I can't find proper chain tension for my singlespeed freewheel bike. My drivetrain ain't perfect so I am looking for tightest spot and making it so there's around 1/4" play there(btw, I'm not using tensioner). Point is, once I start to freely turn cranks(with rear whell unloaded), they always stop in the same "tightest spot" position(how many revolutions can they make freely depends on chain tension). In order to make them turn around all the time like in fixed-gear bike I would have to increase chain slack in tightest spot to more than 1/2" which is probably too lose, given that the slack on other side would go over 1".

I don't know if I should be riding like that, or what's better - more loose chain but with proper crankset rotation or overtightened chain(but still with 1/4" play in tightest spot)? What could go wrong with chain overtensioned, faster component wear(and what exactly would wear, sprockets and chain? rear wheel bearings?).

As my chain, freewheel sprocket and chainring are in very good condition, I don't know what could be causing it. The only thing that comes to my mind would be that my drivetrain is not perfectly clean as I don't mind riding on wet roads.

nothing is "perfect", including the concentricity of your crankset and freewheel.. or your standard for "slack".

set the chain so that there is no over-tightness (binding, drag, noise) or yes, you will damage parts faster.

You just discovered why some bike parts cost more than others.

With that being said, there is still a thing or two you can do. The first being to loosen the chain ring bolts, finding the tight spot and then retightening them such that the chainring is a little more concentric to the crank axle. A rubber mallet will help. This may take a few tries as each attempt gets the chain ring a little more centered.

You might also check that your wheel axle isn't bent. This can cause havoc with how the cog on the wheel hub remains concentric to the axle. The combined action of both cog and chainring being off-center can make things quite out of whack.

There are other things that can be done involving hammers, round files, & a lot of finesse to overcome inexpensive component manufacture. I do not recommend them at all except for the most desperate. Try the above suggestions first & report your results.

Nothing expensive damage will happen. Maybe the chain will break and you will replace the chain with a new one. You are overthinking. Feel free to drag the axle back in the dropout so that there is 1 inch to 1-1/2 inches of chain slack.

Check out this video.

Chainring and cranks are Campagnolo(don't know exact model), chain is Izumi 410 and freewheel is Shimano MX30. Indeed not expensive but I'll take those over any overpriced carbon replacement. What I've just noticed is that freewheel(and most likely chainring) is 3/32" while chain is 1/8", does it matter?
As for chainring bolts, I think that's part of the problem, but I don't want to do it now(I've tried it but the force I have to apply is so huge that I'm just scarred that I will strip those bolts). Wheel axles are new(bought new rear wheel few weeks ago), and the oldest part of my bike is bottom bracket, could is also add to issue?

Upon further inspection, I've noticed that freewheel was making odd noises. Applied some WD40 on the outer parts but it's even worse now. Lubricant didn't help. Now I tend to think that freewheel is dirty or worn but I don't know if it's possible to clean it or replace bearings, probably not worth the effort?

Video of the noise please. You could upload the video on to google drive or one drive and post the link here.

My first guess is the worn-out bearings are the culprit for the noise, because bearings wear out earlier than the freewheel. You can regrease and repack a set of new bearings. However, a video may help the further investigation and to determine the exact cause.

WD-40 is not recommended, not a great lubricant at all and we do not allow it in our shop. A good penetrating substance like PB Blaster is our go to for getting rusted bolt apart. Your freewheel should be using a lightweight oil for lubrication.

Campagnolo has a reputation for making good stuff. I would bet against the chainring or the crank arm not being round. It's probably as good as any that ever was.

Chain is fine. 4/32 (⅛) is bigger than 3/32 so fits over all the gear teeth just fine. What you can't do is go the other way. A chain not fitting over the gear teeth is pretty obvious. Your good sense would've eliminated that before getting to the Internet.

The MX30 is a standard 3/32 freewheel. Assuming it's not defective or worn out, there shouldn't be any issues there either. The good news is they are ridiculously cheap. As is the tool to remove/replace it. Make sure you use the axle nut or quick release skewer (as appropriate) to keep the tool in place when you use it. And be sure to re-grease any threads upon freewheel reinstallation.

WD-40 is a great solvent for other greases. Don't use it on bikes except for that purpose. It's not, nor is it meant to be a great lubricant.

So, we're back to the chainring & cog concentricity about their respective axles.

If you just bought your rear wheel a few weeks ago, it may be worth removing it from the bike, removing any cap style rubber seals (if so equipped) and spinning the axle between your index finger & thumb to assess the bearings actual preload and the systems actual condition. It should be smooth and free with essentially zero resistance and no free play. A bent of broken axle would be obvious upon close inspection.

On a related note: Wheel bearings on new wheels have a tendency to come over loaded and under lubricated right from the factory.

To adjust the chainring bolts you will need a 5mm hex (Allen) wrench and this tool https://www.parktool.com/en-int/product/chainring-nut-wrench-cnw-2

Do not use the "ball" end of the Allen wrench.
There is a difference in torque values between steel & aluminium chain ring bolts.
Steel bolts can also withstand a lot more torque than aluminium.
The presence of thread locker increases the realized clamp up force/bolt tension for a given torque about 30%
A good middle-ground safe bet for just about any chainring bolt situation is Loctite 242® and 5nm of torque.

Loosen the bolts a ⅛ to ¼ turn, just enough that the ring will move if tapped with a whammer, but not so loose it will move by hand. Spin the crank to the tight spot and give the chain ring a tap with a rubber mallet to relieve the excessive chain tension. Repeat until satisfactory results are achieved. Tighten the bolts to 5nm with Loctite242.

I've made videos but have trouble with uploading them from my phone, but I don't think it's necessary. As I said, after riding on wet road I found strange noises coming from freewheel, sort of uneven grinding like some dirt or grain of sand, which wasn't there before. I did silly thing by applying WD40 into that round fissure of freewheel thinking it would help. Next, I've lubricated it with CRC chain oil. It only made things worse, the sounds are still there albeit more smooth but now cranks stop turning really quickly(before they used to get at least few revolutions before they stop and when bike was in near perfect condition they worked almost like on fixed-gear bike). I've talked to mechanic and he told me that I should never use any WD40 or lubricant on freewhel as those can destroy freewheel quickly and suggesting blowing all those fluids out with compressed air. I did what he told me but it's didn't help much.

My guess is that some sort of dirt got inside freewheel and it's still there making noises while my WD40/CRC messed bearings grease and now there's even more friction preventing smooth rotation(which affects whole drivetrain). Good thing is that freewheel is the oldest part of my drivetrain and I was considering to use it through whole winter or until it dies(whichever comes first). I've even purchased Sturmey Archer replacement freewheels and fixed-gear cogs as hub is flipflop.

I've initially thought that it's chain tension preventing cranks from rotating freely but now I have came to conclusion that it must have been freewheel.

As for concentricity on my bike, it's simply ugly, on one side the chain looks like it would be too lose and on the other like it would be too tight. Worst part, it has to be done this way or it might fall off. Some time ago I had issues with chainring bolts coming loose, I've even purchased blue Loctite but never got into doing it this way and instead I am just periodically tightening those bolts with normal allen key. Now I think this might be making things worse(and more concentric), not to mention that I have to apply so much force, that it might strip those bolts. In fact, I'm not even sure if I have properly sized hex key for them as from what I know US/Euro/Asian hex keys are bit different in size and not interchangeable.

You don't want the chain tight on a single speed bike with no chain tensioner. You think you are hearing noises from the freewheel. That might be the rollers shifting position in the rear cog. And you are hearing them because they are telling you the chain is too tight. If you have a little sag in your chain then it's about right.

WD-40 is a Water Displacer hence the name. I don't really recommend it for much in the bike world. For lubrication of a freewheel some lighter lube like Triflow might do the trick. However I would get the proper freewheel for your set up. If you are running a 1/8 chain run a 1/8 freewheel and stick with White Industries and it will last a tong time and is rebuildable. It sounds like you have some decent parts so it would be worth it.

There will always be some variation in the chain tension. The loosest point should not be "too loose", or sooner or later you'll drop the chain. However loose this is, I can't tell you because I don't measure it... If running the chain this tight this results in a tight spot, so be it. Assuming you have decent parts, the tight spot should not be so tight that it stops the whole thing from spinning on the bike stand (I'm thinking of a fixed gear.) Hopefully it does not "hang up" on the tight spot.

I've ridden over 40,000 miles fixed gear, mostly with Biopace chainrings, which always have loose/tight spots, and haven't ruined any bottom brackets or hubs. Except for very cheap ones which were junk to begin with.

Small update: Believing it was my freewheel I have decided to try to melt solid greese and apply it through the crevice. To small degree it worked, now the wheel works smoother, there are less sounds and more revolutions before it stops but the problem isn't gone. So back to the drawing board, I will have to re-tension chainring bolts. I'm also wondering could it be bottom bracket? It's oldest part of my drivetraing and I haven't been inspectig it for years.

Why don't you post a few pictures of it. Different brands and models of bikes do things differently and might need other considerations. Your general descriptions are just getting you general and generic advice.