Farmall Cub Low Idle Pop

R

RunningRed

Well-known member
Hey fellas my low idle pop has returned. I spent most of the day today inspecting everything to try and find out why it's doing that. Seems to be happening some at mid throttle as well. It's very random, sometimes it will do it 2 or 3 times in a row then will go maybe 30 seconds before it does it again and then it might only do it once or twice. I went through the entire ignition system, checked the timing again, adjusted the carburetor and I previously adjusted the valve lash. I also ran another compression check since I bought the tractor a few years ago. I warmed up the tractor for a few minutes before the test and the results are #1- 96 #2- 95 #3- 100 #4- 99. Compression seem good, the #2 cylinder seems to take longer to build up pressure though and bleeds off a maybe 2-3psi over a few minutes. This popping noise drives me nuts, last year before I adjusted anything when I would be unloading firewood into my basement and the tractor was at a low idle sometimes it would pop enough that it would shut off or very near would. Is it possible that I may need to adjust the timing a few degrees to see if it helps? If I adjust the low end I wonder how that effect the timing for the high idle timing.
 
Bill V in Md said:
Is the popping sound a cylinder missing? Does it disappear at high rpm’s? Sorry I forgot, is this a 6v magneto ignition?
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No I don't hear it at high idle, it's also a distributor. The popping doesn't sound like a cylinder miss to me, I double checked to make sure I had all the wires hooked to the right cylinders. When it pops I can hear the engine load up and I also heard the ticking in the distributor slow momentarily. The rpm drops just for a split second and it sounds like it's under a load.
 
Don McCombs said:
I second Jim’s comment. Try Seafoam in your oil in addition to the gas. See can for recommended rate.
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I can and see what happens but I don't think its a sticky valve but it could be. I remember before when I turned the distributor some I could get it to stop but that was over a year ago and I don't know that that's the problem. It runs great at high idle and sounds really good.
 
I would stop using the Seafoam in the gas, and use it only in the oil.
Just a possibility that the Seafoam in the gas is causing the popping.
You could drain the gas out, and put in gas with no Seafoam.
I don't know if you noticed when the popping began, if it was after you put Seafoam in the gas.
 
Glen said:
I would stop using the Seafoam in the gas, and use it only in the oil.
Just a possibility that the Seafoam in the gas is causing the popping.
You could drain the gas out, and put in gas with no Seafoam.
I don't know if you noticed when the popping began, if it was after you put Seafoam in the gas.
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The gas I have in right now doesn't have sea foam in it. I only put enough gas in to test and make adjustments before I fill it. The popping was always there since I brought the cub home. Once I adjusted the governor, idle speeds and the carburetor the popping noise wasn't as bad. Now it's back to where it was before, only now it seems to be at about 800-1000rpms also but nowhere near as bad as it is at low idle. I tried giving it more or less gas at the carb and it sounded a little better but still popped. I tried feathering the choke out while it was running and it makes the rpms increase very slightly but still was popping at both low and mid idle. It just makes me wonder if my timing needs adjusted slightly. I mean after it's warmed up it's dead on 0° at 500rpm and 16° at 1830. It's driving me nuts and I want to find out why.
 
Ok I went over the tractor again today. I messed with the carb, and checked all of the wires and coil. The coil is making a snappy purple ish blue spark and I can make it jump almost half an inch. I checked the lines on the distributor shaft to check the point gap on each one and they are the same. The distributor shaft bushing was next, I tried side to side play and it's no more than .005 of an inch. Then I checked the end play and I think they be some or all of the issue. There is a touch over .085 thousands in and out movement? Isn't the spec like .005-.010?
 
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I have to agree with Jim on the valves, either sticking, or set a little tight. They are better to be a little loose than tight which will allow for expansion when it warms up.
 
I guess you have checked the valve clearance adjustment. Here is info for others.
I would check them if you haven't. They are manually adjusted, if nobody checks them, they can get out of adjustment. They need .013" clearance at the lower end of the valve stems.
You have to remove the valve cover on the left side of the engine to get to the valves.
The Cub operator's manual tells how to adjust the valve clearance.
The valve cover has a gasket, you may need a new gasket if you remove the valve cover. TM Tractor Parts has a new gasket.

Running Red, you could check this below.
Also, use the hand crank, if you have one, and a light, and look at how high the valves are moving, they should all move the same amount, up and down.
I remember a person I knew had a pickup with a V8 engine, and one or more lobes on the camshaft wore down, it made a popping sound when running, he said.
 
seen a few with one lobe worn on the distributor , check that and also there is not to much slop in the distributor bushing, that can cause the point gap to wander all over the place
 
Had a random putt years ago at idle when warm I thought might be water causing mischief. Stopped to speak with a neighbor and was rewarded with a only slight delay between a double putt putt to inspire looking into it more.

Depending which Cub it was , it was either Champions not being champs ; or the one I had a single plug gapped too tight on one plug somehow and the others were spot on.
I am guessing it was the too tight a gap on the one plug Cub. ( I might have bonked it putting it in.)
 
tst said:
seen a few with one lobe worn on the distributor , check that and also there is not to much slop in the distributor bushing, that can cause the point gap to wander all over the place
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I checked the bushing and there isn't much side to side play maybe .005 thousands. Now the end play of the shaft is a different story, it's .085 thousands in and out. I checked the point gap on all of the lines and they seemed to be very close to each other, no detectable different that I could tell.
 
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