Yamaha 9.9 4-stroke backfires

[chazebow]

I have a 91 Yamaha 9.9 4-stroke motor that backfires when you open up the throttle. It idles ok but when you give it gas it backfires for awhile before it reaches full throttle. I took the carburetor off and cleaned it out real good then put in a new carb kit before putting it back together but that didn't correct the problem. Could it possibly be a bad thermostat? It seems to run fine when you first start it up but starts spitting and sputtering after it is warmed up some.
Any ideas as to what the problem could be would be greatly appreciated.

Thanks!

[boats.net]

Check compression to make sure valves are seating, check belt timing, check spark plugs and fire to each cylinder. Check ignition timing.

[arny]

Motor runs to lean,turn mixture screw 1 turn out.