Modern Vespa

Been riding in sub 20 degree (F) weather for a few weeks and all has been well. Now that we've had a few days of warm weather my LX seems to have lost it's "snap". It smells really rich and the exhaust pipe is soot-ed up even after a hard 30 minute ride home. Acceleration has suffered and the bike sounds flat and feels sluggish. When the weather turned cold I had to back the idle screw out a half turn so I suspect that the automatic choke stays on to some degree when it's cold. Is it possible that the choke or enriching circuit is staying on and causing my problem? The bike is usually quite punchy even in cold weather. Now riding in the 40's and 50's the past few days it's riding like a slug. Any thoughts on this? All input appreciated. Thanks.

Cold air is denser than warm. In a perfect world, you set up the carb richer to compensate. If you set the idle higher, that would not have anything to do with it running rich or lean. It would just raise the idle speed. I suspect you - 1) backed out the idle mixture screw and not the idle speed screw, or 2) The choke circuit is going a bit wonky and there are ways to check it by doing a search on here. Good luck.

Check your air filter as well. If it is dirty/ too oily, etc.. it will definitely cause it to run rich/flat.

Idle speed screw. (throttle stop). Mixture screw never touched. I'll check the filter and if that's not it I'll look into the choke circuit. This is a problem that developed just two days ago. Idle was backed down over two weeks ago. I've ridden this bike daily for the past year (7500 miles) so the abrupt change in performance and It's sound had me worried. Finding the soot in the exhaust and rich smell has me hoping it's just a fuel/air issue.

I have to agree that it probably is a fuel/ air issue, so go ahead and check the filter first as it is not all that hard to do. It also may be worth your time to pull the spark plug as well to check the condition of the plug as well as the cap. If the exhaust is sooty, the plug will be as well and could probably use a cleaning.

Have you swapped out your fuel and vacuum lines at all? They may look good but if you haven't you need to do it, even if it isn't the problem. Mine last 4yrs (replaced mine twice so far). When they start to go the bike tends to feel a tad sluggish. I learned my lesson on tweaking the idle screw. Took forever to get it dialed back in right. See if that helps. How old is your belt and rollers ?

New plug and cap about 2000 miles ago. Dr. pulley sliders (9g) and Malossi belt (I know, stock is better) 3400 miles ago. Raining on and off so I probably won't get to dig into it till Monday. Ran out to the store and for gas this morning. Two miles and 15 minutes total. When I shut it down the exhaust actually hurt my eyes it's so rich. Gonna cover all the suggested bases but have the feeling it has to be the enrichment circuit. I'll let you know what I find.

If I ever get another 150, I think I'll put on a carb with a manual choke. I've got no love for automatic chokes on PTWs.

Feel pretty much the same. I have a Malossi 70cc kit to install on my Malaguti Phantom F12. Picked up a Dellorto 17.5mm carb and Yasuni pipe and all the other go-fast bits for it. I made sure I bought the manual choke kit for it too. My RD 400 and XS 1100 both had manual chokes. I guess the automatic ones work well when they work and don't when they don't. Go figure...