Engine starts hesitating only after several hours motoring...

[jkenan]

Despite my best efforts (including those efforts derived from the valuable suggestions here), my motor seems to get 'tired' after motoring for more than 4-5 hours. It runs at 2000 RPM steadily, and the first four hours it sounds completely steady and strong. Then, it slowly exhibits hesitation, which gradually increases until everyone on the boat is certain the engine is going to die. But it never does. In fact, it never goes below 2000 RPM!

Here is everything I've tried to resolve the issue:
1) Verified there are NO leaks in the fuel line. I even installed a priming bulb between the tank and the primary fuel filter, and when I pressurize the system, no leaks occur anywhere in the fuel line (although fuel does spill out of the carburator when I pump the priming bulb hard, but this does not surprise me).
2) Replaced the epoxy filled coil with a brand-new oil-filled coil bought from Don
3) Cleaned every electrical connection on engine and at panel, and sprayed each with anti-corrosion spray.
4) Checked and verified all of the following are in order: Temperature 140- degrees and Oil Pressure 40psi steady throughout motoring, Timing (via RPM optimization at warmed-up cruising speed) AND spark on all four plugs, Compression (90PSI in all four), Carburator (thorough cleaning of all jets), Plugs (dry and grey after several hours of use).

I have the crankcase ventilation kit and adjustable carb, both bought from Moyer Marine. Carb is set to exactly 1.5 turns from closed, and one thing I do notice every time I clean the carb is that the flame arrestor/throat area that feeds the scavenge tube usually has an oily, blackish gas mix in it. I think this is fairly normal though, but wonder if my gas mix could be too rich (or too lean??).

I'm stumped. Why would it motor fine for several hours, and then start to hesitate? When I start it back up (after it has cooled, it runs fine for at least 4 hours, and then repeats with the hesitation). Any help would be greatly appreciated.

Thanks in advance.

[JimG]

Any chance your fuel tank vent is plugged? The time frame seems a little long for fuel starvation, but maybe your fuel pump can draw against the vacuum until the pump accumulates enough air bubbles (that the struggle creates) to begin cavitating? Long shot, I know! The simple test is to open the fuel fill briefly when the problem shows up.

[tenders]

You've been very thorough--and what a bear to troubleshoot when it takes so long to get a symptom. I'd guess that 80% of all boats could have the same thing wrong and nobody would know!

Does tweaking the choke when it happens have any effect? That might help rule fuel in or out.

It sounds like something electrical that goes bad only after it gets really hot for a while. The only things I can think of are: old condenser, old plug wires, or tired electrical fuel pump. I'd swap out rotor and cap while I was at it.

[jkenan]

I've got the the Ignitor Electronic Ignition kit installed, and rotor, cap and wires are new this year (I replace them every year). After hesitation starts, I am able to verify there is spark on all four wires, and it does not seem to be skipping (though it's hard to tell because they occur so fast). I thought electrical may be the issue, which is why I replaced the coil and cleaned all connections.

As far as the fuel supply goes, closing the choke will be my next test, along with opening the fill cap to test for vent line blockage - good suggestions.

Regarding the fuel pump, I would think that if cavitation were the issue, the engine would eventually die. Even if it cavitates slightly, but is still able to supply enough fuel to keep the engine running, wouldn't it stand to reason that the engine would run smoothly? If the cavitation is enough to interrupt supply, the engine would eventually die, right? The pump seems to be working steadily when I check it , but I'm afraid to turn the engine off after it starts hesitating because I'm usually en-route where I don't want to be stranded in case it doesn't want to restart (but this may be the test I have to do). And why don't my RPM's go down through all of this (I've had fuel supply and coil issues before, and have always lost power when hesitation occurs that is associated with these problems)?

Here are some additional thoughts:
1) If there is an oily/black gasoline mix in the scavenge pick-up area, would that mean that some cylinders are not firing regularly, causing fuel delivered to the cylinder to be pulled though the open exhause valve and put back into the carb via crankcase ventilation?
2) Is it possible that as the engine runs warm for a while, the block could expand enough due to it's own heat to stifle smooth operation of one or more valves? I treat valves with MMO every month or so.

[rgoff]

I had a similar problem and it ended up being the coil. However, you've already replaced yours. Frustrating situation I know, as I spent a couple of years replacing various things to get my A4 running reliably.

Good luck,

[tenders]

Valves that stick when the engine is really hot, or a weak valvespring, could explain the oily gunk in the carb and, to a limited extent, the roughness. But when I've sticky valve issues, the engine has run only _slightly_ rough--not _really_ rough. Do you put MMO in the gas as Don recommends? You might try that, if what you're doing now is putting it into the cylinders.

It is possible that an electrical failure in the fuel pump could cause the rough running. This happened to me a few years ago after 10-15 minutes of running, but the engine ran almost normally when the engine was throttled way down.

[jkenan]

'Slightly rough' definitely explains the way my engine runs. I say that based on the fact RPM's don't go down, nor do I seem to loose any power. It is a feeling of slight hesitation, about every second or two. The cadence of the engine waivers in tandem with the hesitation, but always hovers around it's full blown cadence. So based on this, and the oily gunk in carb, I think we've narrowed it down to a sticky valve(s) for now.

I do use MMO in the gas. Beyond that, what can one do to fix a sticky valve? The springs are all brand new (I rebuilt the engine in Jan '06 as a major overhaul replacing all bearings, rings, springs, and having all blocks and housings flushed, cleaned, rebored, ground, etc).

Thanks.