Runs only at full choke

[Radec Aksarben]

My 48 cub sat for a month or 2 after running fine. Went to start it and it would only run at full choke.

I did the little wire in the jet trick. Nothing.

I put a different carb on from a 52 non-runner just to see. Then it was worse. I took the 48 carb apart and found gas in the float. The female threads also fell off the fuel inlet of the carb.

I rebuilt the 52 carb. I put it on the 48 today and it does the same stinking thing. Only runs at full choke and can't adjust the throttle when in that state.

I took the manifold off to see if it was cracked or obstructed and I see nothing wrong with it. But now I need gaskets....

Where do I go from here?

[Glen]

Hi,
You didn't say if you did a gas flow test.
I would do a gas flow test, to see if gas is flowing from the tank to the carburetor fast enough.

Use a clean 2 quart or so can, and remove the drain plug from the bottom of the carb, catch the gas in the can. At first the gas in the carb will come out. Then the gas in the gas line.
It should have a steady flow, in a stream, not stop and go.
Check the flow for about 1 minute.
Be sure the gas valve is fully open for the test.
When done, you can put the gas in the tank again.

You didn't say if you have cleaned the gas strainer under the gas tank.
I would clean that.
It has a screen in the top of it, above the bowl and gasket. Sometimes the screen gets plugged, or is gone.
Sometimes if you try to reuse the bowl gasket, it won't seal if it is old.
TM Tractor has a new gasket and screen for the gas strainer.
If the Cub has Touch Control, it is easier to get to the gas strainer if you move the Touch Control arms to the rear or down position before working on the strainer.

If the Cub has an inline gas filter someone put in, it has to be for a gravity flow gas system.
A filter for a system with a gas pump can slow the gas flow.

The inlet to the gas strainer could be partly plugged with debris, look in the tank with a good light.
Be careful to not make a spark when working in the tank, or where there is gas.

[Don McCombs]

Use a clean 2 quart or so can...

Using a clear glass jar makes it easier to see any debris and/or water in the fuel.

[Radec Aksarben]

Sorry I didn't mention that gas flows freely from the line. That would have saved lots of typing.

But I did not check flow though the carb valve.

[Radec Aksarben]

Would there be an electrical reason for this to happen?

[Glen]

Hi,
You could do a spark test, of the spark going to the spark plugs, so you know the spark is good, it should be blue or white, and jump a 1/4" gap. Yellow is weak and the engine may not run right, or any.

You could check the ignition points, and file them, if you haven't looked at them lately. They should be flat and shiny.
Wipe off all filing with a clean rag.

[Eugene]

You could check the ignition points, and file them, if you haven't looked at them lately.

Suggest. Compression tests, then the igntion system. When those items are checked and satisfactory, come back the fuel/carburation.

My guess, valve sticking, engine running on 3 cylinders.

[Crimson Tim]

I will leave your question about whether an electrical issue can produce these symptoms to someone with more electrical experience, but here are a few more thoughts on air/fuel for your consideration.

The choke biases the air/fuel mixture for more fuel than the normal operating ratio. If it will only run with the choke on, then something is causing that mixture to lean out, such that having the choke on corrects the ratio. As mentioned, a restriction in the fuel supply will do this, and has to be the most common reason for this to happen. How clean is your fuel tank? Any rust or other debris? It doesn’t take much.
Another thing that can lean out the mixture is an air leak at the carb to manifold gasket or if any of the seals in the carb itself are allowing it to suck too much air. If a spritz of starting fluid in the vicinity of the carb alters how it is running, then an air leak is suspect.

Also worth checking: the choke rod connection at the carb can slip such that the choke plate in the carb is actually operating opposite what you would expect based on the position of the rod. Take the air hose off and verify that the choke plate is operating correctly.

Has anything happened (carb swap?) that could have gotten the float height and drop settings out of whack? Most replacements don’t come set right.

[Waif]

2 month set , then needs choked to run. Open choke and stalls out....Seems the case.

Valves were mentioned. Sticking is a possibility from setting.
What else changed or may have changed?
Yes the carb and fuel are suspect. So is anything feeding /circuiting them.

Air.
Check for mud daubers plugging up the air intake screen on your cap.
Check oil in air cleaner. How the bugs got in one of mine I don't know. But they were thick!
As mentioned , inspect choke plate and linkage for proper orientation. Would be odd just from setting , but one never knows till they know.

Pulling plugs to inspect is not out of the question. How an engine is shut down in what condition factors. Rarely an issue. But bad is still bad.
A moisture wet plug can grow funk setting. And certain fuel additives (what does our fuel include for ingredients? Deposits occur. ) can do strange things too.

Ignition clean and functioning?
Points clean and proper adjustment?
Points can and do get funky from setting in certain conditions.
As can ignition cap contacts. Wire ends ect..

Wanting to be run rich by choke can suggest gas is limited. Or rather air fuel mix is out of whack.
I'd study exhaust maybe for a hint of if it is lean or rich. The why of which is another question to follow.

[Radec Aksarben]

I will let you know how this turned out.

I installed the exhaust manifold gaskets and left the muffler off and the air cleaner disconnected from the carb just to elimate them from consideration.
I checked the fuel flow out of the carb bowl through the needle valve and that was consistent and strong stream.

I knew from running on full choke that all cylinders were firing because it ran smoothly.

I started it and it only ran on full choke... again! just on a whim I put my hand in front of the carb intake to block it off and just before the engine stalled I removed my hand and it was running differently so I adjusted to half choke. Then I blocked the intake again and was able to adjust to no choke. The idle was way to fast since the carburetor was a different one, so I just adjusted the linkage to proper idle and put the tractor back together the rest of the way and I was off and running.

Now I can start it with half a crank without choke just as before! My best guess is my hand caused some vacuum change and dislodged something I failed to clean out of the carb.

Thanks
R

[Urbish]

I will let you know how this turned out.

I installed the exhaust manifold gaskets and left the muffler off and the air cleaner disconnected from the carb just to elimate them from consideration.
I checked the fuel flow out of the carb bowl through the needle valve and that was consistent and strong stream.

I knew from running on full choke that all cylinders were firing because it ran smoothly.

I started it and it only ran on full choke... again! just on a whim I put my hand in front of the carb intake to block it off and just before the engine stalled I removed my hand and it was running differently so I adjusted to half choke. Then I blocked the intake again and was able to adjust to no choke. The idle was way to fast since the carburetor was a different one, so I just adjusted the linkage to proper idle and put the tractor back together the rest of the way and I was off and running.

Now I can start it with half a crank without choke just as before! My best guess is my hand caused some vacuum change and dislodged something I failed to clean out of the carb.

Thanks
R

I have had this technique work for me many a time. Glad to hear you got it resolved!