Rough Idle And Gummed Up Pluggs

[stevem1961]

Sorry if someone has covered this topic before. I haven't been on here for quite a while. I have a 1986 175HP Mercury Black Max that is running really rough when idling. It overheated on me first so I replaced the thermostats (waterpump checks ok)then it was running really rough through the full rpm range and, leaving what appeared to be a pretty long vapor trail out the exhaust. I couldn't keep it running at that time and had to be towed. I checked the water jacketts on the heads and removed the heads for inspection. The heads were covered with a black tar looking substance (thick and sticky). I replaced the gasketts and pluggs but still have the problem. The problem seems to be concentrated mainly on the left three cylinders and is bad enough to easily foul out the pluggs. The motor seems to rev-up really quick but idling is a real problem. Does anyone have any ideas?

Thanks in advance

PS: Sorry for the long email.