Modern Vespa

I have an 2006 LX 150 (carburetor model) with 13,000 miles.

The bike runs pretty well, but can sometimes be hard to start. The starting problem is especially noticeable after the bike has been sitting for a few hours. For example, if I ride it to work, the morning startup is alright and the afternoon commute home ~10h later can be finicky. The bike will turnover and crank, but will not catch.

After about 30 seconds of this (3 or 4 cranking attempts), it will always start up (so far!). But this makes me nervous that it'll strand me one day, so I'd like to make it run better. My GT200 always starts easy, so I know better starting is possible for a carbureted bike.

Any suggestions on what I should consider to improve this?

I've read a few similar threads on MV and am considering the following:
* carb is newer, but maybe needs a cleaning (but then why would it be harder to start sometimes than others?)
* replace the starter motor (I've read a few times that the starter motor can weaken with age. But again, why not always hard to start then?)
* maybe a bigger issue, like poor compression (but then bike should run crappy all the time?)


The bike is up-to-date on maintenance (list below). Here are some recent items that I think are relevant:
* new carburetor Jun 2020 (Keihin OEM)
* new battery Jun 2020 (Yuasa) - periodically maintained on Battery Tender
* adjusted valve clearances Jun 2020 (0.15mm exhaust and 0.10mm intake)
* new air filter May 2021 (OEM; pre-oiled)
* new spark plug Aug 2021 (NGK-CR7EB @ 0.7mm)

One thing that comes to mind is a vacuum leak. At this scooters age, possibly you have some cracking in your rubber intake manifold or vacuum hoses. Easy to test for if you start the scooter and start spraying areas with carb cleaner while listening for resulting idle changes.

I would hazard a guess that the air/fuel screw was not adjusted or adjusted when the bike wasn't fully warm, which takes a long time, and isn't correctly set. In order to compensate for this, the idle adjuster on the throttle is open more than it needs to be. This makes carb bikes hard to start.