Ducati Monster Forum

I have a Vespa LX50 with the carbed 2 valve 4 stroke engine. Aka "the dog". I have souped this thing up as far as I possibly can - 78cc jug & piston(up from 49), hot cam, freeflow exhaust, freeflow air filter, performance variator assembly, Bitubo adjustable suspension front & rear, stainless front brake hose.

Whereas it used to struggle to hit 35 now it struggles to hit 45. I use it to commute to work and I feel like it's just about usable now after the mods. I love this thing.

However I find I'm having trouble tuning the carb. It came with a 75 main jet & a 35 idle jet. After the mods I'm at 82 and 38. The 82 appears to be the jet that gives the best pull at low mid and high rpms, as I've tried smaller and larger, both return a bit of a flat spot in those ranges. Plug always seems to be a light grey every time I check it, no deviations.

The trouble I'm having is from a stop. I can achieve decent sustainable idle, that's not the problem. The problem is when I twist the throttle to move off from a stop. It burbles & bogs from a stop unless I preload the throttle a few rpms before the light turns green. I have tried adjusting the mixture screw of course but hell I don't know wtf I'm doin'. I've just been living with it but I'm getting tired. Even bought a new carb, vacuum petcock, hoses, intake manifold, CDI, put a Shorai battery in it, still have the problem.

Of course when it was stock it ran fine albeit frustratingly slow. I'm at wit's end. Is there anyone here that has innate carb talent who can offer some advice to help me tune this dang thing properly?

Easy first.  Turn the idle mixture screw (it might be blocked off) a specific amount of turns.  If it will idle at 2 turns start there.  At 1/4 richer does it get better or worse?  Better go another 1/4.  Worse?  Go lean.  Beyond that I have no familiarity with your scooter.

Thank you. There is nothing about this engine that is out of the ordinary, just a lawnmower engine basically. I suppose I was hoping that someone who had the same issue once upon a time could give me a solution that worked for them. I just hope I can somehow iron that burblebog from a stop out finally.