What year is your Yamaha?
I had a similar issue with my 06 yamaha 50 hp 2 stroke, it ended up being that i had an air leak where fuel was coming into the engine (the seal at the ball valve that plugs into engine was allowing small air leak)
couple things i learned while trouble shooting it were.
1) make sure that your emergency auto enrichment was in the right position (its the red switch on the front of your carbs) it should be in the completely up position.... not on or emergency...
2) change your plugs.. it's amazing how much better performance you get out of an outboard after changing plugs.
you said that you started running an oil and gas mix does the engine not have an oil mixer? if it does have you confirmed that its hooked up right after your carb work and that its pumping oil?
if your running double oil that could foul your plugs pretty quick.
sending you a PM as well that may help you troubleshoot it.
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