Have a 1981 Mercury 70hp bogs at full throttle ,

by Arnold
(Makwa , saskatchewan )

So I took my boat out for its second outting this year , the first time outting we were on the water for 4 hours motor worked awesome , second outting , I switched tanks , which had alittle old gas in it , that I topped up with new gas , motor ran good til half way across the lake , then bogged out at full throttle , and now it only runs good til half throttle , any farther past that it Boggs out , I'm thinking maybe carbs sucked in something from gas tank , ? What yours guys thoughts , how could I fix this ,