Occasional stumble, Merc 25hp four stroke

"Stumble" is probably the wrong word. My pontoon boat 25hp four stroke egine occasionally dips rpm for half a second or so, and does so at intervals anywhere between six or seven minutes, on the long end, and 45 seconds or so on the low end. I'll be cruising at 2500 rpm or so, and the engine will drop to 2100 rpm for maybe a half second, then return to 2500. Otherwise, it runs perfectly.

Here's the weird part: If I remove the engine cover, it might dip once in a half hour. That's not speculation. On a three hour journey yesterday, I made all sorts of changes I could think of -- including idle mixture -- but the major variable was whether the engine cover was on or off, and I repeated that operation enough to firmly believe that my observation that the cover removal affected the engine is accurate.

I have carfully inspected to determine whether the engine cover was somehow mashing a fuel line -- it isn't, or restricting air flow to the carb -- it isn't.

In another thread ("my bowl runneth over") I described a far worse problem, with the carb flooding and causing real serious engine stumble. A carb rebuild kit fixed abo0ut 98% of that issue. What I'm left with is this intensely annoying rpm dip, that seems to sense when I'm nearing a leeward shore, or docking the boat.

The rpm dip occurs over the entire rpm range.

Things I have not yet checked:

integrity of the o-rings in the intake path. But it idles too well for a vacuum leak.

Possible accumulation of water in the fuel. (tho' I have drained the bowl more than once with no discernable effect.

Spark plug condition

Fuel pump output. But it runs WFO for thirty minutes w/o a stumble with the cover off.

The rpm dip is not an ignition miss, which would be somewhat more pronounced. This dip is as if I had throttled back momentarily -- for less than a second -- then returned to cruise.

While the boat goes everywhere I want it to, and docking is not overly affected, if it were an airplane, I would not fly it until it was fixed.

The engine cover thingy really has me stumped. It coudl affect the temperature of the things like the coil, but hard to believe it would be that sensitive/marginal.

Comments? Suggestions? Remedies?

25hp-four-stroke-model-1f25412dd-serial-1b169120

Re: Occasional stumble, Merc 25hp four stroke

momentary lack of fuel would happen at hi speed through the high speed jet. Being momentarily blocked or at low speed through the low speed jet. Being your saying it does it at high and low speeds indicates possibly some debris in the float bowl blocking the main port somewhat or lack of the proper amount of fuel in the bowl. a semi clogged filter. Water in the fuel. Fuel oil and gas not properly mixed. Reed valves not sealing properly. Losse carburetor or leaking gasket or o ring. On the electrical side Plugs not fireing correctly or fading under increased pressure plug wire arching loose coil or plug wire. semi grounding wires bad trigger coil Just a few things to consider. Fuel lines or primer bulb leaking air Lastly water intrusion. By removing the cover you are making it easier for the engine to take in more air with less effort and therefore allowoing better air fuel mixture that would overcome any water intrusion to a point.

Re: Occasional stumble, Merc 25hp four stroke

Maxz695 said:

momentary lack of fuel would happen at hi speed through the high speed jet. Being momentarily blocked or at low speed through the low speed jet. Being your saying it does it at high and low speeds indicates possibly some debris in the float bowl blocking the main port somewhat or lack of the proper amount of fuel in the bowl. a semi clogged filter. Water in the fuel. Fuel oil and gas not properly mixed. Reed valves not sealing properly. Losse carburetor or leaking gasket or o ring. On the electrical side Plugs not fireing correctly or fading under increased pressure plug wire arching loose coil or plug wire. semi grounding wires bad trigger coil Just a few things to consider. Fuel lines or primer bulb leaking air Lastly water intrusion. By removing the cover you are making it easier for the engine to take in more air with less effort and therefore allowoing better air fuel mixture that would overcome any water intrusion to a point.

Click to expand...

LOL, a shotgun blast. I was hoping for a rifle shot. I've considered most of these things, except the oil/gas and reed valve observations.

It's a weird one, for sure, and I'm gonna concentrate on the vacuum issue and fuel quality and supply for the time being. I'm not your basic right-wing lunatic, but I will say that ethanol in motorfuel is just about the dumbest thing the gummint has been herded into. "Marine" gasoline is available at my marina and I'm gonna go for a tank full of that good stiff if my current supply ever runs out. (a 25hp runing at about 20% throttle takes a while to burn 12 gallons)

Re: Occasional stumble, Merc 25hp four stroke

Take a fuel sample from the bottom of the tank.Most fuel system problems are caused by water/dirt in the gas.Adding clean gas to a dirty tank only makes more dirty gas.If you don't have a water separating fuel filter get one.

Re: Occasional stumble, Merc 25hp four stroke

It,s always a great thing to use a portable 6 gal or the like tank for testing Mine has been a great help in diagnosing issues related to lines hoses vents etc. My trip went fairly well yesterday. I had some plug issues probably carburetion when I got it home though. Too dang windy and choppy to test run in my lil bass boat on the bay. It,s even worse today. Another thing to try is get the tank higher than the engine and gravity feed it straight to the carbs. This eliminates everything eles so you can see if it is the engine or the supply system.Just a thouhgt