhere.

New smaller jets on order, I avea feeling that even a 70 will be too big but ill start there and work my way down, plug chopping.

Cheers

Yeah... a 66-68 will probably be the right one

Due to my experience, not higher than a 72 main jet.
Also check ignition... have you fit a new spark plug?
If after adjusting the breaker, it doesn't run well, also change the ignition capacitor.