I have a 1983 Mercruiser 898 Chevy 305 We went out for a cruise today on the way out she ran beautifully. The were out for about an hour and when I turned to head home I I brought the RPMs up to about 3000 from about 2000 and after about 3 minutes the engine started to back fire through the carburetor. I had to back down the RPMs to about 2500 and then after about 5 minutes the engine started to do the same thing again so I had to back it down again to 2000 it ran for a little while longer and then I had to back it down again 1500. I ended up getting towed back to the dock but I kept the engine running the whole time all the gauges were in the normal range. New plugs and tune up and wires. Gas is suspect. When I got back to the dock the engine ran fine. ????? Any ideas? I did check the times and it was good.