My ongoing issue (last attempt)

[maxifields]

Vespa sprint 150 69

.I've had this problem on and off all year.It all seems to be with the mixture.Spark plug is always wet when inspected.My last effort I  put a new carb on main jet 102 slowrunning 42,Went out for approx 20 miles all good.Pulled over left scoot running for about 5 mins started off again 30 seconds down the road it dies.I kick it over and then it won't start just backfires.Look at the plug which is black put new one on and it starts again.This is the same issue I had with the old carb.(dellorto 20/17)Standard 1 and half turns on mixture screw 2 and a half on idle but have tried different settings to see if any difference can be made.And mixing 2% oil via jug in fuel. Any ideas anyone before I have to pay someone to look at it. 

[pxguru]

Sounds very much like a timing issue to me. Have you strobed it to rule that out?

[maxifields]

timing checked last year if I can get hold of a light will check.If the timing was out would it be backfiring all the time ? Thanks for the suggestion though much appreciated.

[shipscat]

if its still on a points ignition it could be the condenser

[willow]

Can you borrow a cdi unit of someone I had similar problem and that was the cause