Strange Engine Behavior

[l008com]

I went for a boat ride tonight and my engine was acting very strange.
It's an '89 Merc 3cylinder 2 cycle. Carbureted of course.
So the engine was kind of sputtering. But instead of sputters of a lack of power, when it sputtered, it would have these little bursts of power. It felt like, instead of misfiring, it was randomly firing with extra power. I was hoping it was just because it was cold out, and because it has carbs. It was in the low 60?'s tonight, which is pretty cold for me to be out. I tried opening it up for a bit but it didn't seem to help. It didn't ever stall, but it was acting so weird that I didn't go very far. I know generally how a carb works, but I don't actually know the specifics.

So I guess what I'm asking is, is this odd engine behavior normal, or is there something wrong? If it's just because it's a carbureted engine running in the cold, is there a way I can make it run better in cooler temps, without effecting the way it runs when it's 90?*out?

 

[emoney]

Re: Strange Engine Behavior

Not convinced that temps in the 60s would be "cool" for an outboard. Best to confirm if the issue is still there by firing it up and see if it does it on muffs in the driveway. "Sputtering" sounds like there's a fuel restriction or a timing issue, as it would have to come from either the spark or the gas. Hard to tell, of course, because we weren't there to hear it. If you start it in the driveway, and it's still doing this, posting a little video would be most helpful. When's the last time it was serviced and what have you done to it?

Do you run a fuel/water separator? How long has the gas been in the tank? If the problem is still there, do you have an alternate tank you can try to see if it remains or goes away? These are just some questions to get you started.

Any motor that "sputters" or "surges" has something amiss. Usually isn't a major thing, and more likely just minor adjustment or maintenance.

 

[l008com]

Re: Strange Engine Behavior

I took a few videos of it last night actually, but from the audio, you couldn't actually hear what it was doing. I don't think it was a fuel restriction, because it was a very sharp brief burst of power, not a drop in power. As far as the tank, it's just a 6 gallon unattached tank. 3 gallons were brand new, the other 3 gallons are about a month old. So the gas should have been fine.

 

[Georgesalmon]

Re: Strange Engine Behavior

Guess; Sounds like not firing on all cylinders all the time? When it has a burst of power all cylinders firing? Might check spark plugs that would be simplest. I would think electrical and not a fuel problem at least that is where I would start. I don't think outside air temp has anything to do with this.

 

[CharlieB]

Re: Strange Engine Behavior

Ditto, inspect the spark plugs to see if or how much carbon is on them. Clean or replace the spark plugs.

While the plugs are out it would be a good idea to verify that compression and ignition are acceptable.

Some auto parts store loan tools, a compression test should show close to the same pressure in each cyl.

NOTE: GROUND all the spark plug wires while cranking the motor, the high voltage must have a path to ground or may damage the ignition system.

Test spark on each plug wire, one at a time. Spark should be a hot blue arc across 7/16 inch in free air.

Once both of these pass you can safely assume your problem is either a fouled spark plug or a drop of water in a carb,

Remove the bras drain plug from the bottom side of each carb, one at a time. Catch the fuel that drains out and inspect for a clear 'bead' of water in the bottom of your cup. Pump the primer bulb to 'flush' any remaining water or trash out of the float bowl. Repeat on remaining carbs.

If you found ANY water, install a water separating fuel filter to prevent future occurrence.

 

[joed]

Re: Strange Engine Behavior

Are you sure it's not a fuel issue. I know my chain super revs just as the tank runs out of fuel and and leans out.

 

[l008com]

Re: Strange Engine Behavior

I went for another ride yesterday.
It's definitely not a temperature issue. It was doing it for the 3 hours I was out.
It was day time, so another thing I noticed was that there was a lot of blue smoke. More than usual, and it was there all day, not just when the engine was cold. It kind of made me wonder, is it possible that my chock is stuck on? It feels like it runs just fine once i'm plane-ed out. But at slower speeds it's very rough.

I have not checked out the plugs yet, I'll probably do that this week. But it doesn't feel like a plug issue because it feels like a little burst of power, not a little drop in power. But they're easy enough so I'll check em anyway.

I did take a recording of the engine. It was in the water, the "video" is just a still photo. It's hard to hear but it just revs for no reason.

 

[aussieflash]

Re: Strange Engine Behavior

Sounds like its just misfiring.Start with the basics...Check Compression and check spark on all cylinders.
Winding out and reving could be leaning out(fuel/carb) problem.

 

[aussieflash]

Re: Strange Engine Behavior

Sounds like its just misfiring.Start with the basics...Check Compression and check spark on all cylinders.
Winding out and reving could be leaning out(fuel/carb) problem.
Am I repeating myself????

 

[Maxz695]

Re: Strange Engine Behavior

The inside of rotting ethenol eaten fuel line hoses will mix rubber residue with the fuel, and cause the fuel to burn blue smoke and missfiring, as well as particals getting into the filter and sometimes the float bowl. Just something for you check into GL. There is a chance the exhaust divider plate has rotted trough (Happened to me on my 40HP )enough to let small amounts of water from the jacket of the outer plate into the exhaust port and getting into the cylinders hitting the plug which creates the misfire. A missfire would be more like a cough and not a loud backfire. Because of the misfire, the unburnt gasses leftover from the missfire combine with the new intake of new gases will obviously have a semming surge affect when the cylinder fires

 

[l008com]

Re: Strange Engine Behavior

Shows you what I know. I took out the plugs, cleaned them with carb cleaner, put them back in. Now it runs like a champ. I ran it on the earmuffs but its hard to tell. So I ran down to the river and dropped it in real quick. Went around for 15 minutes or so. Ran great. And it's cold today, about 54?. So there you have it.

Which makes me wonder, do you guys normally just replace your spark plugs every year? Mine look like they're starting to get kind of crappy on the business end, and the external part of all three were starting to rust. For the $2 it costs each, I figure I might as well just replace them every spring, no?