I recently purchased a used boat that has a 1993 Merc 115 4cyl motor on it. The serial # is OD190240. The motor seems to have been maintained well, it's very clean and everything appears to have a good amount of lube. I'm planning to replace the water pump and thermostat just for peace of mind since I have no idea how old either one is.<br /><br />Due to my limited time recently I've only had the boat on the water a few times but have toyed with the motor quite a bit on a hose in the driveway. I purchased the Clymer manual for this motor and have found it helpfull but it doesn't answer all my questions. I've noticed a few things that I'd like some help on. This may be a little long but I figured it would be better to ask all my questions in one thread versus starting a bunch of new topics.<br /><br />From what I gather this motor is refered to as a 2+2, that is at low rpm only the top 2 cylinders are firing and the final 2 cylinders kick in somewhere around 1600 rpm. I'd never heard of this setup prior to owning this motor but I now think I understand how this works. Anyone have any feedback on this system, pro or con? The previous owner had the idle set at about 1800 rpm, I'm assuming this was to keep all 4 cylinders running at all times. I didn't like this as the motor seemed to slam into gear with such a high idle. I adjusted the idle timing screw on the throttle arm to lower the idle but I don't feel I got it right. The lowest rpm I can get the motor to idle smoothly is about 1000 rpm. If I try adjusting the rpm lower the motor runs fine for for about 30 seconds but then the rpms will occasionally dip down to about 400 rpm for a few seconds and come back up. After a few mintues of this occasional dip in the idle the motor will shut off unless I give it some gas. It starts right back up everytime this happens. Is there another adjustment that I need to make to get the idle down to the proper rpm?<br /><br />On the rear upper left part of the powerhead there's a cover for the thermostat. This cover has a weephole that's leaking a small amount of water whenever the motor is running. I have been told that this is probably just a diaphram associated with the poppet valve. Does this sound right? Could this be the sign of a larger problem? Could someone explain to me what the function of the poppet valve is? <br /><br />My boat doesn't have a temp gauge or water pressure gauge mounted on the console. Any opinions on adding one of these versus relying on the alarm to let me know of any temp problems? The boat does have a mechanical oil pressure gauge on the console however it doesn't appear to be working, the needle barely moves with the motor running. Is there anyway to check this gauge for proper operation? What oil pressure should this gauge be showing if it's working properly?<br /><br />Sorry that this got so long but as I said earlier it seemed better to ask all my questions in one thread. Thanks in advance for any and all help you guys can offer.<br /><br />Rob
