Need help diagnosing

C

clintmo

501 Club
I’ve been having this issue with my ‘49 for a couple years now. I can’t really use it as is. It starts right up and will run great and then unpredictably it will stutter and act like it’s gonna stall out, and under a load it will. And then it’ll straighten out and be fine for a while. Sometimes a good while. And then It’ll happen again. Every time I think it’s over it, I’ll take it out and it’ll do it again and I’ll have to limp it back to the building. I got the suggestion at the bash to try Marvel Mystery Oil and this evening I did that. Started it up and just let it run for a while. I’m sure it’s too soon to know what effect that will have, but in the meantime, here’s a video of what it’s doing. I’ve checked over everything I can think of. It seems fuel related. I’ve checked out every point from the tank to the carb, gone over the carb and tried a known good carb and nothing changes.


Thanks!
Clint
 
Hi,
I would replace the condenser, and see if it runs better. I saw one that ran similar to yours after putting in a new condenser that was defective.
You could file the points and check their gap too, they may have gotten corroded when sitting, or they could be burned from use. Wipe off all filings with a clean rag. :)
 
They have said on here that NAPA has good quality points and condensers for Cubs.
Moisture in the distributor cap can make it run bad. Dry the moisture if there is any in it.
It could be the main jet in the carburetor partly plugged. It is low on the side of the IH carb.
Tighten it gently, they can break if overtightened. It has a gasket, the gasket might not seal if it is old, and you reuse it. TM Tractor Parts has a new gasket.
Below is a picture of the main jet, and cleaning it with a small wire.
 

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It's been a while (maybe a couple years) that I remember Dale Finch posting about similar symptoms that you are having. I can't find the post but it came down to finding a small part of a bug that had gotten into the fuel system and at random times would restrict the fuel flow through the shut off valve. You could try blowing back into the tank with air pressure and maybe install a standpipe strainer/filter into the valve where it screws into the tank. I think there's a HOW TO on the subject. Good Luck!! JMHO Stan
 
Thanks guys. I’ve cleaned out the main jet multiple times. One of the first things I tried, haha! (I have learned a few things!) I’ve removed the strainer and taken it apart and cleaned it all out. I’ll admit that I haven’t opened up the distributor recently, so I’ll be doing that, although I’m pretty sure I did when this problem first occurred. It just seems so much like a fuel thing that that’s where I keep turning back to.

Keep those ideas coming!
Clint
 
seen this a couple times with the zenith carbs, this may sound strange but easy to try, when it runs rough , take it back to the barn, shut it down, pull the plug for the main jet, then take a air hose with a blower on it and give it a couple quick blasts back through the main jet , do not get wild or you will blast the oil right out of the air cleaner, do this with the gas on, you will see the air bubble back in the bowl and hear it in the tank, wait a second and see the gas run back out the hole for the jet, if it has a nice flow put the nut back on, start the engine, may run crappy for a few seconds then be ok, this has worked for me several times, only with the zenith carb, if it is happy after you have a fuel flow problem
 
Does it do this only when the engine is warmed up good or randomly, when cold or warm? Does it just cut out clean like you hit the ignition switch, or does it sort of sputter and crap out?

Based on what you've said it sounds more like a fuel delivery issue. You could have some debris floating around in your tank that is randomly floating around and blocking the fuel flow. It may also be something floating around inside the carb. I can't remember if there is a screen in the carb where the fuel inlet is, but if so check that as it could be clogged up with debris too.

You mentioned you tried a different carb and it did the same thing though, so perhaps your current carb is okay.

Glen mentioned the condenser. It's possible it's an ignition related issue too. Have you tuned it up recently or no?

Maybe it's an issue with the governor? It's not allowing the engine to rev back up to maintain the desired RPM based on the throttle setting? Just tossing out ideas.
 
what I would do in order (again this is just me) : first I would take off the air cleaner hose to the carb. I would let it run and if it starts to stumble I would hit it with a dash of starting fluid. If it smooths right out you know it’s fuel delivery related. If it does I would hook up an external fuel tank with know good fuel. If it runs there then you know it’s in the valve (could be water). If it doesn’t run well then you know it’s in the carb.
If the starting fluid doesn’t help then you know it’s on the ignition side. I have a device that lets you see the spark plug getting spark through a clear section of insulated wire. If it has a bright spark you know it’s not the ignition. If it’s weak you have a good clue and you can start working on the ignition!
 
Update: I tried offrink’s process of elimination and the starting fluid didn’t help, so I got into the distributer. Everything looked ok so I went straight for the condenser. Put in a new one and it seems to have solved the issue. Ran it at high RPMs and then low idle for probably 15 minutes while I tinkered around on some other things and it never faltered. Fingers crossed that that’s that.

Thanks for the help as always!
Clint
 
clintmo said:
Update: I tried offrink’s process of elimination and the starting fluid didn’t help, so I got into the distributer. Everything looked ok so I went straight for the condenser. Put in a new one and it seems to have solved the issue. Ran it at high RPMs and then low idle for probably 15 minutes while I tinkered around on some other things and it never faltered. Fingers crossed that that’s that.

Thanks for the help as always!
Clint
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Awesome!
 
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