RPM drop while warming up the engine...

Yes sir,

I repalaced the spark plugs on 200 hours.

I used NGK.

I will take your advice and will check gaps.

Nadav.

I haven't heard of carb icing during a ground warm up. I think I would look for another cause.

Thank you Roger,

I check any direction.

If you have any idea, I appreciate if you tell me.

Nadav.

I agree with Roger!  Carb Ice is not likely.
If the carbs are cold enough to freeze internally they will be showing frost externally.
At a minimum, the carbs would be condensing (Sweating) great amounts of water over their surfaces.
Are your Carbs Frosted over or dripping Wet during the misfire events?

If you do not have EGT probes, use a non-Contact IR Thermometer to determine which cylinder(s) is/are colder than the other.
If an IR Thermometer Guns is not already in your toolkit, one can be obtained starting at less than $20.

Nadav, let us know what those spark plugs look like when you pull them.  By the way when you feel the vibrations at 2,500 rpm if you continue to advance the throttle does the vibration increase or decrease? Does rpm hold steady, and do you make your full static rpm? Also, have you complied with the two most recent carburetor Service Bulletins?