Farmall Cub – Persistent Carb/Fuel Issues After Major Troubleshooting

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Hi all, I’ve been working on reviving my 1951 Farmall Cub and have run into a persistent problem I can't seem to fully resolve. Earlier this year I decided to take on this challenge and got it running in just a couple weeks of tinkering here and there and learning about engines and old tractors. I even drove it around the yard briefly, which was exciting. But, as I recall, that’s when things got a bit more complicated for me. It took me a while to realise there was a grounding issue, which I fixed, but then I ran into what I think is a carburetor and/or fuel delivery issue. I'm hoping someone might recognize what’s going on and have a suggestion for what to try next. Here are some details:

What’s I’ve Done So Far
  • Stuff that I don’t think is causing the issue:
    • New battery (and as long as I remember to push in the ignition knob, it’s fine)
    • I rebuilt the starter – new bushings, brushes, new terminals, new switch
    • New ignition knob
    • I replaced the rusty bolts connecting the battery box to the tractor and ground down the metal where they meet and this solved an earlier issue where nothing was working.
  • Carburetor:
    • I’ve rebuilt the original carb, and taken it apart and cleaned everything so many times, and adjusted the float height carefully (tried ~1.4"–1.75" closed/open) too many times.
    • I installed a new aftermarket carb, which ran well briefly, but in short time had the same issues as the original carb (see below).
    • The carb bowl often floods – especially after running or failed starts – with fuel dripping or spraying from the weep hole.
    • Sometimes starts fine once or twice, then won’t restart (engine cranks but doesn’t catch).
  • Fuel Delivery Line:
    • Steel and rubber lines inspected and replaced — no obvious kinks or blockages. Right now, I have a small auxiliary fuel tank connected so I don’t need to bother removing the big tank to check things (which I felt I had to too many times)
    • Confirmed tank is venting (tried with cap off).
    • Added inline fuel filter, and also an inline shutoff valve.
    • Fuel flows freely to the carb inlet when disconnected.
  • Spark Plugs:
    • Clean, spark, and not wet after no-starts
Current Behavior
  • Tractor will start and run once for a couple minutes or more and I think sounds okay, then either dies or I shut it off. The last time I tried this with the new auxiliary fuel tank set up, it seemed to run well and no smoke coming out of the stack, which has sometimes happened.
  • Restart attempts usually fail, with fuel starting to drip from the carburetor weep hole after a few tries. Many times the fuel would spray out of the weep hole, but this last time with the auxiliary fuel tank setup, fuel only dripped.
  • Spark still looks good, and plugs not fouled or soaked.
  • Feels like something is causing erratic flooding or inconsistent fuel flow into the carb bowl, even with multiple carbs, various fuel height settings, and different fuel line setups tested.
  • If I take off the carb, let everything set for a while, then try again, the tractor starts but with the same results.

Questions / Help Sought
  • Does anyone have any helpful suggestions on what I should try next?
  • Is there anything I’ve missed?
  • Might there be anyone near Chambersburg, PA who would be keen on taking a look at the tractor and tell me what I’m overlooking and need to do?

Thanks for any help! I’d really like to get this tractor reliably starting and running again!
 
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Have you check to see if your carb has an inlet screen? The original IH carbs did, and that might be plugged up from crud in the tank. The zeniths also have an inlet screen, but it's part of the 90° inlet fitting.
 
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Fuel dripping from the carburetor after a failed start indicates fuel is getting to the carburetor, but maybe not clear through it. Unscrew the main jet out of the side of the carburetor and make sure it is totally clear. Poke a single strand of telephone wire through it then blow it out with air.

Fuel spraying from the weep hole sounds like a whole different thing. Check your spark plug wires to be sure none of them are crossed.

Starting briefly then refusing to restart is often ignition. A decent spark might go away after starting. It can be caused by a bad coil or bad condenser. Check for a good spark IMMEDIATELY after it has run a few minutes and quit. Coils tend to take longer before they quit. A failure within a couple minutes is more likely to be the condenser.
 
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I had a similar issue years ago. Double check the linkage adjustment between the carb and governor. Sometimes adjusting it just a couple turns one way or the other to find the sweet spot can make a difference.
 
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I ran into the same problem a few years back. It really helps to carefully check the connection between the carburetor and governor sometimes tweaking it slightly in either direction until you hit the right setting solves the issue.
 
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Dale Finch said:
Have you check to see if your carb has an inlet screen? The original IH carbs did, and that might be plugged up from crud in the tank. The zeniths also have an inlet screen, but it's part of the 90° inlet fitting.
Click to expand...
I realise now that I haven't had an inlet screen in this whole time. Therefore, not the issue I suppose. But I"ll put the one in that I just found in the rebuilt kit at some point. Thanks!
 
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Jim Becker said:
Fuel dripping from the carburetor after a failed start indicates fuel is getting to the carburetor, but maybe not clear through it. Unscrew the main jet out of the side of the carburetor and make sure it is totally clear. Poke a single strand of telephone wire through it then blow it out with air.

Fuel spraying from the weep hole sounds like a whole different thing. Check your spark plug wires to be sure none of them are crossed.

Starting briefly then refusing to restart is often ignition. A decent spark might go away after starting. It can be caused by a bad coil or bad condenser. Check for a good spark IMMEDIATELY after it has run a few minutes and quit. Coils tend to take longer before they quit. A failure within a couple minutes is more likely to be the condenser.
Click to expand...
OK, thanks! I feel I've made sure the carburetor was totally clear so many times, and blown everything out with an air compressor many times. But I'm keen to check for a spark after a run and quit. Thanks!
 
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k hutchins said:
I had a similar issue years ago. Double check the linkage adjustment between the carb and governor. Sometimes adjusting it just a couple turns one way or the other to find the sweet spot can make a difference.
Click to expand...
Oh wow! This was something I was considering, as the throttle's movement only affects the rod to the carburetor 2/3 or 3/4 (i.e. I move the throttle lever from low to high and by the time it gets to about 2/3 - 3/4 full the rod to the carburetor stops moving. After a while of messing (naively) I figured it couldn't be the issue. I'll definitely look into this linkage adjustment stuff next! Thank you!
 
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If you haven't done a dry & wet compression test to gauge the general health of the engine I suggest you do it. First check compression dry and record your readings, then squirt about a tablespoon of oil in each cylinder then recheck compression and record your readings.
If you don't see significant differences in the readings, it's time to adjust your valves. Low compression will make the engine hard to start, run poorly and may even make it difficult to draw the fuel mixture up from the carburetor.
A significant difference in compression readings indicates worn piston rings. If you don't have a compression tester, a loaner can usually be supplied by a local auto parts store.
 
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ricky racer said:
If you haven't done a dry, wet compression test to gauge the general health of the engine. First check compression dry and record your readings, then squirt about a tablespoon of oil in each cylinder then recheck compression and record your readings.
If you don't see significant differences in the readings, it's time to adjust your valves. Low come will make the engine hard to start, run poorly and may even make it difficult to draw the fuel mixture up from the carburetor.
A significant difference in compression readings worn piston rings. If you don't have a compression tester, a loaner can usually be supplied by a local auto parts store.
Click to expand...
Oh gosh! That sounds like an adventure. OK, I'll put this on the list of things to try next! thank you
 
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Ditch the inline fuel filter and put on an OEM sediment bowl. Those inline filters work until they don't. Make sure the needle valve is sealing completely by turning the carb upside down and blowing air into the carb. Sometimes those needle valve threads/ composite washer don't seal 100% to the carb top so you need to put some sealant on them.
 
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yellowcub said:
Ditch the inline fuel filter and put on an OEM sediment bowl. Those inline filters work until they don't. Make sure the needle valve is sealing completely by turning the carb upside down and blowing air into the carb. Sometimes those needle valve threads/ composite washer don't seal 100% to the carb top so you need to put some sealant on them.
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I have the sediment bowl and may ditch the inline fuel filter at some point. Thanks. The needle valve is sealing completely - I've tested it many times, including the way you suggest. I like your thought on the threads possibly not sealing, but it's not always a flooding issue + the air tests indicate there's a good seal (with both carbs. Thanks for your thoughts!
 
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Rocketman Two said:
Even though you have cleaned it many times, have you tapped the side of the carb with a screwdriver handle to make sure the float isn't sticking?
Click to expand...
yup. thanks. I'm pretty convinced the issue isn't to do with the needle or float at this point. I'm keen to try this governer adjustment idea in the next couple days and see where that takes me
 
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Hi,
I don't see that you have looked at the Battery Ignition unit, maybe I missed it above.
The ignition points burn with use, and can corrode when sitting. Either condition can prevent power from going through them, or make the engine not run right.
I remove them and file the surfaces flat with a large flat file. Wipe off all filing with a clean rag.
If they are too bad, they need replacing.
They have said on here that some new points have a clear coating on them, power won't go through the coating, it has to be removed by filing the points. Wipe off all filing with a clean rag.

The Battery Ignition unit timing is important, the engine can start hard, or not have as much power as it should if the timing is wrong. The Cub service manual says to use a timing light to check or set the timing. Cars used a timing light in the past.
There are pictures of the points and condenser, in the 1952 Cub operator's manual.
The coil may have quit, or partially quit.
It is sort of common for the old original Cub coils to quit. Some of them work when cold, then quit when warm, then work again when the engine is cold.
The original coils IH used on Cubs in 1951 had posts at both ends. The modern coils have all the posts at one end, like automotive coils had in the past.
The modern coils are a bigger diameter then the old coils. You would need a different coil bracket too. TM Tractor Parts has a new coil and bracket. They are on here, near the bottom of the Forums page. You can look at the pictures.
I sent you a message, you get to it near the top right of the page. :)
 
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Glen said:
Hi,
I don't see that you have looked at the Battery Ignition unit, maybe I missed it above.
The ignition points burn with use, and can corrode when sitting. Either condition can prevent power from going through them, or make the engine not run right.
I remove them and file the surfaces flat with a large flat file. Wipe off all filing with a clean rag.
If they are too bad, they need replacing.
They have said on here that some new points have a clear coating on them, power won't go through the coating, it has to be removed by filing the points. Wipe off all filing with a clean rag.

The Battery Ignition unit timing is important, the engine can start hard, or not have as much power as it should if the timing is wrong. The Cub service manual says to use a timing light to check or set the timing. Cars used a timing light in the past.
There are pictures of the points and condenser, in the 1952 Cub operator's manual.
The coil may have quit, or partially quit.
It is sort of common for the old original Cub coils to quit. Some of them work when cold, then quit when warm, then work again when the engine is cold.
The original coils IH used on Cubs in 1951 had posts at both ends. The modern coils have all the posts at one end, like automotive coils had in the past.
The modern coils are a bigger diameter then the old coils. You would need a different coil bracket too. TM Tractor Parts has a new coil and bracket. They are on here, near the bottom of the Forums page. You can look at the pictures.
I sent you a message, you get to it near the top right of the page. :)
Click to expand...

Glen said:
Hi,
I don't see that you have looked at the Battery Ignition unit, maybe I missed it above.
The ignition points burn with use, and can corrode when sitting. Either condition can prevent power from going through them, or make the engine not run right.
I remove them and file the surfaces flat with a large flat file. Wipe off all filing with a clean rag.
If they are too bad, they need replacing.
They have said on here that some new points have a clear coating on them, power won't go through the coating, it has to be removed by filing the points. Wipe off all filing with a clean rag.

The Battery Ignition unit timing is important, the engine can start hard, or not have as much power as it should if the timing is wrong. The Cub service manual says to use a timing light to check or set the timing. Cars used a timing light in the past.
There are pictures of the points and condenser, in the 1952 Cub operator's manual.
The coil may have quit, or partially quit.
It is sort of common for the old original Cub coils to quit. Some of them work when cold, then quit when warm, then work again when the engine is cold.
The original coils IH used on Cubs in 1951 had posts at both ends. The modern coils have all the posts at one end, like automotive coils had in the past.
The modern coils are a bigger diameter then the old coils. You would need a different coil bracket too. TM Tractor Parts has a new coil and bracket. They are on here, near the bottom of the Forums page. You can look at the pictures.
I sent you a message, you get to it near the top right of the page. :)
Click to expand...
Thanks, if adjusting the governer mechanism doesn't work, I'll give this a try. Thanks for all the info!
 
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Were you able to figure out the issue?

I ran into a problem last winter working on a chainsaw where I worked through everything I thought could be the problem. Crank seals, rebuilt the carb, new points, new condenser, new coil, new spark plug, new spark plug wire, etc. I ruled out everything. New fuel line and filter. Cleaned the carb multiple times. Would never run right.

I finally swapped a different carb onto the saw and it ran perfect! I know a Cub and a chainsaw are different, but they both need fuel to run. Perhaps something is funky with that carb you're using? If you rule everything else out, maybe check into that.
 
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cdahl383 said:
Were you able to figure out the issue?

I ran into a problem last winter working on a chainsaw where I worked through everything I thought could be the problem. Crank seals, rebuilt the carb, new points, new condenser, new coil, new spark plug, new spark plug wire, etc. I ruled out everything. New fuel line and filter. Cleaned the carb multiple times. Would never run right.

I finally swapped a different carb onto the saw and it ran perfect! I know a Cub and a chainsaw are different, but they both need fuel to run. Perhaps something is funky with that carb you're using? If you rule everything else out, maybe check into that.
Click to expand...
I had to work on other things, and then it got too cold to mess around with the governer in the cold shed. Yeah, it could still be a carburetor issue - I bought a 25$ one to test whether earlier issues with flooding and not starting was due to problems with the original carburetor. Putting on the new carburetor allowed the tractor to start right up. But eventually the issues returned and now they're a bit different from then anyway 🤷🏻‍♀️
 
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