91 Marathon leaking Fuel and Oil and Backfiring

[Sherman]

I recently acquired a 91 gas powered Marathon. The oiler has been disconnected and we are running premixed gas. It runs fine for about 10 to 15 minutes and then when you try and start it in forward the starter will spin but it will not fire. If you switch it into reverse it will fire right up and go without any problem. While it sits for about 20 minutes some fuel and oil leaks out of the exhaust pipe and then it will start. It also backfires occasionally when you let off the gas. I have some experience with small outboards and lawn equipment but this is a little different to me. I am thinking the float or needle in the carb is the problem. Any suggestions would be appreciated. Thanks

[Sherman]

Update: I checked the fuel pump, cleaned the carb, replaced the spark plug and fixed a loose spark plug wire. Now there is no leaking or backfiring and it runs great for about 15 minutes or so. When you stop and then try and start it in forward the starter will spin but it will not fire. If you switch it to reverse it fires right up and runs without a problem. Is it possible for the forward microswitch to malfunction in this way or should I look elsewhere?