Over the weekend I tried to replace the high speed needle valve assembly on my ST .40 (Italian made), since it was sweating some fuel . I purchased a valve assembly and all the components match exactly as my old valve, except the point of the needle, that is a bit longer than the old one. The result was that the engine pics up RPM and then decreases RPM over and over.
I dont want to mess yet with the low speed needle, without knowing if that is the trouble. With the old valve the engine runs great and the idle is smooth.
Any suggestions will be appreciated

Have you tried the old needle in the new body?

Thanks Minnflyer, Iīll go ahead and try switching needles to see if thatīs the trouble