I just purchased a used 1989 Mariner 6hp long shaft outboard. The outboard won't idle in neutral worth a darn. It seems to run rough a low speed but slightly higher speeds no problem. Every couple hours I would run it up to burn it clean. But was still able to troll 7.5 hours yesterday no problem. I checked the plugs and they were good. But will change them over to NGK's. I also switched it over to synthetic AMS oil, as there was a lot of staining of unburned oil below the exhaust port just under the power head. I guess my question is this just a carburator adjustment or a possible combustion chamber carbon build up problem, or normal with this 6-8 hp mariner-merc line of 2 stroke persuasion. Thanks for any ideas. 