Hi,

 

Newly built Polini130-engine with full circle crank, Egig clutch, PHBH24 on reed and original gearbox. 
 

Engine runs very very good in 1st and 2nd gear, but jumps in and out of gear when in 3rd. And very bad noise in 4th. 
-big «clunks» like someone is hitting the casings with a hammer. 
 

The same noise in 4th without engine running, also with clutch engaged. 
 

Judging by the "bad noise" you're describing I'd highly recommend to open the engine to see what's wrong.

I've already rebuilt the engine with a new gearbox, and the noise in 4th is now gone.

Just have to mount it in the frame and test it on the road.