50HP Mercury 4 Stroke EFI RPM issue

Hey all, wish my first post was not an SOS call but it is what it is. Hope I can return the favor with info at some point but I am a novice at this point.

Bought a new to me boat, 2010 Legend 16 XGS with a 50HP 4 Stroke EFI engine and here's what is happening.

Engine starts and will run at idle with no issues. Engine in the water in neutral can rev up past 5K with no issues and while in gear in reverse past 5K with no issues. Put the boat in forward and could not get it past 3500 RPM. There are no beeps or sounds for engine issues coming from the mercury, it will move along at this RPM doing about 11mph with no issues. At first I thought that was just the way it worked being novice and all, wasn't until the father in law told me that is not right. after turning off at one point and doing a little fishing, started it back up and the engine ran at 5000 rpm, hit 25mph and continued to work no issues for about 2 hours.

Que the next morning, get in and start out and the engine is down to 3500rpm again but this time before I get a half mile it is running fine and moving along no issues, up and down the rpm line. I fish for about an hour at 3 MPH and head back to shore and no issues. About 1hr later we load up and head out and the same thing, 3500 rpm but it does not come out of it after a couple miles and we go back to doc, drop the crew off and head back out with just a buddy to fish close by and we are not a half mile out and boat works fine lol. Head back to shore, boat is off maybe 5 minutes and head back out, back to 3500rpm max. Stays like this for the rest of the drive, about an hour.

So again, no issues starting, no issues in neutral or reverse, only in forward will the engine do this. To me it seems like a fuel issue, the fuel had about a half of tank from last year but the guy stated it was stabilized and I topped the tank up with fresh supreme. The gas filter has some dirt in the fins, I do not think this is the cause but will replace. The priming bulb seems fine, pumps up fine and remains hard to squeeze. There are no issues with the throttle cable and linkage as I checked with cowl off. The prop spins freely with no restriction when in neutral and out of the water so no strain there causing it, plus at time it comes out of it. I have heard that there is a screen in front of the fuel pump that has caused issues like this and may need to be cleaned. Also heard a neutral switch can cause this but was not able to locate it on the engine to disconnect and try. Also heard a battery may cause this, volt meter is showing 13 volts and the batter is new but have a second new battery I can switch being used for the trolling motor. Also tried loosing the gas cap to see if it was a vacuum issue.

I am thinking my next move is check the fuel pressure, new Fuel filter, gas stabilizer, more new fuel and fuel injection cleaner before I start trying to disconnect vapor separator tank and clean the pump.

Anyone with any other thoughts?

After talking with a close freind who used to be a Mercury mechanic and has the same engine at 40HP he is point at the fuel. States his 3 year old 40HP does the same thing and also advised to avoid gas with ethanol in it. I did use premium fuel but after researching it is the only provider in my province that still uses ethanol in their supreme fuel.

Fingers crossed. Will be a week before I get out again.

Got out to service the engine today and do not think other than oil changes much had been done over the years. Fairly certain the low pressure fuel pump filter was never changed, the gas was brown running out and it looked to be secured from factory as a wire tie needs to be cut to access. Gear oil was not great, bit thick and milky, one of the plug gaskets were missing so I filled it, all new washers and will check it in a couple weeks, may need to pull the prop and replace the prop seals. Replaced the high pressure fuel filter as well. Ran out of time to test so will let anyone interested know next week.

To the boat out today and after a half hour test run doing he same thing. Changed the plugs and still the same, tried bypassing the neutral switch and the same as well. Works fine in neutral, works fine in reverse, while in forward gear power is limited, hit 3500rpm and playing with he trim I change a better plane and hit 4000 rpm. Has not come out of it in a while. Going to try running the fuel through it and see what happens. Thinking a trip to a dealer may be in the future, any other suggestions?

About a half tank through it at this point with seafoam in it and still the same. Let it idle today at the dock for 45 minutes and purrs without so much as a shake. Been told to check the exhaust next, the guy before me let it sit in the water for the season and barnacles built up on the haul and engine which were removed, said they may have got inside and be restricting the exhaust.

To check the exhaust, you will have to drop the lower unit; have a 2 stroke 40 that had extreme carbon buildup in the exhaust cavity and the exhaust manifold; not hard to drop the lower unit - I think you have to have the shift lever in forward but that is very old info.

RickM46 said:

To check the exhaust, you will have to drop the lower unit; have a 2 stroke 40 that had extreme carbon buildup in the exhaust cavity and the exhaust manifold; not hard to drop the lower unit - I think you have to have the shift lever in forward but that is very old info.

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Thanks for the tip, I was going into it thinking I could just take off he prop.

When you had the exhaust block did you have reduced RPM’s? The item after the exhaust I am going to check is the timing.

No to reduced RPMS. When I had the extreme buildup of black carbon stalactites in my exhaust channel and large buildup of carbon in the exhaust manifold, my Merc 2 stroke 40 engine would not idle. It would not idle because the 2 relief exhaust ports at the rear base of the head were blocked by the stalactites. However, in debugging this issue, I made sure the timing was set to specs. After I FINALLY saw that the relief exhaust ports were blocked, I dropped the lower unit and chipped/scraped the carbon from the exhaust channel with a metal rod and removed the exhaust manifold and scraped the carbon out of it. THEN, I readjusted the mixture and idle in the carbs (had the shop manual). It then idled. Since you have EFI, you don't have to worry about carbs - maybe just the injectors.

I don't know if the latter will help you but the shop manual led me through a bunch of checks on the engine to eventually debug my problem.

Praying this is the last update!

As mentioned put seafoam in the gas last week and let a 1/4 tank or so run through before leaving for the week. Started the boat for the day and the first start had a puff of black from the exhaust that smelled, took the boat out and each trip around the lake it would come out of it and work than go back to 3500 rpm. I accidentally hit the kill switch and when I started back up the boat ran great for about 2hrs. Let it sit for an hour and went out again, started fine and took off phone but lost all power and for the first time ever the engine quit. Started back up and ran fine for 2hrs with multiple on and offs. Hoping it was gunk in the system and will continue to work.

Well it is fixed, was a plug wire.

Glad to hear you got it fixed! Welcome to the club of debugging. The good thing is that your efforts got a lot of other things up to snuff. You may want to drain and fill the lower unit with new lube; other than that, have a good season.