Modern Vespa

So the engine is an LML 150, the entire carb was taken apart and cleaned and put back together. The engine runs well, lots of power...thing is, if I give it a liiiiiiiiiiittle bit of throttle, the engine doesn't raise in RPM, it goes down, but if I give it lots of throttle it revs normally. Same thing at all speeds. When I'm stopped at a light I have to pull the choke or it's likely to stall.

The mix screw facing backwards is unscrewed one and a half turns. There is no smoke, engine is running clean...it has 3K miles so well passed break in period.

Any suggestions? I'm not sure if unscrewed adds more gas than air or vice versa because I'm a n00bie. Please go easy on me. haha. I'm still learning. I set it to 1.5 out because that's what my friend suggested.

Weird. Clogged pilot jet?

It cannot be the jets, the entire motor is stock and I just cleaned the entire carb including the jets thoroughly. (but thanks for the suggestion!)

Maybe it's just the mix screw. 1 and a half turns out is just a starting point. Continue to adjust that back screw to the right 1/4 turn to allow more mixture and see what happens.

ahhhh kemosabe... if the only thing you have played with is the carb and it ran fine before than the carb is more than likely at fault.

check the jets again your cleaning may have dislodged something that has now come free and jumped into your jet.

it sounds like you may just need to adjust the a/f screw.... for the 5 mins it takes I would be pulling the jets and checking them tho

Thanks all, I will try pulling the screw out by a 1/4 and seeing if it helps, followed by another 1/4 out if not. But before I do so, please lemme know if this is making it more rich or lean? I'm assuming it needs more fuel if pulling the choke prevents bogging when the throttle is only twisted a little?

^^^ if you are going to adjust the air-mix screw, and have the patience (good to) go out 1/8 turn at a time.. the screw can be pretty effective/responsive and you don't want to blow right past the sweet spot!

Excellent suggestion! I'm on my way home now, wish me luck!

good luck

One other thingi'd recomend- those LML engines tend to respond very well to a swap to a 55/160 idle jet.

+1 on re checking your jets. Just because you cleaned the carb, doesn't mean something else could find its way from the tank to the carb. I've been battling this problem for a few weeks now. I'm about to have my tank coated to rule out rust deposits.

I may be incorrect, but the 55/160 (2.9) SI Pilot/idle jet is stock, isn't it? When I replaced my filter & exhaust, my CHT readings at mid range (30-40 mph) were hot so I had to and replace it with a richer 52/140 (2.7).

Winding it out is richer, in is leaner.

you are incorrect. 55/160 is stock on most P's, but the stellas shipped with something ridiculous like a 40/130.

also the spaco carbs have a different pitch to the mix adjustment screw- the thread is swine as steep. on those you start at 3 turns out instead of 1 1/2. and each turn has half the effect.

Okay so a quick update cause I got it working perfectly again:

A friend of mine suggested I put the mix back the way it was and see what jets it had. I did that and discovered it had a 40/120 and a 160, I threw everything back together and took it for a drive assuming I'd have to continue using the choke but it ran perfectly...so I suppose I must have made a tighter seal or something. Perhaps there was a small piece of crud that I dislodged by accident...either way, the 150 LML is running smoothly again after months of bogging so if you have similar symptoms with the first 1/4 throttle response then check out the jets.

2 months ago a 200cc LML 4 stroke had some kind of same problem in the road during a trip, the problem was the spark plug cap.