Engine starts to run rough after 1 hour

[Softuk]

A few days ago I took my new (for me) boat out on its first trip. The engine started up and ran beautifully for about an hour and a half. Then it started to miss every few seconds. Then it got worse over time to the point where it was really struggling to stay alive. After dropping the throttle and giving some choke it smoothed out for a while but continued to get worse until it finally died about 10ft from the slip. I replaced the aged fuel filter and lines and tried it again the next morning. Same exact thing happened an hour and a half out. Limped back to port and called a mechanic. He immediately suspected the coil and sure enough there was oil all over it like it had leaked and over heated. Its replaced now but I don't feel confident that was the problem. It seemed much more like a fuel problem. If it was just the coil would giving choke make the engine run smoother?

[67rway]

Does it have any gauges on it? Water temp, oil pressure, volts/ammeter? What were they reading?

How does the oil look?
How do the spark plugs look?

[Softuk]

After warm up and at cruising speed oil pressure is at 40 and water temp is around 155 degrees. Voltage is at 14v. Oil is very fresh, spark plugs are good.

I took the boat out again today to test the new coil. The same exact problem persisted. To be specific the engine speed would wobble in a range of about 100 rpms. This range would get bigger the longer I drove. Occasionally the engine speed would just drop by 100 rpms and would then wobble around that speed until I would bump the throttle to bring it back up to cruising. Giving it some choke would smooth out the wobbles but I had to keep giving more and more choke as time went on.

[tenders]

Sounds to me like a failing fuel pump. Second guess would be an air leak somewhere in the fuel line, but if you replaced all the line and the problem didn't change, I'd look elsewhere. Third guess is a failing condenser, if you have analog ignition. The coil wasn't a bad guess, but I agree that the choke helping the situation suggests otherwise.

[Softuk]

I don't think its the condenser. I replaced that just before that first trip along with new points. The fuel pump is mechanical. How would I diagnose that? Also I did replace some fuel line but there are some fittings on the tank and such that might be a culprit. What would be a good test for that? Thanks.

[tenders]

I'm not an expert on the mechanical pump but I believe it does have a diaphragm which wears out and can be replaced. This situation sounds like that actually: it's pumping just below what it takes to run the engine. There's some extra fuel in the carb float bowl making up the difference, but once that's gone the engine slowly starves.

I had a similar problem with an old electric pump a few years ago, though the problem manifested itself after about 15 minutes, not 60.

To check the fuel lines, look carefully at each joint from tank to engine to make sure the hose is matched to the fitting and doesn't have cracks, kinks, or crimps in it that would hinder a solid seal.

[Marian Claire]

When you changed the fuel filter was it full of fuel? How is your system set up? Tank, line, main filter, line, pump, inline filter, carb? It does sound like a air leak. You could disconnect the line into the fuel pump and plug it. Then use a temporary, gas can and line, to feed the pump. This would take the main tank, pickup tube, fittings and old fuel lines out of the equation. There is another option but it uses an unapproved clear filter.
Dan S/V Marian Claire
Welcome to the forum.

[Loki9]

Maybe the fuel tank vent is blocked? Try loosening the fuel fill cap and run it that way as a test.

[Dave Neptune]

Gage again

Softuk, my first question is what is the fuel pressure? A simple "cheap" gage will save a lot of futzing around. And I agree with it is something to do with the fuel delivery not keeping up with the engine. If you have a mechanical pump you can work the bail by hand when she begins to die out and if she picks back up you found the issue.

Dave Neptune

[sastanley]

You need the Mr. Gasket 1561, & 3-way female tee and two threaded barbs.

Link: http://amzn.com/B000BWCFLO

[Overdraft]

air leak experience

Someone suggested an air leak, which rings a bell with me. Some years ago I had a problem similar to yours where the engine began to run rough and stumble, and which gradually got worse until it wouldn't run. Discovered by chance that filling the fuel tank solved the problem for a while, then the problem would return. Turned out to be a slight crack in the solder joint for the fuel tank pick-up tube. As the fuel level dropped it would start sucking more and more air until it couldn't pull enough gas to run. Just one more thing to check.

[Softuk]

Some new developments today. I hooked a primer bulb up to the fuel line and pulled full from a separate gas can, bypassing the filter and main fuel tank. The problem persisted even with pumping the primer bulb, so that makes me think the pump isn't the problem.

A mechanic came down shortly after. He seemed to dismiss the carburetor as the problem pretty quickly. Not sure why. We pulled the cap off the coolant while the engine was running and it was bubbling like crazy. This made the mechanic think the head gasket is blown or the block is cracked. Would this cause the engine to run rough like I described? Its not over heating and giving choke still smooths it out.

Thanks for your help!

[Mo]

Was there any steam (white smoke) out the exhaust. A head gasket leak could be putting the fire out in the combustion chamber causing a miss, choking might help a bit....hard to say.

Run the engine for a minute, then pull the plugs and take a picture of them right away. Plugs can tell allot. Gas will not form a droplet on a plug but water will. A compression check is in order when you get to it and use a gauge when doing it. Right there, on those few things to look for, you can pretty much make the call on the head gasket. Also have a look at the oil and see if there is any water in it

[ndutton]

This mechanic of yours is either very good or very bad, not sure which yet.

He seemed to dismiss the carburetor as the problem pretty quickly. Not sure why.

I'd sure like to know why. Do you suspect he dismissed manifold leaks at the same time? If applying choke improves the symptom, even temporarily, that alone screams over-lean mixture. Guess what controls the mixture - - carburetor (and it's fuel supply) and intake manifold.

We pulled the cap off the coolant while the engine was running and it was bubbling like crazy. This made the mechanic think the head gasket is blown or the block is cracked.

I'm not so sure the presumption fits the symptom. Typically when cylinder pressure enters the cooling system there's a puff-puff symptom under the coolant cap. The reported bubbling is certainly interesting and I'd like to know the cause. My cooling system (electric pump) doesn't bubble at all.

As suggested previously, a compression check is in order. Based on the comments of the mechanic and giving him the benefit of the doubt for the time being, I'd pressure test the cooling system too. The results of those two tests will tell us quite a bit.

[Mo]

Good points Neil...I agree, sure needs a further more in-depth look.