I hear a puff of air and the engine stumbles

1986 Mercruiser 260 5.7

Here is a weird one. Last year before I wrapped my boat I changed the oil and noticed some oil on the back of one of the heads. I figured it to be a bad valve cover gasket so I pulled the valve cover to find out the valve cover was rotted. I hade two small block chevy valve covers so I got new gaskets and swapped them out. That's all I did besides draining the engine of water. Just the oil change, valve cover swap, took batteries out, drained the water and covered it up. No other changes since last year when it was running fine.

This year I have taken the boat out twice and only when in gear and under 1500 RPM's (Trolling through a no wake) I every so often hear a puff of air come from the engine compartment and the engine stumbles for a brief second. Other than that, it runs smooth and strong, no other problems. All tuned up and happy. Idelling (not in gear) fine, motoring over 1500 RPM's fine. Just trolling, every about 1 to 1-1/2 minutes I hear a puff of air and the engine stumbles for a very quick second.

The breather tubes appear hooked up correctly and are obstruction free.

Weird right?

I am wondering if at low RPM crankcase pressure is building up in the valve covers and overcoming the breather tubes and blowing out through the valve cover gasket every once in a while causing the puff sound, I mean I guess a quick drop in crankcase pressure would cause a slight stumble right?

I think I will tighten the valve covers next time I take it out and see if it goes away.

I find it very odd, I thought I would post to see if anyone else ever heard of anything similar???

Re: I hear a puff of air and the engine stumbles

Ayuh,.... I'd sooner think the carb is runnin' Lean, at idle, 'n yer hearin' a slight backfire, 'n stumble...

Re: I hear a puff of air and the engine stumbles

You know, that makes sense Bondo, perhaps I adjust out the idle screws a turn and see if it stops. Good idear ya have there. Funny thing is that it just started doing it this season. Never last season. I was only looking at the things I changed since then. Perhaps the carbe gummed up a bit over the winter. The plugs look nice and tan, but that could be an indication of a good mixture off an idle circut. This whole puff of air and stumble, its the oddest thing.

One other thing I thought of last night after I posted this. The only vaccum port I see aside from the choke pull off is a line running to the mechanical fuel pump. I have never inspected a marine fuel pump, I imagine it is just a car fuel pump shrouded by a casing in case the pump leaks the fuel would drip into the case and be sucked up by the vaccum ling going to the carb thus running so rich it would force someone to inspect it. Seems like a good idea. If I am correct, then the vaccum line I see must go to a tube that reaches down to the bottom most cavity of the "casing" I speak of so that it would suck out any potential fuel, there would have to be a small hole or valve atop the casing to let air in to suck the fuel out. Perhaps there is vaccum going to the fuel pump and if there indeed is a air hole or valve in top of the fuel pump, perhaps it is clogged and opens briefly enough to suck in a burst of air thus emitting that "puff" noise I hear and stumbling the engine. Seems a long shot, especially if I don't truly know how how the fuel pump is designed. But seeing as how it happens at low idle when the engine has the most vaccum, it just seemed like maybe it could be a vaccum leak. I will try making the carb a little fatter at idle per your suggestion. Thanks

Re: I hear a puff of air and the engine stumbles

On your fuel pump....the clear line is for...when your fuel pump Diaphragm ruptures it will send the fuel back into the carb to burn instead of gas in your bilge or motor.

If the line has a crack near the carb it could cause a vacum leak.

Re: I hear a puff of air and the engine stumbles

Turns out the PCV fitting on the starboard was partially obstructed with crud. I should have checked it more thoroughly. The puff of air I heard was likely pressure building up in the valve cover and escaping every so often. I cleaned it out and the puff of air sound and stumble went away.

Re: I hear a puff of air and the engine stumbles

Mine does that when it's cold and I pull out of the dock and give it. Only ever does it after I take it off the trailer. I got pop. Hesitation and then she's off. Doesn't come back again till I put it in the water on the next tour