Folks...2020 500 trail.  Always been stored outside, covered, etc but at outside temperature.  For several winters it started A1, could just reach in at 20 below freezing, turn the ignition on, let the fuel pump prime, then crank and it would fire right up and run smooth.
All of a sudden, last winter, it wouldn't do that anymore and the only way to get it to fire up was to feed it about 1/4 throttle. Then it would start up, and run ROUGH for about 15-20 seconds, then suddenly run smooth and not have another problem all day.  It would only act up on a cold start.  After the weather warmed up, the issue remained...regardless how warm it was outside, it would start the same way cold.  (first start only).
I replaced the coolant temp sensor because the temp gauge had also stopped working (just the default 1st bar lit) but it would warm up normally, and the fan would kick on when expected, turn off again, etc.  Also, twice, the temp gauge shot right up to full bars (not in a situation where overheating would be a possibility) but then dropped straight back down to the 1 default.  So temp sender was on its way out...  After replacement the gauge is reading as normal again.

I still have the hard start issue.  Dirty MAF sensor?  Checking I found there isn't a MAF, but rather a TMAP.  Got some MAF cleaner, popped out the TMAP (was oily), cleaned it, put it back in...same.   Crap.   Oh, and have never seen a MIL/Check Engine light come on.    So, still thinking TMAP, I unplugged it, and tried another cold start.  MIL comes on when the ignition is turned on (which it would), and it starts better cold (a little better, but still not great) with the sensor unplugged.  I assume it sees a hard TMAP fault so just throws best-assumed values at it (open loop? Is that still a thing?).   
I don't have a scan tool (Yeah, I know...I ought to)...but if it's starting and running a little better with the sensor unplugged vs plugged in...I figure I'm in the market for a TMAP. 
And, after starting it a few times with the TMAP plugged back in, the MIL goes out as soon as I start cranking, but is on solid with ignition on before cranking..which, I'm figuring is normal. (Lit up while key on, engine off is likely saying there's a stored code...right?) 

So in the end, my question would be....If it cold-starts BETTER with the TMAP unplugged, that pretty much tells me that I need a TMAP....make sense?

And yeah...I will change to a stickier air filter oil.  I've been using the Honda filter oil but it's pretty runny. And gets tacky after drying but not nearly as tacky as other oils I've used on other ATVs in the past.   With the Honda stuff I've found that even after blotting the UNI out well, it would still fill up the airbox drain with drippies.  Why I was a numptie and never said "I bet that's gooping up my MAF sensor..", well, I'll never know.

Edit - Valves.   Yes, I did have my first service at the dealer and the valve check was done.  I have 3000kms on it now, and they haven't been checked since.  I likely should just check/adjust them now to be sure, and that might also be my hard start issue....but dunno.  Looking for feedback from the experts.  :-)

[2015 Z Force 800 53/Trail EPS]

One thing people don't realize is that not only does that foam filter oil coat the MAP/Air temp sensor, but it also enter the Idle Air Control (IAC) valve port and gum it up..which makes for hard starts and erratic running..and dieing when coming back to idle when cold sometimes. You can clean it with regular spray carb cleaner and the straw. Just shoot it into any port you see on the intake side of the TB, let sit a minued ot two and start it, run it a few seconds then shut down and repeat three or four times. The best filter oils we found were BelRay and Maxima FFT and their Fab1. Still want to roll-up a few paper towls inside to get the extra and let completely dry before starting the unit. While off you will also want to clean the entire intake track from filter to TB. Good luck

Well....I fixed the cold start issue.   Tight valves.  Exhaust had clearance but were about 6 thou too tight...intakes I had to back the adjustors off 3/4 of a turn before they no longed had tension.  Yeesh
I had wondered if the dealer even checked them like they were supposed to on the first service...yup, they must have.  They left a trail of stripped bolts as proof.   
All 4 bolts on the fan cover, they had tightened them until the nut in the plastic backing started to turn.  Thanks guys!   Tried everything...finally ended up just spinning them with the impact driver until the nuts melted the plastic and pulled through.  Put longer bolts and washers/nuts on the backside of them.
2 valve cover bolts were loose, and two were stupidly tight.  One of the tight ones came out, the other went about a half turn and broke off in the head.

But, it started right up cold and runs great.   And, it's honestly never run this good.  From a dead stop it's got way more pull, ripped a couple donuts (THAT was never possible) and the highest top speed EVER was 90km/h with a tailwind.   Hit 101 on the first test run.