1986 Evinrude 90hp loss of power, fueling problem?

Hi all,

The engine starts and runs well right through the rev range for about 20mins and then looses power and won't rev much over about 2000rpm. I've found if I activate the electric choke by pushing the key into the ignition I get full power back, this suggests its a lack of fuel problem. The VRO unit is removed, running a new diaphram fuel pump off the crank case. Am I best to remove and rebuild carby's? the manual suggests carby's are rarely the problem and adjusting can cause more problems... is it worth using an injector cleaner or spraying carby cleaner... what should I look for in carby's? blocked jets? any help appreciated, thanks