A puzzling problem

[PFord]

I've got one for the advance sleuths among you! Let me set this up:

I bought a 1975 Cub. Mechanically seems to be very sound and well cared for. It's always had the slightest of a miss even from the beginning. A couple of weeks ago, it began to fall on it's face after easy starting. Start it, it would die 2-4 seconds later. I found that I could keep it running by running it a hair under full choke. But it made little power. Eventually this progressed to the point where it was barely possible for me to release the clutch and keep it running. As of last Thursday, I had to feather the throttle just to get it back to it's parking spot. I had noticed over the past week, that there was smoke coming from the dipstick. I suspected piston rings or a head gasket issue. Over the weekend I pulled off the hood so I could start diagnosing the issue better.

So symptoms:

1. Smoke coming from the dipstick
2. Engine does shortly after cranking
3. Engine requires nearly full choke to run at all
4. After pulling the plugs, 1 and 2 look like they are running quite lean. 3 and 4 look like they are running full rich
5. I ran a compression test to see if it might have been rings or a head gasket. Cyl 1: 116, Cyl 2: 118, Cyl 3: 118, Cyl 4: 120. This was done cold, but that's about as tight and as even as you're going to see in an engine.


My thoughts:

First thought was a fuel restriction at the bowl or the carb, but with 3/4 running VERY rich, I'm doubting that. I did pull and clean the main jet anyway.

Second thought was head gasket but that seems unlikely. I may pull the head anyway just to take a look around

Third thought was piston rings, but I think the compression test rules that out.

After looking at images online of a bare block and the manifold, I've got some new thoughts.

1. A blockage in the manifold toward the 1/2 runner.
2. A leakage of air over on the 1/2 side of the intake runner. This would draw in unmetered air and let the engine run lean even with me on nearly full choke
3. Weak spark on 3/4 causing fouling, but with 1/2 acting completely opposite to 3/4 that just seems unlikely.
4. Head gasket or some other air leak into 1/2 causing lean running.


I'm open to other ideas here. Anything I missed? In the Air/Fuel/Spark triangle, especially with a carb, it's likely going to be a problem north of the carb if it's only affecting 1 set of pistons rather than 2. Until this morning, I hadn't realized the engine had 4 exhaust ports and only 2 intake ports. But that made sense of why I saw the split personalities when examining the plugs.

And by the way, the oil is not milky, and the coolant looks perfect, so I'm even less inclined to think it's a head gasket issue. My money is on an air leak into that cylinder pair or a blockage of some type. Either way, that manifold is going to have to come off.

So what say the collective mind here?

[Slim140]

If you have to run the choke out, it's starving for fuel. Check for restrictions in the line from the carb to the tank.

[PFord]

If you have to run the choke out, it's starving for fuel. Check for restrictions in the line from the carb to the tank.

See that's the thing. If it was REALLY starved for fuel, I wouldn't be running way rich on 3/4. I'm running lean on the front two, and way rich on the back 2. That's why I think the problem is after the carb.

[Slim140]

If you have to run the choke out, it's starving for fuel. Check for restrictions in the line from the carb to the tank.

See that's the thing. If it was REALLY starved for fuel, I wouldn't be running way rich on 3/4. I'm running lean on the front two, and way rich on the back 2. That's why I think the problem is after the carb.

These engines are simple. You have good compression. When you have to run it with the choke out it's starving for fuel. Start with checking the line from the tank to the carb. If that isn't restricted, clean the carb.

[PFord]

These engines are simple. You have good compression. When you have to run it with the choke out it's starving for fuel. Start with checking the line from the tank to the carb. If that isn't restricted, clean the carb.

LOL! It's the simple things that bite ya!

And I just checked the fuel line. Ran 3ft of string trimmer line through it and pulled it all the way through. Clean as a whistle.

So yea, maybe carb. Could be that if it's fuel starved as you say, 3/4 are getting the lions share of the mix for some reason.

Is there a recommendation for a cheap carb I could use for testing? I'd like to do a simple swap over, see if my issues magically fix themselves, and if so, I'll rebuild my IH carb and put it back. I don't want to spend the $70 on rebuild parts, hours of labor, put it back on, and still have the same problem.

[Slim140]

These engines are simple. You have good compression. When you have to run it with the choke out it's starving for fuel. Start with checking the line from the tank to the carb. If that isn't restricted, clean the carb.

LOL! It's the simple things that bite ya!

And I just checked the fuel line. Ran 3ft of string trimmer line through it and pulled it all the way through. Clean as a whistle.

So yea, maybe carb. Could be that if it's fuel starved as you say, 3/4 are getting the lions share of the mix for some reason.

Is there a recommendation for a cheap carb I could use for testing? I'd like to do a simple swap over, see if my issues magically fix themselves, and if so, I'll rebuild my IH carb and put it back. I don't want to spend the $70 on rebuild parts, hours of labor, put it back on, and still have the same problem.

See if there's a screen in the carb inlet. Check inside the tank for something that moves around and can block the hole.

Here's a carb kit for $13.99

https://www.amazon.com/Carburetor-Inter ... 6157&psc=1

[staninlowerAL]

Have you done a flow test from the tank? Cleaning fuel line as you described is good, but there could still be a restriction in the tank and/or the valve/sediment bowl/screen.

[Lnk]

I am betting you have an air leak in the intake for 1/2, the carb is common to all cylinders. 3 & 4 running rich tells me you don't have a fuel flow issue. Compression numbers look good. Pull the manifold. Either a mouse nest in the runner, or a bad intake gasket. That's my guess. Let us know what you find.

[indy61]

Hook up a pony tank while the hood is off and see how she runs.
The smoking from dipstick is probably just condensation.
The sparkplug difference might be it needs the valves adjusted.

[PFord]

See if there's a screen in the carb inlet. Check inside the tank for something that moves around and can block the hole.

Here's a carb kit for $13.99

https://www.amazon.com/Carburetor-Inter ... 6157&psc=1

There is a screen in the carb inlet. I looked at it last week and it looked ok. Nothing in the tank. It's incredibly clean. I will have a deeper look to see if there is a filter on the inlet and if that might be plugged up somehow.

[PFord]

Have you done a flow test from the tank? Cleaning fuel line as you described is good, but there could still be a restriction in the tank and/or the valve/sediment bowl/screen.

Yea, flow is OK. And for grins I have a new sediment bowl screen that just came in middle of last week that I'll be installing. When I cleaned the sediment bowl last week, and then turned on flow again to fill the tank, it flowed like gangbusters.

[PFord]

I am betting you have an air leak in the intake for 1/2, the carb is common to all cylinders. 3 & 4 running rich tells me you don't have a fuel flow issue. Compression numbers look good. Pull the manifold. Either a mouse nest in the runner, or a bad intake gasket. That's my guess. Let us know what you find.

That's kinda my thought. I'll order new gaskets today for the manifold and carb. Don't want to pull them until I can put them back on.

[PFord]

Hook up a pony tank while the hood is off and see how she runs.
The smoking from dipstick is probably just condensation.
The sparkplug difference might be it needs the valves adjusted.

Yep, that was my thought. I went looking for a pony tank yesterday, but didn't find one. I'll try to order something this week.

My thought on the smoking might have been scorching oil since it was running so lean.

[SamsFarm]

One other thing.

Pull the distributor cap and check the contacts under for corrosion!

Clean the contacts if so.

[PFord]

One other thing.

Pull the distributor cap and check the contacts under for corrosion!

Clean the contacts if so.

Yep, I need to do that.