Engine rpm varies after longer run

[Tripaddle]

I’ve owned the boat for 4 years now. Minimal maintenance (oil, plugs once, impeller, and MMO in gas), and rerouted the flow as described at the exhaust manifold. Still has points and cranks after just a few rpms. Doesn’t like the choke and raw water cooled. The temperature gauge really never moves but we normally just get out of marina and start sailing. In the beginning I installed a new fuel tank, all hoses, fuel pump, water separator, and cleaned carb, also wrapped exhaust. Oil pressure always 30-40. Everything adjusted to spec and no tweaking done. But, always a but, after running 2 hours it seems like the motor rpms will change. Not really trying to cut off or like a loss of power but the rpms will drop and come back. Feels scary and we slow down. Our max is probably 1500 rpm. Maybe the coil? I’ve never touched the points. Am I being stubborn not going electronic? I’m thinking of adding the sender unit for temperature when I place an order so I have it when I go to the boat.

[Sam]

Well your right that it could be coil going bad after it heats up. I would guess you have a common fuel/tank problem. Some amount of water at the bottom that sometimes gets emulsified with the gas depending on the sloshing that eventually results in rpm periodically dropping and the later rising back to normal. If you have direct access pump out the bottom couple of gallons and collect for visual observation. Some boats have a flex pick up tube/line lying on the bottom of the tank [with or with out a screen] while others have a tube 1 to 1i/2 in above the bottom.

You could test all this routing to a back up gas tank and see if problem goes away. It may be easier just to pump out a few gallons or the whole tank if it is low. Later check the seal on the fuel inlet.

[Al Schober]

First, you may be running a bit rich. How do the plugs look? Most A4s when cold like a bit of choke. I liked using the adjustable main jet. If it got clogged, I was often able to clear it by running it closed/open a few times.
In 30 years with an A4, never went EI. Points & condenser - carried spares.
Doubt if it's the coil. When they go bad the engine stops like you shut off the switch. No fading, no faltering, just STOP.
I'd check for water in the fuel tank, particularly if you've been using fuel with ethanol. This check was on my annual list. You do need to be able to access the very bottom of the tank. The pickup for the engine generally doesn't go to the bottom. Pump out a gallon into a clear jug and see what's there. Top layer will be gas - back into the tank. Bottom layer is water and crud - dispose of as your conscience allows.

[roadnsky]

Doubt if it's the coil. When they go bad the engine stops like you shut off the switch. No fading, no faltering, just STOP.

Hey Al-
Please read...

http://www.moyermarineforum.com/forums/showpost.php?p=121444&postcount=5

[Tripaddle]

Well water in the gas does sound like a possibility. The fuel cap is in the cockpit floor that has been full of water. I have replaced the o-ring. I’m not sure in 4 years we’ve ever used a full tank. I top the tank off for the winter. I have decided last trip to try and run it down. I believe I’ll pump some or all out.
I’ve read about coils and carrying a spare. I’m guilty of not carrying one. I’m just considering the electronic ignition but some post makes it sound not very straight forward.

[ndutton]

I’m just considering the electronic ignition but some post makes it sound not very straight forward.

It's really not complicated. The installation is 2 screws and a magnetic collar, the required ignition system assessment is 2 simple measurements and one elementary school division problem.

[tenders]

Electronic ignition is the best A4 upgrade for the money and time required. Simple installation, extremely reliable, and removes two potentially unreliable components, the condenser and points. I don’t know if what you’re experiencing now is an ignition problem, my guess would be no, but if the engine is basically running this is a decent time to swap in electronic ignition.

I would wonder about water in fuel, clogged filter, gunk in carb, or failing fuel pump (the electric pumps can be hard to diagnose since it can take them a while before they start failing).

[Bratina]

What Sam said - rule out bad gas by either pumping out (plenty of work, but allows you to put said gas in a translucent container and look for water), or run of an auxilliary tank. The usual warnings about gasoline vapour and sources of ignitition apply.

Do you have an inline fuel pressure gauge? Mine cost ~$20, and it's something I should have done day one for the quick diagnostics it affords.

I did not take a methodical approach to my engine problems, and ended up chasing my tail for two seasons (missing a LOT of sailing) before I found:

1. water in the gas
2. a bad spark plug
3. a clogged polishing filter

Each of these would have been a ten minute troubleshooting exercise if I had applied simple diagnostic steps.