Intermittent spark problem Farmall BN

Brad_bb

Member
Farmall BN John's forklift conversion. Started having the forklift die on me. Always after it warmed up. Seemed pretty random. I chased some other gremlins thinking they were the problem. I replace the distributor with a new one because I thought I had a cap/rotor issue and and a replacement cap/rotor was not available for the aftermarket distributor that was in there- so I replaced the whole unit. Turns out that was not the problem. I recently replaced the old Zenith replacement carb with a new one (at one point I thought it was flooding, but have finally come to the conclusion that it was the intermittent spark all along). The problem still persists. I had swapped the coil about 2 years ago, but looked up symptoms of a coil going bad and one of them was stalling like I'm having. So last night I picked up a new coil at Napa - 12V, no external resistor required. The problem still persists though. It will die randomly after it's warmed up. I connect my spark tester and can see NO spark while cranking. By the way, it cranks fine and when it runs, it runs great. After I let it sit awhile I try it again and sometimes I then get spark and she starts right up. So it continues to do this. I do notice when it dies that the coil is hot, and the plug wires are hot at the boot when I go to pull the #4 off to check for spark. I have also checked that I have 12V on the positive side of the coil. FYI I have an ON/Off Switch to turn on the positive side of the coil. Anyone have any ideas why I could still have intermittent spark? Is the new coil getting hot a symptom of something or just normal?
 
How hot is hot? The coil would not be cool to the touch after the tractor had been running, but it should not be so hot that you can't comfortably hold your hand on it for at least 5 seconds.

New coils could be bad.

Are you SURE that the coil is marked "no external resistor required."

What is the voltage output on your alternator while the tractor is running?
 
Coils do get warm but if it is so hot that if you spit on it it steams off it is a bad coil. As for the spark plug boot they get hot since they have a spark plug in them and the plugs get very hot. I would run a hot wire from the battery ignition side to the coil ignition side and see if it fixes it. If it does the problem is from the coil back if not could be points or condenser
 
Couple things to check also are the condition of the old push-pull switch for run/stop, and also the condition of the wiring harness and ring terminals where the connect behind the control panel and at the coil itself. There could be old or decayed connections, chafed wires, etc. Brass-brush all of the studs and nuts where the ring terminals seat as well.

I put a weatherpacked 12volt relay into my new wire harness for the ignition. Why add more complexity? The reason is the old push-pull switch now just has to hold milliamps to pull in the relay coil. A better set of dry contacts is actually sending current to the ignition setup. Its my feeling that this setup is more reliable without too much compromise of simplicity, the relay technology has been well-proven in vehicle applications.
 
Check your points, if they are burned and show heat discoloring, you could be pulling too much current through your coil, which could account for the excess coil heat. Coil current when the points are closed should be about 4-6 amps DC. Much higher than that and the coil will get hot, much lower and the coil probably isn't developing enough secondary voltage for the spark.
I would be suspicious of the condenser (it could be breaking down) and the setting of the points (maybe too much dwell). It sounds like you've replaced about everything else.
 
Yes the coil is from Napa. Has the number 905. Says right on it, "no external resistor required".
413502383.jpg

New information. So just before I posted, It had died and the coil was hot. I could touch it, but it was quite hot. The surrounding engine was warm but not hot. So the heat is not soaking from somewhere else. It's being created in the coil.

I walked away and posted. Then I was doing some other stuff for almost an hour. I had left the switch in the "ON" position, which means 12V was still being seen on the positive side of the coil. Now the coil to too hot to touch. It got hotter with power on the + side. Expected or unusual?

I did check for spark and no spark.
413502385.jpg


So then I left it alone for an hour and the coil has cooled down. Check for spark and I NOW have spark. This is all telling me that the coil is getting hot and then not functioning, but when it cools down, it works.

So is the alt voltage relevant if it's heating without running? How do I measure alt voltage? From where to where?
413502384.jpg
 
I have a toggle switch. I did remove and clean all the connections. I had re-wired the tractor just over a year ago. Tractor is kept inside- heated. No corrosion.

The toggle switch is working correctly. When it's on, I've got 12V at the positive coil post. When off, it's open.
 
My 2 cents worth is a bad codensor, or the
connection between the coil and the distributor
 
If the power switch is on, and the points are closed, yes it is conducting current and the coil will get hot.
 
Leaving the switch on will let the smoke out of a coil plus burn the points up. So now you have bad points and good chance a bad coil. Bet if you check the points they will be burned and maybe even welded up and if you spun it over and they had welded up they are now bent so they cannot open and if they do not open you get no spark since it is when they open that you get spark
 
brad, just a wild thought here, i wonder if the alternator is putting out too high of voltage? the large nut on the side of the alt is the output. put the + side of a volt meter on that nut and the other lead to a good ground on the tractor, see what your voltage is with the tractor off. then start it and see what you have at a high idle. should be anywhere from 13.5 on up to about 16 or 17. if its way above that, it may be your problem. just a thought and you dont have to throw parts at it to test.
 
Been away for while and have not been able to address this issue. Today my hand jumped on the machine and started it. It ran for a minute or two and I told him to shut it down. He did. Then a couple minutes later I wanted to check some voltages and told him to jump on and fire it up. It would not start. gas started dripping from the bottom of the Zenith carburetor.
This is probably due to excess fuel cause it wouldn't start. So I connected my spark gap tester and tried to start it. No spark on my tester. So I started hunting around checking voltage at the coil- at first I thought I read 0V at the positive side of the coil. I was hunting around and wanted to check continuity across the aftermarket toggle on/off switch. I then noticed the bottom of the switch was separating. I had my hand wiggle the on off switch as I checked the voltage at the coil and I couldn't see anything definitive. Then I tried it again and suddenly I was getting spark. I decided that I needed to replace the on/off switch and was hoping that is where the intermittent spark was coming from? So then I set out to re-gap the points. I gapped them at .028. I don't have a spec other than what was in the Steiner catalog in the description for their tune up kit which says .020. I then decided to start the tractor. It started up easily not that it was getting spark. I decided to play with the timing to see if I could improve the idle, which is not smooth like it was before all this began. It also has a bit of a stumble when you hit the throttle. I loosened the bolts and and rotated the distributor to see if I could improve. I could not. It's idle is not smooth and steady like it used to be before all this began 6 months ago.

Here's one issue. I don't know how to time this engine. There isn't a reference tab that I know if like on a small block chevy. I have a Sun tune up kit with timing light, dwell/tachometer. What should the timing be and how do you set it on this engine?

Note, while I was doing this playing, idling in open doorway of the shop, the coil did heat up. Getting pretty exasperated!
 
Been away for while and have not been able to address this issue. Today my hand jumped on the machine and started it. It ran for a minute or two and I told him to shut it down. He did. Then a couple minutes later I wanted to check some voltages and told him to jump on and fire it up. It would not start. gas started dripping from the bottom of the Zenith carburetor.
This is probably due to excess fuel cause it wouldn't start. So I connected my spark gap tester and tried to start it. No spark on my tester. So I started hunting around checking voltage at the coil- at first I thought I read 0V at the positive side of the coil. I was hunting around and wanted to check continuity across the aftermarket toggle on/off switch. I then noticed the bottom of the switch was separating. I had my hand wiggle the on off switch as I checked the voltage at the coil and I couldn't see anything definitive. Then I tried it again and suddenly I was getting spark. I decided that I needed to replace the on/off switch and was hoping that is where the intermittent spark was coming from? So then I set out to re-gap the points. I gapped them at .028. I don't have a spec other than what was in the Steiner catalog in the description for their tune up kit which says .020. (So I guess I need to regap it, but I don't think this is the problem). I then decided to start the tractor. It started up easily not that it was getting spark. I decided to play with the timing to see if I could improve the idle, which is not smooth like it was before all this began. It also has a bit of a stumble when you hit the throttle. I loosened the bolts and and rotated the distributor to see if I could improve. I could not. It's idle is not smooth and steady like it used to be before all this began 6 months ago.

Here's one issue. I don't know how to time this engine. There isn't a reference tab that I know if like on a small block chevy. I have a Sun tune up kit with timing light, dwell/tachometer. What should the timing be and how do you set it on this engine?

Note, while I was doing this playing, idling in open doorway of the shop, the coil did heat up. Getting pretty exasperated!
 

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