I am not too clear on the problem being described. From the video, the engine seems to be running fine. The machine it is powering, not so much. The cycle of running making a series of noises, then a klunk seem to be a mechanism that is cycling through some series of motions. If a belt were damaged, for example, i.e., missing a chunk of the 'vee', it could make noises like that as it passed around the drive pulleys, or idler, or other pulleys.
I think the post belongs in a section of the site that deals with the specific brand, not in the small engine area as the engine, to me, sounds okay.
tom

 

I agree with Tomw0. The length of time between clunks would seem to point to an issue with a longer belt.

 

Does the machine move without any jerks while it is making this clunking noise? Hopefully, this clunking isn't something coming from the transmission.

 

Let me give some more info, electromagnetic clutch is off, this is engine working only and actually there is no clunking and no alarming noises from engine at all, it may give that imppresion as microphone is close. What I'm raising is just constant idle up/ idle down. Additionally when this issue appears if I try to accelerate and mowing it stops working after few seconds. Looks like not enough gasoline but it makes no sense, if cold works very well and I can work around 5/10 minutes w/o any issues but if engine gets hot then starts breaking and it happens, engine got overheated ?

 

I would take a look at the intake. If it was loose, it could be ok when cold, but leaner when hot. It might also hesitate when you applied more throttle. Other than that, check the fuel tank outlet for gathered debris that slows flow, plugged fuel line, clogged filter, or possibly a sticking float valve.
tom

 

Thx guys, I changed fuel filter and looks like everything is good so far

Thank you very much !

 

That sounds like governor hunting, so definitely carb/fuel problem.

I would first remove carb to clean out and replace with new gaskets. With the small jets and fuel passages, it doesn't take much to block any fuel delivery within.

When I rebuild these small carbs I use small wires to push through any passage hole and back that up using some carb spray cleaner and blow back the opposite way to make sure that that passage is clear both ways.

Before putting carb back together, blow all passages and carb dry using air.

Remove the fuel line before/after the fuel pump and at carburetor, get rid of this gas. If fuel line is older than 10yrs, replace fuel line from where you now install new fuel filter to fuel pump and from pump to carb. Old line will still have some contamination inside and when you rebuild that carb, you'll just put more crud back into carb when you start it up fresh.

Also, older fuel line can develop exterior/interior cracks over time from vibration and fuel usage, so just easier/cheaper to replace than trying to repair.

Del