So I'll start with a little background on the 2007 Suzuki Boulevard S40 first.  My son inlaw is the 3rd owner.  The first one owned it until recently.  The second owner didn't own it long and didn't ride it much.  It's got ~9k miles on it and looks well kept.  I wasn't around when he bought it, but it was running. Until he gets his license, he's keeping it by my house.  I trailered it to my house and tried starting it and it seemed like it wasn't getting gas.  It eventually started, but was running very lean with little throttle response, I thought maybe it was cold blooded, and tried running it around the my court.  I only drove it a minute or 2 and it backfired hard and quit running.  I suspected the bike had old gas and potentially varnished up jets.  First thing I did was drain the bowl, it didn't have hardly any gas in it and looked cloudy.  I tried turning the gas valve to all positions with bowl drain open, and only little fuel was dripping out. I suspected the needle was stuck closed but when I pulled the bowl off, it was actually very clean and the float ant needle moved smoothly.  So I thought it could be one of 2 things, the float seat screen plugged or the gas valve screen plugged.

Here is where I need some advise, I'm used to really old bikes that had gas valves that were on, off or reserve.  I'm not familiar with the vacuum operated one on this bike.  The tank was probably 1/2 full but wouldn't flow with a decent stream with gas valve on any position.  In my estimation it wasn't enough flow coming through hose.  On the old type, you get an full flow through the 3/16 gas hose.  I drained tank and removed gas valve. The screen looks nice and clean.  I have a hand held vacuum pump to check the diaphragm on valve and it doesn't hold any vacuum.  I took the diaphragm apart and didn't see any obvious issues, the diaphragm didn't look like it had any splits or holes.
So I guess the first question is is there any issues that crop up with the vacuum operated gas valve.  Also in the Prime position, the valve should be in the full on position even without vacuum, is the fuel flow restricted in this type of valve so you don't get a nice fuel flow stream, like it just dribbles out slowly.  I will leave it here for now since this is kind of long winded.  I'm hoping there are some longtime owners of these bikes that know all of the ins and outs.  I'm thinking this is more of a starter bike and that many may move up to bigger bikes and possibly nobody owns these long enough to know them that thoroughly.

Basically ditto and sounds like nice work on the trouble shooting. I'll add my thoughts.

The milky fuel is likely the result of running E10 and the ethanol mixing with moisture. I run only pure gas for that and other reasons.

I don't think the diaphragm itself was ever intended to also act as a fuel barrier. There's a tiny O-ring the attached plunger/piston that fails and allows fuel into the diaphragm chamber. In my case, this allows raw fuel into the vacuum line and made for an insanely rich condition.

Switching to the Raptor petcock cured this. Immediately. It also cured the common power surging at light throttle.

I'd try pure gas and the Raptor switch first. Since you've already checked the float, if this doesn't fix it, you'll probably need to clean the carb/jets. Remember to also tune the idle mix.

Since the bike's basically down, I'd also check that cam chain/tensioner.

Thanks all for everyones help.  The raptor petcock is exactly what I need.  I've never had a vacuum assisted one, but after reading about some of the problems with them on this forum, this seems to make the most sense because it's not that expensive and prevents potential future problems.  It seems stupid (besides the fact they used a vacuum assist petcock) that Suzuki makes you buy an $80 new assembly and not offer a rebuild kit for $10 or $15.  I'm glad someone did the research to find a direct replacement, makes things a lot easier.

Assuming that this fix's the not starting issue, I will start a new thread about jetting.  I had an 2007 Roadstar (last year that was carbed), and it suffered the same leanness off idle, thanks to the emission rules from our friendly EPA, that required some re-jetting to get it to run smoothly without hesitation.

Yeah, start a new thread if you need to. The carb is fairly simple. Bumping up the pilot, removing the mixture plug, and moving the needle up (by using 2x #40 washers) is the common route. DO some searching, it's well documented here.