Dimishing spark

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Luu

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I am working on trying to get a 1948 Cub running that I got from my late father in-law. I don't know much history on it but the battery I replaced (6v) was dated 1996. I've replace a few parts and rebuilt the carb but at this point I am having trouble getting it to start. It has a 6v system with a J4 magneto. When I check for spark, I get a spark and then either a weak second one or none after that. I have replaced the cap, rotor, coil, condenser, plugs, points, plug wires and gaskets. I have double checked the timing many times as well made sure the rotor gear is aligned with the indexing of the other gear. I've disconnected the shut off switch. I hear a good snap when I rotate it. I've also tried swapping back in some of the replaced parts. I'm stuck at this point. What else could it be and how would I test that? Your help is appreciated.
Kris - Wisconsin
 
Luu said:
I am working on trying to get a 1948 Cub running that I got from my late father in-law. I don't know much history on it but the battery I replaced (6v) was dated 1996. I've replace a few parts and rebuilt the carb but at this point I am having trouble getting it to start. It has a 6v system with a J4 magneto. When I check for spark, I get a spark and then either a weak second one or none after that. I have replaced the cap, rotor, coil, condenser, plugs, points, plug wires and gaskets. I have double checked the timing many times as well made sure the rotor gear is aligned with the indexing of the other gear. I've disconnected the shut off switch. I hear a good snap when I rotate it. I've also tried swapping back in some of the replaced parts. I'm stuck at this point. What else could it be and how would I test that? Your help is appreciated.
Kris - Wisconsin
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Welcome to the Forum. From your post I'm assuming that the magneto is an OEM installation (not modified in any way). If that is true, a good/functioning magneto is all you are required to have electrically in order to run the engine as the mag makes the current internally as it operates to start and run the engine. NO BATTERY is needed other than for it to run the starter motor. If the output wire (cable) from the mag to the distributor cap will not produce a spark to ground when it's disconnected and the engine rotating to start you probably have a bad coil which is not unreasonable on a 70 YO tractor. You could have other problems since you've replaced parts, etc but you must have a spark from the coil to ground first. JMHO Stan
 
Recheck points gap and make certain they are clean. How long has it been since this tractor run? From the age of the battery a good while likely. Pull all the plugs out and spin engine over holding your finger over plug holes and see if you have compression on all cylinders. And check spark on each wire while you have all the plugs out spinning over.
 
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Make sure your points have clean contacts, both the stationary and moving ones. New ones often have a coating that needs to be cleaned off before they will pass current properly. Beyond that, spark that initially looks good then drops off is usually caused by a bad coil or bad condenser. This failure is usually heat related. In a coil it almost always takes a while to show up -- like after maybe 20 minutes of running. If it happens quickly, it is more likely to be the condenser. There have been multiple reports of new condensers failing straight out of the box. So be sure to try reinstalling the old condenser.
 
So here is my baseline to check to see what is wrong. I described it in a different post. Check to see if you have spark at the plug. They make a tool that you put inline between the plug wire and the plug so you can see when the spark goes through the wire. If you don’t have good spark then I would start there. If you do then I would take the air cleaner off and give it a puff of starting fluid. If it starts and runs, or stumbles at least you know that it’s fuel related. If it tries to start but just kind of stumbles then I would be looking at timing.
 
staninlowerAL said:
Welcome to the Forum. From your post I'm assuming that the magneto is an OEM installation (not modified in any way). If that is true, a good/functioning magneto is all you are required to have electrically in order to run the engine as the mag makes the current internally as it operates to start and run the engine. NO BATTERY is needed other than for it to run the starter motor. If the output wire (cable) from the mag to the distributor cap will not produce a spark to ground when it's disconnected and the engine rotating to start you probably have a bad coil which is not unreasonable on a 70 YO tractor. You could have other problems since you've replaced parts, etc but you must have a spark from the coil to ground first. JMHO Stan
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Thank you for your reply Stan,

I will retest the distributor cap to ground again. I believe the results were the same as each plug I tested when I did that a week ago. That is, blue spark then little to no spark as the engine is turned over. I replaced the coil with a new one. I'm not sure how to test if the new one is bad or not. I believe the magneto is original. I've attached a photo of before I started cleaning thing up and replacing parts. I don't have a history of how long it has been sitting other then the old battery was dated 1996.
 

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yes good blue spark is needed for a mag, make sure you do not have a wire grounded out inside, is it a import coil you installed ? , seen several NG out of the box. does the mag "Kick " should be able to here a snapping noise
 
Luu said:
..................I will retest the distributor cap to ground again. ......................... I'm not sure how to test if the new one is bad or not. I believe the magneto is original. ..........
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Referencing your pic, to test the coil (which is under the rectangle plastic cover) remove the black wire from the distributor cap center tower connection and hold it about a 1/4 inch from a clean bare metal part of the engine and try cranking the engine over. If all conditions are right the mag spark will jump the gap from the end of the wire to the metal. That's a simple yes or no test for the mag coil. If you don't get a spark likely the coil is going to need replacing. If you do get a spark reinstall the wire in the center of the distributor cap and check for spark from each plug wire to ground. BTW points gap is .013 for the mag and .020 for a battery ignition. Post what you discover and we'll go to the next step. Stan
 
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What kind of plug wires did you get? Copper core or more modern carbon core? I had an issue with an H4 magneto that kept having issues of plug wires burning out resulting in weak spark even after I replaced everything else (coil, points, ect). I believe it had something to do with the fact magnetos put more juice down the wires than more modern ignitions, which can cause resistive problems in modern carbon core wires. Look, I'm not an electrician, but all I know was that after switching to copper core wires, the engine ran much better and the ends of my wires weren't getting darkened.

EDIT: I found my old video on this:

Forgive my potato camera and messy workbench. It was 10 years ago and I was a lazy 22 year old haha
 
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Thanks all for your input and suggestions. I redid the test with the distributor cap center tower connection to a grounded portion of the tractor. I get similar results as the individual plug, strong spark at first then weak spark to none. Before that I made sure the points and contacts were clean and gapped at .013. New wires were ordered specifically for the Cub with the magneto (Copper core).
Before I started focusing on the ignition I checked the compression with the old thumb over the plug hole test and tried starting fluid in the carb. But at this point, without good spark none of that matters and gets put on the back burner.
I've attached a picture of the old and new coil just for reference.
 

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Did you swap your old condenser back in or try a new one like Jim suggested? Because it sounds like you're not even able to run it long enough to heat-soak the coil if that was the issue. He might be right in that its a DOA condenser.
 
If it were mine, I would have someone pull me off and with it in high gear. I’ve seen old tractors that were hard to crank but pull them off start right up. Especially with a six volt system. Just something to try. Good luck finding your issue!
 
havoc1482 said:
Did you swap your old condenser back in or try a new one like Jim suggested? Because it sounds like you're not even able to run it long enough to heat-soak the coil if that was the issue. He might be right in that its a DOA condenser.
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Yes I did, same results. So maybe both are bad? I tested the ohms on the new one while the old one was back in and it gradually built resistance and then showed "OL". The tractor has yet to fire at all for me but I'm just concentrating on getting good (or any) spark.
 
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Farmallsodbuster said:
If it were mine, I would have someone pull me off and with it in high gear. I’ve seen old tractors that were hard to crank but pull them off start right up. Especially with a six volt system. Just something to try. Good luck finding your issue!
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It cranks really well, it just doesn't spark consistently. I've got a new cable and ground strap to the battery and the starter is strong.
 
Luu said:
Yes I did, same results. So maybe both are bad?
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Its possible. Lets reiterate here:
  • New Wires
  • New Cap + Rotor
  • New Coil
  • New Points, clean and gapped to .013
  • New Condenser
And after all that you still get strong initial spark that dies-off gradually. I'd hate to suggest what amounts to throwing parts and money at the wall, but based on the work you've already done and the context of the issue it seems like a DOA condenser.
 
I'm going to go ahead and say that you got a faulty coil if it consistently produces a good spark, then weak, then none. Can you perhaps work with the supplier to exchange it? Seems like there should be some recourse here. One other possibility to be considered is using an external coil to get it started. You would have to remove the mag coil to do that and connect a battery supply voltage to the external coil and then on to the terminal on the distributor where currently the kill switch wire is connected. I've got a 49 FCub with a mag that's working like that. Stan
 
staninlowerAL said:
I'm going to go ahead and say that you got a faulty coil if it consistently produces a good spark, then weak, then none. Can you perhaps work with the supplier to exchange it? Seems like there should be some recourse here. One other possibility to be considered is using an external coil to get it started. You would have to remove the mag coil to do that and connect a battery supply voltage to the external coil and then on to the terminal on the distributor where currently the kill switch wire is connected. I've got a 49 FCub with a mag that's working like that. Stan
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Thanks Stan,
If I do have to go that route, what coil or type of coil would I buy? I know little about the types of coils available or where to get them. Also, would I keep the kill switch connected? Or would there have to be a different type switch I would have to use?
 
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