1948 Cub Fuel System Troubles

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markges
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1948 Cub Fuel System Troubles

Postby markges » Sun May 15, 2022 10:38 pm

My 1948 Cub starts well with the crank, but after 1-2 minutes it shuts down as if it has run out of fuel. Everything is correct: the fuel sediment bowl is turned on, the on/off knob is pulled out, and there is fuel in the tank. Why is it shutting down? There must be a fuel flow problem. Could it be sediment getting into the carburetor? Can someone help me diagnose this problem? By the way, this happens while sitting idle AND while in gear and moving about (1st, 2nd, 3rd, and reverse). My name is Mark if you wish to address me by name.

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Re: 1948 Cub Fuel System Troubles

Postby tst » Sun May 15, 2022 10:56 pm

welcome to the forum, make sure the screen in the sediment bowl is not plugged up does the fuel have good flow to the carb, is the screen in the fuel inlet of the carb plugged up ?

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Re: 1948 Cub Fuel System Troubles

Postby markges » Sun May 15, 2022 11:01 pm

Thank you for the welcome! I'm happy to be a part of the Farmall Cub club! The screen in the sediment bowl looks clean ( opened it up and drained it yesterday), but I haven't opened the carburetor yet. I'll take a closer look at the sediment bowel screen, then open the carb. I don't have much experience with carburetor work. Do you have any suggestions on videos online that I can watch?

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Re: 1948 Cub Fuel System Troubles

Postby Glen » Mon May 16, 2022 1:30 am

Hi,
A 1948 Cub originally came with an IH carburetor.
This info is for an IH carb.

You could do a gas flow test, and see if enough gas is getting to the carb.
Be sure the gas valve under the gas tank is fully open.
Use a large clean can, or glass jar, to catch the gas.
If you use a clean can, or jar, you can put the gas back in the gas tank when done.
Remove the small plug on the bottom of the carb, it probably has a square head.
Catch the gas.
The 1st gas that comes out will be what is in the carb, then the gas flowing from the gas tank.
The gas flow should be steady, and not stop, and be about a pencil size flow.
Catch the gas for about 1 minute, then put the plug in.

If the flow is good, the screens are probably ok.

If the flow was too slow, or stopped, look at the carb inlet screen, and the other info below.

The gas tank should have a vented cap, and the vent holes need to be open and clean.
If the vent is plugged the gas flow will stop in a few minutes with the engine running.
The engine is stopping sooner than a plugged cap vent would usually take to stop the gas flow, but you could check it.

You don't have to disassemble the carb to look at the gas inlet screen.
It is where the gas line screws onto the carb.
It fits into the hole in the carb.
Remove the gas line from the carb, and it should be visible.
Use a good light to see it.

If the Cub still has the original style metal gas line, from the gas strainer to the carb, you will need to loosen the fitting at the gas strainer too, so the tube can come out of the carb.

It is easier to get to the gas strainer if you move the Touch Control arms to the rear or down position, before shutting off the gas.

It might need the main jet cleaned, a sort of common problem with Cubs. If it is plugged, the engine won't run, or if partly plugged, it won't run well.
The main jet is the brass colored, 6 sided bolt like part, low on the side of the IH carb.
It screws out of the carb. Clean it with a small wire, then wash it in solvent.
Shut off the gas before removing the main jet.
It has a gasket on it, if the gasket is old, it might not seal if you try to reuse it.
TM Tractor at the bottom of the page has a new gasket for the main jet.
Or it might seal using a thin coat of sealer on it.
Tighten everything on the IH carb gently, it is made from soft metal, and threads can strip.

If the Cub has a rubber gas line, and an inline filter, that someone put on, the filter should be one made for a gravity flow gas system.
If the filter is for a fuel pump system, it can restrict the gas flow, using it with a gravity flow system.

Below are pics from TM Tractor.
The 1st pic shows where the screen is.
The 2nd pic shows the metal gas line.
The 3rd pic shows an IH style carb on a Cub, and the main jet, at the lower side of the carb.
The 4th pic shows cleaning the main jet. :)
Attachments
Cub carb 7.jpg
Cub carb 7.jpg (12.83 KiB) Viewed 410 times
Cub fuel line 3.jpg
Cub carb 3.jpg
Cub jet.jpg

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Re: 1948 Cub Fuel System Troubles

Postby Clemsonfor » Mon May 16, 2022 6:35 am

I would guess there is a restriction somewhere in between the tank and carb. It clearly is getting fuel but not fast enough to fill the flair bowl. It sounds as if on e full of fuel and you start it it will run till it sucks the float bowl dry and then fuel is not replaced fast enough and it will stall out. Check the areas the previous posters say to look at

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Re: 1948 Cub Fuel System Troubles

Postby Gary Dotson » Mon May 16, 2022 7:30 am

With a flash light, look into your fuel tank, you can see the fuel outlet port. Often times debris will obstruct fuel flow from the tank. Other than the fuel inlet screen, don’t go into the carb just yet. While you have the fuel line disconnected, checking the screen, open the tank valve, catching the fuel in a container, and check the flow rate. It should maintain a steady flow and not drop off to a dribble. Monitor it for a full minute. It’s almost certain that you are dealing with a fuel delivery problem prior to the actual carb.

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Re: 1948 Cub Fuel System Troubles

Postby markges » Mon May 16, 2022 9:53 am

Thank you very much! I’ll follow these recommendations. One thing that I thought I felt was softness like sediment in the bottom of the gas tank (I used a measuring stick to see how much fuel was in the tank). If that’s the case, should I drain the fuel to clean out that sediment? If so, how do I get all of it out effectively?

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Re: 1948 Cub Fuel System Troubles

Postby Don McCombs » Mon May 16, 2022 12:32 pm

Depending on how much fuel is in the tank, you could try a syphon to “vacuum” it up. If there’s just a small amount or it’s old, I’d drain it.
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Re: 1948 Cub Fuel System Troubles

Postby John *.?-!.* cub owner » Tue May 17, 2022 11:10 pm

You have been given a lot of good advice about the fuel system, but do not automatically assume that is fuel problems. A bad coil or a bad condenser on either a magneto or distributor system can mimic fuel flow problems. I learned that the hard way about 35 years ago, and got my memory refreshed on it about 5 years ago. :oops:
If you are not part of the solution,
you are part of the problem!!!

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Re: 1948 Cub Fuel System Troubles

Postby markges » Thu May 26, 2022 5:29 pm

Here's the latest news on my fuel system troubles (Gary Dotson you might be correct--see below):
I removed the sediment bowl with the fuel valve above it turned off. Then, once the bowl was removed I turned the valve back to full open but the fuel only trickled out. I used some wire (down from the top of the tank) to move around inside the drain hole and suddenly the fuel came out at full speed. It seems that the actual fuel tank drain hole might have been so plugged up that the fuel could barely get to the carburetor. The fuel line to the carburetor seemed clean, but I still cleaned it out. The main jet was clean. The gas tank has a lot of sediment inside. I'm waiting for the tank to dry out so that I can get the sediment cleaned out. What's the best way to clean it out? With a vacuum (once it's dry of course)?

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Re: 1948 Cub Fuel System Troubles

Postby Bob McCarty » Thu May 26, 2022 5:35 pm

DON'T use a vacuum. There is a chance that a spark from the vacuum motor could ignite the fumes and cause the tank and vacuum to explode!
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we need to think differently."
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Re: 1948 Cub Fuel System Troubles

Postby markges » Thu May 26, 2022 5:57 pm

What's another way to clean the sediment out of the tank?

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Re: 1948 Cub Fuel System Troubles

Postby Dale Finch » Thu May 26, 2022 6:03 pm

You can remove the cast upper half of the sediment bowl, which will leave a flush hole that will allow you to drain ALL the fuel out. Then "rinse" it out with whatever more knowledgeable folks recommend.
Do you have rust in the tank, or just accumulated "trash"?
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Re: 1948 Cub Fuel System Troubles

Postby Don McCombs » Thu May 26, 2022 10:00 pm

Do as Dale says, then flush/rinse the tank out with kerosene. Higher flash point.
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Re: 1948 Cub Fuel System Troubles

Postby markges » Fri May 27, 2022 11:50 am

There appears to be a little bit of rust in the tank but not significant. Mostly it seems to be trash as Dale Finch mentioned. Should I flush it with kerosene only? If I do that, what's the best way to do it? Does it need pressure? And, a lot of kerosene?


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