Cams in these mower engines are so small, think you are barking up the wrong tree. These are low rev Diesel like engines. Put the valves in factory spec and look elsewhere for your slow revs. As in 97% of all complaints are dirty carbs and or neglected fuel systems.
To more directly answer your question, you should never be at a low/slow 1750rpm like idle. Unless your engine is water cooled with full pressure lube and oil filter. These are not car engines. They are made to immediately go from 0 to 3600rpm all day long. Briggs states for MAX engine life and durability, run at MAX revs ALL THE TIME. At low revs you get way reduced air cooling and lubrication. Slam her at 3600 and never look back.
Sounds like you have a fuel supply issue like a dirty carburetor or a mounting gasket leaking.
Adjusting valves too tight will result in faster valve and seat wear. These clearances are designed in to compensate for heat expansion of the components.
Valve clearances are recommended to be only adjusted when the engine is cold.
Have you verified both cylinders are operating? These engines will run on a single cylinder and not vibrate.
Try using a spray bottle and spray a shot of water on each exhaust outlet tube near the head. The water should boil off immediately. If it does not on one side, there is likely a coil issue or the kill wire harness has a bad diode inline.
