Chain tension varying depending on where the crankset turn is. WTF?

I'm building a new bike (fixed) from scratch.
I just mounted the chain, and I observed that when the wheel turn, the tension on the chain will vary from very tense to normal (+/- 1cm vertical play).

Behind I have a Miche Primato hub (bought second hand, never tested), with a BLB cog that I used before with no problem. On the crankset I have a Sugino RD 2 130, 165mm (second hand, never tested), and a Stronglight 50t, new.

For the cog to be deformed would be really weird, right? And the crank seemed completely fine, not to mention that the chainring is new...

Where could the problem be? It's kinda freaking me out..

Cheap non-round chainring.

You think so?

You can try centering the chainring using Sheldon's method:
http://www.sheldonbrown.com/singlespeed.html#tension
https://velowoodcyclery.wordpress.com/2011/04/14/centering-a-chainring/

I used it on non perfect round (cheap) chainring and it helped so much. Still not perfect but way better.

Or possibly Chainring not centred on spider? Having said that I've had grief in the past trying to cobble together Chainring+Cogs+Chains combo's all used on different bikes as they all wear at different rates in different ways and show varying chain tension just like you've described.

EDIT- sirBenek typed quicker than me.

I'll give it a try! Thanks Mr. Benek!

I have the same problem on my Primato cranks. Apparently the holes on the spider are not completely symmetrical.

It´s a nightmare to tighten the chain.

I've used SB's method with varying success. I also combined SB's method with moving the chainring round one position at a time and re-tested each time, laborious to say the least but I found one position that was OK. Without moving it around I wouln't have I don't think.

Another question is, if the uneveness is there but is not extreme, how important is for it to be fixed to perfection? In some forum I read something like "is a bike, not a rocket", regarding the same issue. What do u guys think?

Not very important at all.

Personally though, if I had a drivetrain with a lot of variation I'd err on the side of having it a bit tight at the tight spot rather than too loose at the loose spot.

Sheldon Brown has a great article on centering your chainring to reduce/eliminate tight spots in your chain.

Of coarse if you've got a cheap chainring and/or cog, you may just have to live with it (Fuck you very much, Surly.)

Cheap non-round chainset.

ftfy. In my experience, pretty much all chainrings made on modern machinery are concentric, but the same can't be said of cranks, especially square taper ones.

the holes on the spider are not completely symmetrical.

↑This. I just moved the same (Stronglight CH3) chainring from some cheap square taper cranks to some Shimano 5700s, and my previously wobbly drive train is now nicely uniform around the crank rotation.