Small Engines (Lawn Mowers, etc.): lawnmower, stop screw, incorrect assembly

Question
I have a 3.5 horse B+S Lawnmower that I put a new diaphram in and cleaned the carb and tank.It fired right up and ran good for a while. Now it has become real finicky on the carb adjustment and the governor seems messed up.I get it to mow and the next time I use it, it is all out of whack again.Are there general directions for adjusting this simple carb and governor.Sometimes the rpm goes up and down by itself.The throttle only works wide open won't idle down or in between.Sometimes it will only run with the aircleaner off even when clean.I have to start it now holding the butterfly open with a screwdriver  and then turn the little shaft that the governor stop screw sits on top of.I can turn adjustment screw almost all the way in and runs best at only half turn out.Can you diagnose anything from this? thanks

Answer
First off, if you get little or no response by adjusting the mixture screw, the carb still has a blockage.  The engine revving up and down also indicates this.  Another issue may be incorrect assembly of the carb.  Go to www.briggsandstratton.com with the engine numbers and locate the parts diagram to ensure you have assembled the carb correctly.  Hope this helps.