I am not a mechanic, but have just completed a major overhaul on my DF140, so I feel like I know it well. The IAC valve controls the air flow when the motor is in nuetral. It's what sets the idle speed. Unless you having idling problems, I see no reason to check it other than maybe making sure the tube isn't clogging up.
As far as the timing chain goes, from the research I did, Suzuki made this chain to last the life of the motor. When I put my timing chain on when I put the motor back together, there was no way to adjust it once it's on. Also, to inspect the timing chain, you would need to remove the powerhead from the engine holder, to get a full view.
Now to move on to way more important issues. Have you replaced your engine holder plug? Just do a quick google search on "Suzuki engine holder corrosion". Hours of fun reading. Long story short. Suzuki placed a plug on the port side of the engine holder in anticipation of using it for an O2 sensor in the future. No O2 sensor ever came, but the soft metal plug they used has cause a lot of problems. The dissimilar metals starts the corrosion process. It eventually will eat thru this plug area as in this case:
Suzuki DF140 "engine holder" corrosion fix! - The Hull Truth - Boating and Fishing Forum
Sometimes, the cancer corrosion will eat thru the exhaust walls of the engine holder first, before any problems occur with the plug. This can cause a multitude of symptoms including: water in the exhaust, exhaust in the crankcase, water in the crankcase, etc. This is what my engine holder corrosion looked like on the inside:
So what I would do is remove that plug ASAP and replace it with an aluminum one. To do this you might need to get creative. The plug is an 18mm by 1.5 thread. Hard to find. I had a machine shop make me one.
If you are not up for this task, I would at the very least watch very closely for water getting into the oil. This will be a sign things have gone south with the engine holder. Another common reason for water in the oil is a corroded seal on the water cooler of the engine filter holder.