Sudden very ragged running problem

[pdecker]

Happy Labor Day Weekend! I did everything I could think of to get ready to motor / sail from Fort Lauderdale to Miami this weekend. Yesterday morning, the engine started just fine and ran great for about on hour, then suddenly quit. I could restart the engine, but it would run very roughly at very high RPM, and not at all at low RPM, as if a cylinder or two were not firing. I tested for spark, and found good spark. I tested for fuel flow to the carb, and found good fuel flow. I'm kind of stumped. Is the carb clogged? Are valves stuck? Plug wires shot all of a sudden?

Here is some more data:

The engine was running very hot, since we were trying to make it through the bridges on time. The gauge was reading about 190. I know I have to flush the exhaust manifold, but I did make sure there was flow of water in the exhaust out the transom.

I changed the oil last week, and verified the right amount of oil was in the engine before starting it yesterday.

I filled the gas tank full with mid-grade fuel and added some MMO. There was no label at the fuel pump that mentioned ethanol, but I don't know if they do that here in Florida.

I installed new spark plugs, rotor, and distributor cap on Friday night, and verified the engine started and ran smoothly afterward. The Ignitor electronic ignition was installed last year. The coil is mounted off the engine, and I have a ballast resistor attached. At the time of failure, the temperature of the coil was 140 F. The temperature of the fuel pump was 120.

The fuel filter is about a month old, and I have a vacuum gauge installed to verify it is not clogged. The carburetor is one year old.

Any help would be appreciated.

[hanleyclifford]

Please (not the coil)

This does sound like a fuel issue. Please verify that you have a "fuel pressure" gauge (you mentioned "vacuum gauge"). A vacuum gauge will not help much here. You mentioned a fuel fill up. Have you verified that your fuel tank vent is clear?

[pdecker]

Gauges

Yes, I have a vacuum gauge for the line off the fuel filter, and another pressure gauge after the fuel pump going to the carb. I'll check the fuel tank vent.

http://www.cruisingworld.com/how/fuel-filter-gauge?page=0%2C1

[hanleyclifford]

Yes, I have a vacuum gauge for the line off the fuel filter, and another pressure gauge after the fuel pump going to the carb.

That's impressive. OK, if you have fuel pressure delivered right to the carb the problem is in the carb itself. A good place to start is the main jet accessed under the bowl. The fact that the carb is relatively new is irrelevant. The odds are stacked big time against clean fuel delivery for us. The boats and tanks can be old. We use the engines (most of us) only to get in and out of marinas. The delivery paths themselves can be torturous, and then to top it off the government puts moonshine into the gasoline. Check that main jet.

[Marian Claire]

Does it run any better if you partially close the choke?
Dan S/V Marian Claire

[Mo]

I'd check the racor for water sooner rather than later. When I had a water in fuel (yes, from a gas station) the boat wouldn't idle and sputtered to beat all with rpms up. Reverbations went through the drive train as well... Once I sucked the tank bottom and changed filters (twice) it sorted out and didn't have an issue following that.

[JOHN COOKSON]

A blocked fuel tank vent is easy to diagnose. Just loosen the fill cap and see if the engine runs better\normally. If you want to shotgun the troubleshooting loosen the fill cap before the next start. If the engine runs normally then tighten the fill cap and see if the problem reoccures or if tightening the cap makes no difference.

Lots of times (but not always) when there is a fuel\carb problem the engine sort of sputters to a stop like it ran out of fuel. Was your shutdown sudden like someone turned off the key or more of a "soft" shutdown like you ran out of fuel? Did you try closing the choke a bit? If closing the choke helps the engine to run better it pretty much locks in the carb\fuel system problem. (In your case the carb)

Are you sure there is not something wrapped around the prop?
Also has the packing not gone to hell in a hand basket and binding the shaft somehow.

You can check for spark by pulling a wire off a plug, putting a screwdriver in the boot and see if you can jump an arc to the plug while someone turns the engine with the starter. Remember to keep the raw water valve closed anytime you are testing a no start engine.

In summary I agree it sounds as if you have a carb problem. These are just some other ideas for your consideration.
Best of luck to you.

TRUE GRIT

[Mo]

Good point to check prop John. Rather than carb I'd be looking to Water....water in fuel...he described exactly what occurs. Just because a fuel filter went on last month doesn't mean there is no water in that tank now. Check for water before you pull the carb off.

[pdecker]

No water in the fuel, and loosening the gas cap doesn't improve running

I started it up today and the ragged running starts within a few seconds. I removed the gas cap while it was running poorly, and that did not improve the engine, so I don't think it's the vent.

I could not remove the bolt from the bottom of the Racor since I put it on too tight. But I removed the whole filter and dumped the entire contents into coffee cups. (Don't tell the Mrs!) I did not see any signs of water.

When the engine shut down yesterday, it was a soft shut down like it was starved for fuel. Same today.

I do not think the problem is a fouled prop since the symptoms readily happen in neutral gear. But, I'll jump in and check it tomorrow to be sure.

So, I think the signs are pointing to the carburetor. I have the Moyer overhaul manual, and Don's video on carburetor troubleshooting. I also happen to have a carb rebuild kit in my spares inventory if need be. I'll probably attack that tomorrow.

Thanks for all your help!

http://www.moyermarine.com/cgi-bin/shopper.cgi?preadd=action&key=CLIP_01_551

[ndutton]

If it turns out there are contaminants in the main jet, idle circuit ports or wherever else, the question shifts to 'where did it come from?' A thorough repair goes beyond addressing the symptom, it resolves the cause.

[hanleyclifford]

Sounds like you've zeroed in on the problem. Still, before going for the full rebuild I would do a quick inspection of the main jet. Just remove the cover plug and then extract the jet with a screwdriver that is big enough to fill the slot in the jet but narrow enough on the shank to not mess up the threads going in. Be careful not to drop the small gasket ring into the bilge. BTW, since switching to the adjustable main jet I have been able to clear this kind of problem several times by just twisting the jet open a couple turns and then returning to normal position. Of course if you have a bowl full of junk removing the carb is the only option.

[tenders]

Would also suggest taking off the distributor cap and checking for cleanliness thereunder, an intact rotor, and solid connections to the wires.

[Al Schober]

Sounds to me like the first thing to eliminate is water in the fuel. Dumping 6oz into a coffee cup isn't telling you anything. What you need is a gallon from the very bottom of your tank into a clear jug. Then you'll be able to see what's there.
I don't know your tank arrangement - in my case I'm able to get to the very bottom through the fill using a 6' piece of PEX tubing. A Jabsco hand pump moves the fuel into the jug. Easy to see what you have. If your tank bottom is not accessible from the fill, you'll have to add a port in the top of the tank. Make it easy to get to, as you'll probably want to check your tank annually.
I'm guessing you'll find two layers. The top layer is fuel - put it back into the tank. The bottom layer is a mix of water, alcohol, and crud - dispose of it. Side benefit of this effort is that your gas tank is now alcohol free - you've dumped it with the water!
I've been preaching this sermon a lot in the last year - based on my own experience. I don't want to count the sailing days I've lost due to water in the gas, clogged main jets, clogged float valves, and overflowing carbs. The clogs due to corrosion products from the water. Even a marine Racor filter has steel in it that will corrode and send crud downstream to the carb.

[romantic comedy]

The original post says that it was running at very high RPM, right?

I would suspect a vacuum leak right away.

[Golfdad75]

Same problem

I had similar problem this weekend. Emptied my racor, cleaned my carb, sucked out the bottom gallon of gas, re fueled and it ran great