leave your bilge blowers on...just a suggestion. I know my boat gained 2mph when I cracked the hatch at full throttle. I added a scoop and hole sawed vents it the back of the hatch.

That is extremely common, believe it or not. Usually, the port engine is the one that vapor locks. Merc puts the fuel filter on the port side of the engine on just about every engine they build. Because of that, the port engine's fuel filter gets heat from not only the engine it's mounted to, but from the starboard engine as well. The starboard engine's filter is on the outside of the boat. It's not receiving heat from the other engine. Also, there are usually vents on the outside of the boat, so it ends up getting a little more cooler air, stopping it from vapor locking.

The absolute fix for the problem is to run the fuel return back to the tank, instead of back to the filter head like it currently does.
Eddie

duplicate

Although vapor lock is a likely culprit there is one other condition that can cause similar symptoms. The diaphragm in the VST can go bad allowing fuel to bleed through and out the secondary vapor line to the throttle body. Check this line to see if its percolating. If so it will actually flood the engine forcing you to go to flood clear mode to get it started but will appear to be vapor lock.

Those VST are problem childs to begin with even the O-ring seal can be hard to install after removal. That fuel set up did not stay around long for Merc and there was reason for it as in problems. Also there was a reason starting with Gen II cool fuel systems to the current Gen III as of fuel & cooler components moved to the bottom of the engine area. It brings it closer to the bilge area in which stays cooler plus the lake water underside of the hull keeps that area cooler as well.

Also for Gen II cool fuel systems Merc added a Boost pump in which helps against vapor lock. Merc also had a couple of Service Bulletin's about vapor lock back in 1997 and a revised issue in 1999 if I remember correctly because many black productions motors were experiencing vapor lock right out of the box back then.

Also the fuel you use can cause vapor lock. If you read up about RVP = Reid Vapor Pressure that can explain a lot. Its value changes in places of the US up to 7 times depending on the weather conditions for a period of time. We experience this here in Texas in the marine end because a customer filled up his boat on the 4th of July and never use the boat again - then we get it to winterize at freezing temps. The (summer gas blend ) does not work very well in temps below 40 degrees. It has to do with RVP of the fuel itself. RVP and specific gravity of fuel can make your engine run not so great sometimes.