Check the obvious, or the faulty application of Occam's Razor

[Bratina]

I've been reading obsessively on these forums to try and solve a problem with my engine. It ran rough, and wouldn't generate above ~1600 rpm. I removed the leads to each cylinder and - lo - the engine didn't change pitch at all when I removed #4.

Fearing the worst, I added MMO, hand-cranked, and let it sit for a couple of days. I ran the engine - nothing out of #4, a cold plug/lead, where the others were hot. I switched leads. No change. I prodded valves, added MMO, cranked (yes, through-hull closed), read more online and in the Atomic Bible, and MMO-d more. No change.

I decided to look in the hole of plug #3, to see if anything looked different; not much. On a whim, I put the plug for #3 in #4, and vice versa. I suspect you can guess the rest.

With a new plug (RJ12C) in place, she's running like a champ! I should have checked the obvious first.

With all that said, a question. Is there something I might have done that caused the plug (also a Champion RJ12C) to have failed? I used the correct socket, etc., although not a torque wrench. The plugs have ~5h on them, and were new earlier this season. #4 plug was sooty, suggesting it was running at some stage, but was wet with gas when I swapped to #3.

[Dave Neptune]

Bratina, it is not real common but I have seen many faulty (won't fire) plugs over the years and it appears that may of been your issue.

You may have a weak ign system that just would not fire a weaker plug/cylinder.

Check your spark. pull the coil wire out of the cap and have someone crank the engine. When they crank hold the wire about 1/4" or less from the head or head-bolt. You should see a crisp blue spark, if not it may be time for some other checks within the ignition system. Insulate yourself and turn off the water valve while cranking.

Dave Neptune

[Mo]

I've had plugs fail prior to the ones I use in the boat now. Keep a spare set aboard the boat

[hanleyclifford]

You might also have a bad ignition wire. A new set is available from the online catalogue on this site.

[The Garbone]

On my old air cooled motorcycle it is common for me to replace the plug once a year, just get hard to kick start for a little bit then stops firing all together even though it looks nice. Of course I run a bit on the rich side since it is air cooled.....

How this relates, ehh, I don't know.

[Bratina]

Thanks, everyone, for the replies.

You might also have a bad ignition wire. A new set is available from the online catalogue on this site.

Good question - I switched the ignition wires between #3 and #4 and the problem didn't follow the wires. Can I take that to mean that the wires are ok? I would have assumed that if the wires were faulty then the problem would follow the wire. They're from our host, purchased in 2013 (or early 2014).

Bratina, it is not real common but I have seen many faulty (won't fire) plugs over the years and it appears that may of been your issue.

You may have a weak ign system that just would not fire a weaker plug/cylinder.

Check your spark. pull the coil wire out of the cap and have someone crank the engine. When they crank hold the wire about 1/4" or less from the head or head-bolt. You should see a crisp blue spark, if not it may be time for some other checks within the ignition system. Insulate yourself and turn off the water valve while cranking.

Dave Neptune

I will run this check when I'm next at the boat (this weekend most likely). I have to ask though - what kind of / level of insulation?! I pull the plug wires by hand and, although I'm not standing in water or anything I'm not insulated...

It's a good call-out though. When I first got the boat, and knew almost nothing about the Atomic 4, I left the key 'on' for ~30 minutes after the engine stalled. The battery was almost flat when it stalled, but I guess I may have damaged the coil. I'll check the spark once I've been able to make sure I'm insulated enough

[hanleyclifford]

Please test your voltage at coil +, hot and running, just to eliminate that possibility.

[Dave Neptune]

Bratina, you can hold the wire in position with a plastic handled screw driver or rubber coated plier. I usually just hold it by hand and 99% of the time all is well and the other 1% or so I let go of the wire in a hurry~it's really no biggie.

Dave Neptune

[JOHN COOKSON]

Check for corrosionin the spark plug terminals in the towers in the distributor cap. Sometimes the little rubber boots don't work as well as they should to keep moisture out.

TRUE GRIT

[Bratina]

Check for corrosion in the spark plug terminals in the towers in the distributor cap. Sometimes the little rubber boots don't work as well as they should to keep moisture out.

TRUE GRIT

That's a nice easy check - thanks. I'll pull them and take a look. If there is corrosion can I clean it up, or should I replace the cap? I'm honestly scared of messing with the distributor, any more than the 'field tuning' I ran under power by twisting it a little.

Bratina, you can hold the wire in position with a plastic handled screw driver or rubber coated plier. I usually just hold it by hand and 99% of the time all is well and the other 1% or so I let go of the wire in a hurry~it's really no biggie.

Dave Neptune

No pacemaker for this 38 year-old, so I'll just "hold and hope"

Please test your voltage at coil +, hot and running, just to eliminate that possibility.

Thanks - I will add to the list for diagnostics this weekend and report back. I'm a novice on 12V. Multimeter lead to the coil +, other lead to ground, e.g. a stud on the engine block? Sorry, that's a real basic question...

[roadnsky]

Thanks - I will add to the list for diagnostics this weekend and report back. I'm a novice on 12V. Multimeter lead to the coil +, other lead to ground, e.g. a stud on the engine block? Sorry, that's a real basic question...

Correct.
The - terminal on the coil is NOT a ground...

[JOHN COOKSON]

That's a nice easy check - thanks. I'll pull them and take a look. If there is corrosion can I clean it up, or should I replace the cap? I'm honestly scared of messing with the distributor, any more than the 'field tuning' I ran under power by twisting it a little.

You should check all the secondary circuit, coil->spark plugs. There may be something inside the distributor cap that is causing the spark plug misfire such as a burned rotor or burned contacts inside the distributor cap.
Put a piece of tape on the tower for cylinder #1 as a marker. Pull the leads off the spark plugs, loosen the screws that hold the cap on and lift the cap straight up. Remember the firing order is 1,2,4,3.You can make a dawning or take a picture before removing the distributor cap if you need a memory refresher.

TRUE GRIT

[Bratina]

You should check all the secondary circuit, coil->spark plugs. There may be something inside the distributor cap that is causing the spark plug misfire such as a burned rotor or burned contacts inside the distributor cap.
Put a piece of tape on the tower for cylinder #1 as a marker. Pull the leads off the spark plugs, loosen the screws that hold the cap on and lift the cap straight up. Remember the firing order is 1,2,4,3.You can make a dawning or take a picture before removing the distributor cap if you need a memory refresher.

TRUE GRIT

Thank you, too. I have a long list of step-by-step diagnostics for this weekend. I'll report back!

[Bratina]

Diagnostics run.

Coil+ 10.5v idle and in gear, rising to 10.9v at around 1400 rpm. What's an acceptable range? I have points.

No corrosion on the tower inserts under the plug wires. I took the cap off and it looks ok. A little bit of dirt on the rotor maybe, but clean cap with no carbon trail or visible corrosion. The contacts looked like the plastic was very slightly melted. Just a tiny bit, and I have no frame of reference for how it should look.

The only thing I couldn't check was for spark, since I'm at the boat on my own. I put everything back together and it all ran nicely, so at least I didn't do anything terrible taking the distributor cap off....

The engine ran well to around 1900 rpm. My prop is likely filthy (Great Lakes). Not sure if I should continue to be concerned?

[hanleyclifford]

Diagnostics run.

Coil+ 10.5v idle and in gear, rising to 10.9v at around 1400 rpm. What's an acceptable range? I have points.

No corrosion on the tower inserts under the plug wires. I took the cap off and it looks ok. A little bit of dirt on the rotor maybe, but clean cap with no carbon trail or visible corrosion. The contacts looked like the plastic was very slightly melted. Just a tiny bit, and I have no frame of reference for how it should look.

The only thing I couldn't check was for spark, since I'm at the boat on my own. I put everything back together and it all ran nicely, so at least I didn't do anything terrible taking the distributor cap off....

The engine ran well to around 1900 rpm. My prop is likely filthy (Great Lakes). Not sure if I should continue to be concerned?

With that kind of voltage (very close to the numbers I run on my engine) you probably do not need to run resistance spark plugs. You may be ok while the plug is new but later the problem could come back. Try using a non resistive plug. Edit: don't know if this has ben discussed, but you should know what your dwell reading is.