Why exmark loses power when blades are turned on

Engine valves need adjusting.
symptoms: starts ok, runs until engine reaches max temp loses power then stops. can't restart when hot. once restarted runs for a while stops again.

There is very little clerance in these valves and temperature alters the lenth by a few thou and the valve don't close fully losing compression, once cold the valves shrink back enough to create enough compression to start again.

Try adjusting belt tensioner.

check to see if power coming out of switch if not bad

you mean it dies or the engine slows down ???

When you say 'going forwaed', is that while mowing or does it do it without the blades engaged or 'not mowing'?

clean your carb and put new fuel filter inline, and a set of plugs !

My problem is that I don't know what your problem is exactly.

the springs that control carby is there leaves or grass stopping it from moving ,some lawn mowers have bad desinge for air flow off ****

THE answer that FINALLY worked for me. Torque the bolt that holds on the blade to the recommended torque specified in your owners manual (for my Exmark this torque is 120 ft-lbs). When I first used the torque wrench, i was quite surprised by the amount of addional force I needed to apply to reach 120 ft-lbs Exmark blades have a round blade hole with a washer and therefore, relies only on friction provided by torquing bolt to the proper torque to prevent slippage so that the blade rotates at the same speed as the spindle. When slippage between the blade and spindle occurs, blade tip speed slows leaving uncut grass.
Note: this solution will NOT work for mowers with a blade and spindle design which uses a positive means (e.g., a star shaped hole on the blade) that assures the blade rotates at the same speed as the spindle with no slippage.

ck for 12 volts at switch in off and on I think switch is bad