Have you checked the oil? Has it been recently changed? Dead man cable stretched? Are the fins on the engine block and cylinder head clean?

Yea,oil is good.I change it every year and use Briggs recommended.Oil is not problem,it's hard to start since I bought the mower.
I will check brake cable. So I need to open engine to check fins? I'm not so good for that.But why would they not be clean,engine is very young and don't have much working hours? Thanks for reply!

As for the prime requirement , this is normal on most newer primer units due to epa reg's (lean) As for the harder than normal recoil , perhaps your model has a decompression feature (check owners manual) If it is not functioning properly than your description of pull resistance fluctuating sounds accurate. What manufacturer is the mower itself ? Husky , Toro or What ! Most newer model mowers today have reduced pull effort . The only other possible issue may be that your timing has been bumped a few degrees , but then pull effort would be consistantly more and engine performance would have been effected. Have you hit any obstructions , stick or branch ..etc ?

Mower brand is MTD it is 51 BC model.I have never hit something,i work with mower just on easy grass,everything that is hard to approach or on lean ground I do with trimmer.I think timing is not problem,like I sad when engine is running it works without any problem,don't have rpm drops or something like that,just clean sound.

I'll bring mower to service because it is in warranty but my problem is that service is faraway from me,and I don't have time,but I will try somehow.That is why I want to try to fix problem myself.
My old mower is MTD 51 NC(like BC,just BC is some new version,but everything is same)and the pull on old one is so easy,and it was never hard like on this.Problem of old mower is that it makes hard vibration that destroy the chassic,nothing special it's 16 years old :D

Oh good to hear that's it still within warranty , definitely bring it for servicing then ! If you alter something then you will be questioned . Better safe than Sorry lol .