Might just be crap in the fuel that has blocked some of the passages in the center carb metering block, follows RapidRobert's suggestion that might help, at the same time screw in the fuel idle mixture screws on the center carb one at a time, if the motor stumbles/stall the Power Valve is fine, if idle quality pretty much stays the same with both screws in, the Power Valve is toast and/or leaking fuel via it's diaphram or gasket


Sometimes just a blast of compressed air in the idle mixture screws orifice after you remove the screw completely can dislodge crap, a clean idle screw passage can be determined by feeling the compressed air your blasting in the screw orifice and it's exiting the outer air bleed located in the carb's throat for each corresponding idle screw L/R....100-125 psi air won't hurt the carbs passages

Mike