Farmall BN Will Not Start When Warmed Up

Rusty Knuckles · Jan 17, 2011

I have a Farmall BN rebuilt the engine about three years ago. Runs like a top. Now just this last month or two after you run the tractor for a while and shut it off it will not start. Pull it or jump it with the truck and it will fire right up. The battery has plenty of juice to crank the engine but it will not fire. If you let it sit and cool down for an hour or so it will start. This tractor has the distributor with the attached coil on the top or is that a Mag I don’t know (I going off of pictures). I don’t have the tractor to look at it was my uncles. Could it just be the battery? He said he rechecked the valves and they were all within spec. I believe it was a 6V but I will have to ask. Any Help?

old · Jan 17, 2011

Well #1 to tell if it is 6 volts or 12 volts count the number of cell caps/holes it has. If you have 3 it is 6 volts if 6 you are 12 volts. Sits for and hour and then starts again sounds like a coil going bad. Both a distributor and a mag have coils just a distributor has one that has a wire running from it to the distributor cap and on that coil is a wire that goes to the switch and has battery power going to it and a mag has an internal coil and no battery power going to it and may or may not have a wire hook to it going to a shut off switch

Bob · Jan 17, 2011

We need to figure out for SURE if it's a mag or a distributor.

In either case, check the spark when COLD with a spare sparkplug with the contact bent open to 3/16" (or a spark tester), then check it warm, when it won't start.

<img src = "https://photos.yesterdaystractors.com/photoads/upload/35914.jpg">
IH Magneto

<img src = "http://www.collectibles-articles.com/antique/collectible-image-large/ih-farmall-distributor-a-b-c-h-m-super-working-cond_370461956271.jpg">
IH Distributor

Rusty Knuckles · Jan 17, 2011

It is the Mag I will find out if it is 6v for sure.

Bob · Jan 17, 2011

If it is a mag, it could be that the impulse is not "catching' at cranking speed, making for a weaker spark, or a coil that gets weak when warm. It's POSSIBLE for the condenser to act like that, as well.

Checking the spark when it will and won't start will tell for sure if the spark gets weak and the mag will have to be repaired as needed.

ScottyHOMeY · Jan 17, 2011

All good suggestions, Bob. The only caution I'd add is that if the problem is the impulse and the condenser and coil are good, you'll still get a strong spark when you crank it, it will just be at the wrong time.

Been havin' the same problem with mine, and that's what I'm down to. When it won't start hot, I've tried the hand crank just to listen, and I don't hear the impulse clicking like it does when it's cold. Gonna have to pull it here shortly and either clean/free it up or take a road trip up to Madawaska to see about repairs.

holmesw · Jan 18, 2011

My dad has an allis chalmers B, that is a crank start magneto ignition. When we first got it, it would normally take one pull of the crank, when it was cold, and it would start. After it was warmed up, you would get blisters trying to restart it. When we would roll it down a hill, and pop the clutch, it would take right off. We eventually got frustrated enought, that we took it to a mechanic, who fixed it. What it was, was the impulse coupler on the magneto. The mechanic said it was only catching on one of the two nubs, and was thus getting half of the needed spark. I do not know if this is your problem or not, but it brought back enough memories for me, about our B.

teddy52food · Jan 18, 2011

You will only get a weak spark if the impulse isn"t working. Unless you try & spin it over & that is a good way to hurt your arm as it will try to kick back. The impulse should retard the spark for starting and when it snaps, it turns the rotor fast enough to produce a hot spark.

Rusty Knuckles · Jan 18, 2011

It Is also a 6volt system.

Dave from Texas · Jan 18, 2011

I had an H that did the same thing. As long as i didnot let it die it would run strong all day long. If it died I was stuck. My problem was that the rotor was not the right one for the mag. The timing would move just enough to keep it from starting. So I would recommend you check to see if it is good and tight and then check the timing.

mkirsch · Jan 18, 2011

(quoted from post at 10:38:31 01/18/11) It Is also a 6volt system.

The voltage of the battery is irrelevant if it has a magneto ignition.

Magnetos generate their own electricity.

You should be hearing a series of *SNAP* noises from the magneto when cranking the engine over with the starter. If you don't, the impulse coupler is bad.

ScottyHOMeY · Jan 18, 2011

Thanks, Teddy. Still livin' and learnin' here.

Farmall BN Will Not Start When Warmed Up

Rusty Knuckles

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old

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Bob

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Rusty Knuckles

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Bob

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ScottyHOMeY

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holmesw

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teddy52food

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Dave from Texas

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mkirsch

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