Modern Vespa

I have just restored a " basket case " ET4 125 back to its former glory . I had to fit a new CDI and a good used carb which was in a clean condition , new starter motor and ......hey presto , it works ! I adjusted the mixture screw on the carb and the throttle screw to get a decent idle , which it does perfectly . The problem I have now is that accelerating from idling speed up to about 25 mph the engine splutters . After that it runs smoothly , pulls up hills no problem and starts easily from hot or cold . I have checked the inlet manifold and airbox hoses and everything seems ok . Has anyone any suggestions for a solution to the problem , please ?

You say it idles ok? How many turns out from bottom are you on the mixture screw?

Two and a half , that seems to give the best idle .

when it sputters at 25mph are you WOT or partial throttle?

when it comes on, is it sudden or does it build, then kinda fall on its face and pick back up?

Hi Greasy , at 25 mph I am at partial throttle . When I start it on the stand it idles perfectly . As I gradually increase the revs the engine splutters up to around 1/3 throttle , after that it runs fine . Any thoughts ?

although the carb was "in a clean condition" did you clean it?

to me, this almost sounds like a dirty carb or a needle not secured in the slide or maybe a diaphragm not properly sealed/seated or possibly torn.

I did'nt give the carb a very thorough inspection , so I think that I must give it a strip down and clean in the next few days . Thanks for your suggestions and I'll keep you posted .

Update : I stripped the carb down and inspected it . I found a few small black flakes in the float bowl . When I looked at the piston that holds the needle , I noticed an area on the surface where the plastic seems to have degraded and was flaking off . I gave this a polish with 800 grit wet and dry paper which got rid of the flaking . So , carb back on and after a few coughs and splutters the performance seems much improved , only time will tell if there are any more flakes of plastic floating around inside . There were no obvious blockages in the jets so I gave everything a clean with carb cleaner . Once again , thanks for the helpful comments .