Small Engines (Lawn Mowers, etc.): Briggs&Stratton 80202 1720 01, air fuel mixture, briggs stratton

Question
Dear Mr Jones,
This horizontal shaft 3HP engine is fitted to a cylinder lawn mower.The engine is 20 years old but had little use. It has a metal carb mounted on top of the fuel tank,a manual choke,a throttle control on the mower handle.The problem is I can't get a reliable 1750rpm idle.If I set the needle valve for correct mixture at the governed 3000rpm, I end up with the screw about 3/4 turn out. But then the engine won't idle smoothly. I notice the governor is moving back & forth slightly in time to the vibration of the engine at idle,but if I hold the throttle quadrant against its stop I can then get a steady idle.When I release it,the idle rises to 1900/2000.
I am puzzled as to how ONE mixture screw is supposed to be capable of setting an adequate full power AND idle adjustment.It doesn't seem possible to me.
I have changed the small diaphragm and both fuel suction pipes.
Your advice would be most appreciated.
Regards,
James Jarratt

Answer
Many Briggs idle speeds are 1800 rpm.

Is the air/fuel mix jet removable?  Not just the needle, but the jet?

Have you checked to see if the carb, tank and diaphragm cover are warped?  

We used to find a lot of carbs warped.  A warped carb can cause an air leak which may be why the air/fuel mixture needle only needs to be open 3/4 of a turn.

Let me know.
Eric