There is one microswitch bellow pedal for forward and one bellow pedal for reverse. And there is a rotary position sensor 0-5 V linked with levers for throttle, who drives one step motor which fisicaly move lever and control how much rpm's will engine deliver. I replaced brushes on step motor and he works fine now. The only way to adjust idle is to adjust lever lenght and increase it a bit. Everything else goes from one control box located below fuel tank. Tommorow i try to adjust idle and see if this can eliminate that suddent start. If anything else you want to add please do. So far you helped me a lot. Thank you very much.

You may have a bit of miss adjustment between the throttle and transmission pump levers/linkage. The engine should slightly increase RPM (from about 650/700 rpm to about 850 rpm) before the transmission tries to move, and then they should be working pretty much together. I am not speaking about this particular model and serial number, (so the factory specs for RPM may be higher or just different) but for hydrostatic transmission powered equipment in general.
I don't think I can recall how Clark put the linkage together for a Hydrostatic.

O.k. Thanx 4 that, 'cause this Clark is first one 4 me to deal with. Usualy i work on TCM. Main problem was that i repaired engine malfunction(full throttle on start-up, caused by jammed fuel pump). And now customer says that this kind of movement is not normal. I tried to drive it and in whole range of throttle he goes fine. Sometimes there's a little pause to throttle response, usualy on small pressure on throttle pedal.

this is normal for any hydrostatic transmission. you will have to get used to the motion changes, and soon you will find that it is even easier to accurately, quickly and safely move pallets.
It is different, but there is considerable reason to believe it is better, once you are used to the difference.