Carburetor Icing? (Merc 25hp four stroke, 2005)

[halmc]

Brace yourselves. In an earlier thread I described how my engine runs nicely, but from time to time, and todate, inexplicably, drops 2-300 RPM, then takes off again. Um, unless I take the cover off.

Take the cover off the engine and it runs peachy. No rpm drop at any rpm range. Runs like a new one.

BUT (and that's a big butt) put the cover back on and it's same ole, same ole.

So: along I go, with cover on, engine starts to lose rpm gradually (over two or three seconds) then suddenly drops maybe 300rpm, does NOT cough, then back to normal. Until next time.

Cover off: no issues.

I have taken the cover on and off maybe fifty times over about 28 hrs of running time.

I've never heard of carburetor icing on any marine engine. But the symptoms are absolutely identical to those on a light airplane when carburetor icing is experienced.

And I'll be cussed if I can figure how the engine cover would enhance carb icing -- in fact, seems like it would inhibit it.

Hithereto, I have used fuel with "up to" 10% ethanol.

Observations?

(the air passage through the cover is unobstructed; nothing on the cover touches any part of the powerhead, other than the bottom cowling; there are no modifications to the intake path; the engine never quits; with the cover off, the engine runs perfectly.

Why me?

 

[oldman570]

Re: Carburetor Icing? (Merc 25hp four stroke, 2005)

From the way you state that it runs fine with the cover off, and not so good with it on, Makes me think that there is an restiction in the air to the motor. Could be that exhaust is getting into the motor cover and depleating the fresh air that is needed. I have had motors stumble as such when the vent blown on my test tank took a vacation and the exhaust fumes would come back on the motor I was testing. Check the gaskets between the powerhead the lower mounting aera for any signs of a leak that could cause a buildup of exhaust into the carbs when the hood is on. Also check to see if the fuel hoses could be getting smashed or a sharp bend in the hoses could be the trouble.JMO
Oldman570

 

[halmc]

Re: Carburetor Icing? (Merc 25hp four stroke, 2005)

From the way you state that it runs fine with the cover off, and not so good with it on, Makes me think that there is an restiction in the air to the motor. Could be that exhaust is getting into the motor cover and depleating the fresh air that is needed. I have had motors stumble as such when the vent blown on my test tank took a vacation and the exhaust fumes would come back on the motor I was testing. Check the gaskets between the powerhead the lower mounting aera for any signs of a leak that could cause a buildup of exhaust into the carbs when the hood is on. Also check to see if the fuel hoses could be getting smashed or a sharp bend in the hoses could be the trouble.JMO
Oldman570

(the air passage through the cover is unobstructed; nothing on the cover touches any part of the powerhead, other than the bottom cowling; there are no modifications to the intake path . . . )

 

[Laddies]

Re: Carburetor Icing? (Merc 25hp four stroke, 2005)

A lean condition will cause icing on a marine engine, it use to be common on up draft carbs.

 

[halmc]

Re: Carburetor Icing? (Merc 25hp four stroke, 2005)

It's pretty bizarre. If it is icing, I'm not sure exactly how to cure it. The engine does not run as if it were lean, but ya never know. I have shifted over to non-ethanol fuel, hoping -- w/o any scientific theory to rely on -- that it is less prone to icing than today's auto fuel. Haven't had a chance to try it yet. Noted, too, that the last few carbureted Mercs were probably jetted lean to meet emission standards. So maybe if the fuel doesn't cure the issue, I'll richen it up a bit.

In the meantime, removal of the cover is part of my "preflight" procedure. Running around w/o a cover on the engine makes me look like a water borne version of the Clampetts.

 

[quicktach]

Re: Carburetor Icing? (Merc 25hp four stroke, 2005)

You have looked closely at the air intake side of the motor. How is the exhaust side of the engine ... is the exhaust restricted in any manner ... in there any sludge anywhere in the exhaust system on the way out of the cylinders? I'm thinking of an upset in the net air flow through the motor by putting the cover on. It may be worthwhile trying some decarbonizer in the engine.... following the instruction on the spray can. This stuff is available from your Merc dealer or any auto parts store.