F115 im stuck

[Big john k]

I just recently got a 2001 f115 4 stroke. i did all the usual stuff plugs, oil , new injectors cleaned screen on high pressure pump, new fuel lines, new filters had it running great for the last month. Then it started to idel at 1600 to 1800 rpm and slam the lower end when I put it in gear. So i pulled it form the lake and noticed the crank case was full of gas. I replaced the thermostat it was stuck open and checked low pressure pump and it dont seam to be leaking in the crank case . Change the oil. But i cannot figure out the high idel. When tinkering i noticed that if i pull the air intake silencer tube off of the air box and plug it with my thumb rpms goes back down to 1000 were it was when it was running good. Any ideas please help i cannot stare at this motor much longer summer is half over. Thanks john