Short version: Replace your head temperature sensor. (It's the one on the port side.)
Longer version: The port side temperature sensor tells the ECU the engine's temperature. The ECU uses that information to control fuel injector pulse width. (There is also a temperature sensor on the starboard side, but that is used to sense the cooling water's temperature.)
The port side temperature sensor has a reputation for failing after a while. I've never experienced this, but the symptoms are many and varied, and usually pretty dramatic. I have a spare sensor that I picked up "just in case" from a guy who was parting out a 240EFI, but haven't ever noticed any of the dire symptoms reported by most victims of a bad temperature sensor.
This weekend we were running the boat and I realized that I've been seeing a bit more smoke lately, even when the engine is fully warmed. It seemed to smoke at idle more than it should, and definitely more than I remembered. Nothing serious, nothing dramatic, but I noticed it.
If the ECU were being lied to about the temperature, and thought the engine was still cool, it would lengthen the fuel injector pulse widths to richen up the mixture and help the cold engine warm up. On a warmed engine, though, that could cause the smoking I was seeing.
So, having a few spare minutes, I thought what the heck - let's swap out the sensor. So I did. (It takes a 13mm socket and about three minutes.) We then ran the engine for the next two days.
WHAT A DIFFERENCE! The smoke was gone - totally. That alone made it worthwhile. But the engine also ran smoother. The change had been so gradual that I hadn't noticed it, but snapping back to like-new performance was amazing. I couldn't believe what we'd lost.
We also gained 3-4 MPH on the top end. I had attributed that to air resistance from the waketower I installed, but now we're back to 54-55 MPH from the 51-52 we were seeing. That may be due to the ~100-200 RPM we gained at WOT. We had been at 5900-6000 and now we're getting at least 6100.
We also gained efficiency. We used to hold 20 MPH (wakeboarding speed) at exactly 4000 RPM. Now the engine purrs along at ~3600 RPM for that speed, a full ten percent less.
It is now my opinion that these sensors can fail gradually. I haven't plotted curves on the old one, but I bet they don't match the ones in the manual anymore. I now think they should be considered a consumable that should be replaced every few years, particularly since they only cost around $40 IIRC.
I wish I had replaced this sensor at the start of last season. My advice: Don't wait. Buy a replacement and swap it in. If you don't get any improvement, at least you have a spare for a component that is known to fail quite often. If you do see improvement, pat yourself on the back for the fastest and cheapest tune up imaginable.
