low speed on max throttle

I have an old 3 hp toro 92908, self propelled, auto choke, fwd, b&s eng #1842-02. I have kept it running over the years with routine maintainance only, no major repairs. I keep the gas tank empty in the winter and fuel it in the spring. A year ago I replaced diaphragm with oe B&s, and it has run perfectly since. This spring, it started on the first pull after winter, but has a low, even idle speed at max throttle. The choke plate is closed, but the idle becomes full speed when I manually open the plate to full vertical position while the engine is cold, not hot, on a fresh start. I let it get hot with the choke plate held open, and once hot, I was able to mow at full speed, and stop and start with no problem. I get no surges or roughness. There is no reason to think I have accidentally turned the idle speed or mixtures screws while in storage, and I find it hard to believe the NEW diaphragm has failed. Any suggestions to solve this problem appreciated

low speed on max throttle