A perplexing problem

[ol'George]

Originally Posted by Ken52

John,
I wondered about a sticking valve too. I did run some of the Marvel's Mystery oil through it a few years ago. I don't recall that it made any difference though. Around that same time I removed the exhaust valves and really polished them up well, as well as removing a few thousandths off the length of the valve to ensure adequate clearance when hot. Also installed new rotators.
Wouldn't a sticking valve cause some backfiring either through the muffler or carb? I don't think it's doing that. I think I'll try the Mystery oil again though. Can't hurt.

George,
I went out and looked at the oil pressure switch. I noticed that the wire going to it is rubbed bare in one place. It makes me wonder if it might be shorting out somewhere. I'll check it in the morning.
I didn't think that an oil pressure switch would cause the ignition system to hiccup, but I'll certainly follow up on it.

Thanks for the suggestions!
Ken

They are just an oil pressure switch,-- on/off around #3-5 or so,
well if it is gimpy and ever so slightly/intermittently making ground contact
it will cause the ign to ground.
also when you turn off the ign switch it grounds the mag.
if there is a slight ground path from corrosion or a heat induced problem you can loose ign. down that path also.
Not saying that it is you problem but something to check, I've seen some weird things in my years.
People tend to bring me things others have given up on or in a bushel basket.

[Ken52]

Just in case anyone's still following this mystery, here are the things that I've tried today:
1. disconnected the oil pressure switch wire and made sure that it was not grounded anywhere. Unfortunately, I couldn't notice any difference in the symptoms.
2. disconnected the fuel line at the carburetor and directed the fuel line into a gas can. Turned the key on, and the electric fuel pump pumped at least a gallon of gas into the gas can in less than a minute before I shut it off. The pump is a very low pressure unit (about 2 - 4 psi), so the float easily keeps the fuel bowl from overfilling. One nice side benefit is that the tractor starts quicker than it ever has. I think that this test confirms to me that I don't have a fuel starvation issue.

My next step will be to run some Mystery Oil in the fuel.
Any other suggestions are more than welcome!

[J-Mech]

I still think you have an exhaust valve sticking.

When it acts up, check compression.

[Ken52]

Jonathan,
I don't doubt that you're right about a sticking valve. I think I've just about ruled out most of the other possibilities.
I really can't check compression accurately though, because it will start running good within 30 seconds after it starts missing. That's probably too fast for me to get it back to the shed, shut down, remove spark plug, etc.

Assuming that the valves are sticking due to crud buildup on the valve stem where it goes through the valve guide, I wonder if there is some kind of solvent that I can apply through the valve spring covers without removing the heads, valves, etc.

[J-Mech]

Originally Posted by Ken52

Assuming that the valves are sticking due to crud buildup on the valve stem where it goes through the valve guide, I wonder if there is some kind of solvent that I can apply through the valve spring covers without removing the heads, valves, etc.

NOPE. Take them apart and clean them is the best way. If they really are sticking nothing will penetrate it good enough to clean it.

In your first post, you said you replaced the magnetos, but then you say that the second one is genuine Kohler. Is one a Kohler and the other not?

[Ken52]

Jonathan,
Regarding the exhaust valves sticking, I actually did remove them, clean them thoroughly a couple of years ago. They were gunked up before that, but the cleaning, etc. freed them up pretty well, I thought. But the first time I ran the tractor after that, it had identical symptoms as before. That's why I've been slow to jump into that process again. I've had good experience with engine work, so I'm confident that the process was done reasonably well. Still, I value your input, and although I may not get to it right away, I think it's good advice to act on.
The coil on the 1862 Magnum engine has two leads, so there's only one component involved.

[J-Mech]

I was going to suggest that you may have a failing ign switch, but since you put on an electric fuel pump, you can't just unplug it and run the tractor.

I'm not a fan of electric fuel pumps on Kohlers unless you need more flow than the mechanical puts out, like in a puller.

I'm assuming that your sure that you don't have a vapor lock issue? Where did you mount the pump?

[Ken52]

Hmm. Ignition switch.... I hadn't given that component a thought. I suppose that I could figure out how to bypass it with a temporary wire. It might be worth a try. Or would a malfunctioning switch cause the engine to die, rather than miss as though it was running on one cylinder? I don't know if I did a very good job of describing how the engine sounds, but that's what it appears to be doing - running on just one cylinder for about 30 seconds or so, until it starts smoothing out and running fine for another 5 - 10 minutes.

I mounted the electric fuel pump about where the original one was mounted, to the rear of the firewall on the LH side. I had seen comments in some posts about vapor lock, but I'm not sure what causes it, or how it happens. After I read those posts, I did try putting some insulation between the carburetor and the engine to isolate it from the heat, but again, it didn't affect the misfire problem.

[Leadslingingdaddy]

What's your compression side to side? What do the plugs look like?

Is there a difference in the head/combustion area?? Seems when it heats up... something is expanding and creating a loss somewhere.. Do you have excessive crankcase pressure?

Good seal at the manifold and carb base....

[Ken52]

Those are good questions.
I can check the compression later today. I think that it will test good because when it's running well, which is 90% of the time, it runs smooth and has good power.
Plugs are new (again) Autolite this time.
It's been a couple years since I had the heads off. At that time, I didn't see any significant differences side to side.
I don't think that I have high crankcase pressure. At least it's not indicated when removing the dipstick.
I've had the carburetor and manifold off more than once, and I've just reused the gaskets. I know it would be better to replace them, but I've inspected the gaskets and mating surfaces, and they are not showing any indication at all that there is an air leak.