Farmall model A, no spark at plugs

adambeck3475 · Jun 10, 2021

Ive been trying to get my grandpas 39 farmall model A running with no luck. The issue Im having is no spark at the plugs. I have replaced the rotor, spark plug wires and spark plugs. Im getting spark from the coil put nothing at the plugs. Any information would help, Ive never worked worked on anything with a magneto so Im at loss. Thanks

wore out · Jun 10, 2021

My first GUESS would be the the rotor is not timed correctly.

You can find full details on correcting that and full info on any other mag issues at the link below. Your mag should be an H-4, if as-original.

You will have to copy it and paste it into the address bar of your browser as it will not post here as a clickable link.

http://farmallcub.com/rudi_cub/www.cleancomputes.com/Cub/Blue%20Ribbon%20Service%20Manuals/GSS-5035%20Service%20Manual%20Magnetos/index.html

This post was edited by wore out on 06/10/2021 at 12:22 pm.

Fritz Maurer · Jun 10, 2021

You didnt state in your post whether you replaced the cap. If by chance you are using anything but copper plug wires, the additional resistance will make a borderline cap leak to ground somewhere.

DaveBN · Jun 10, 2021

When turning slow as in starting speed the magneto should make a clicking noise. That is the impulse tripping. Impulse mechanism spins the magneto fast enough to produce spark at low engine rpm. Are you hearing this clicking noise? If not you need to work on the magneto. Post your results so we can help you.
Davd

adambeck3475 · Jun 10, 2021

Just answering some of the questions. I have not replaced the cap because it was previously replaced and the contacts look brand new. When I turn the engine over I am hearing the click. Like I said I know Im getting some spark from the coil, could it be that even tho Im getting some spark from the coil it wouldnt be enough?

old · Jun 10, 2021

Some spark doesn't tell us much.
You need a blue/white spark that will jump a 1/4 inch gap or more. Does it have a shut down switch at the dash of the tractor?? If it does you might want to take the wire off the side of the mag and try starting it and checking for spark that way. Doing that takes out things that could be a problem

adambeck3475 · Jun 10, 2021

Well I definitely dont have that much spark. What do I need to replace on the magneto to increase the spark?

wore out · Jun 10, 2021

(quoted from post at 12:41:05 06/10/21) Well I definitely dont have that much spark. What do I need to replace on the magneto to increase the spark?

Have you availed yourself of a look at the OEM factory manual I posted a link to previously?

Did you understand what I meant by "timing" the rotor?

adambeck3475 · Jun 10, 2021

Are you talking about timing the magneto to top dead center? Ive
never worked on a magneto.

rustred · Jun 10, 2021

you do not need to replace anything. clean the contact points and start the tractor. a point file is the best to keep the points flat and square and matching. or use emery and then clean with brake clean on a paper towel pulled through. points when sitting closed make a film on the contacts, which will cause a weak spark or no spark. if the engine stops with the points open this will not happen.

old · Jun 10, 2021

Most of the time weak spark is caused by bad or dirty points. Plus point gap has to be correct because that can cause weak spark and the timing to be off

Charlie M · Jun 10, 2021

Since the points have been changed means the plate that turns the rotor has been removed. Make sure the rotor isn't off 180 degrees. Its very easy to do. You can check by putting the number one piston on top dead center on the compressions stroke and see if the rotor is about to fire the plug for #1 cylinder. With it off 180 degrees you will have spark but not on the plug you are expecting. Make sure your tractor is wired correctly from the cap to the plugs. The sequence I believe is stamped into the block above the mag and number 1 cylinder is closest to the radiator.

Janicholson · Jun 10, 2021

The rotor is mounted on a shaft. that shaft is geared to a second shaft. this set of gears is behind the center cover with the points. If you replaced the points, you had it apart. (wore out, posted the timing methodology for those gears. making sure they are correct is foundational to it passing spark to the cap and wires!. If it is sparking it will likely run when timed at those small gears. Jim

rustred · Jun 10, 2021

here u go, did you time it? always wonder why people would have it out of time when there is marks to follow. L FOR LEFT HAND ROTATION, and yours is RIGHT HAND ROTATION. ... easy peasy.

wore out · Jun 10, 2021

(quoted from post at 15:03:12 06/10/21) Since the points have been changed means the plate that turns the rotor has been removed.

"Since the points have been changed means the plate that turns the rotor has been removed. Make sure the rotor isn't off 180 degrees. Its very easy to do."

In reality that's NOT easy to do at all as the rotor engages with a notch on the shaft it fits on and the drive to the rotor gears will only go together one way!

IMPOSSIBLE to muck that up without mangling/jamming parts together, IMHO, ANYTHING but "easy".

On the other hand, if the mag was removed from the engine or the little rotor gearbox cover was removed by mistake stuff can happen!

adambeck3475 · Jun 11, 2021

Update: I took the magneto apart, cleaned the points, fixed the connector that goes on the wire that connects the condenser to the points, and timed the gears. I have a little jolt at the plugs, Im guessing it not enough to cause a spark. What do I need to do to get a bigger spark? Im assuming this is still the reason it wont start.

Janicholson · Jun 11, 2021

Rotate the engine slowly (neutral with brakes locked, hand cranking is best.) each rotation of the crank should produce 2 very clear solid clicks. Like pinching a wooden spring cloths pin and letting it snap shut. if that is not happening, or if it is lacking crispness in the snap, the impulse coupling may be failing. An impulse coupling is located in the drive to the mag, just inside of the mounting flange and part of the mag, not the tractor. The device (bear with be on this) is like holding a kid back by the suspenders, then letting them go. In the mag, there is a certain rotation speed of a magnet that generates the power to make spark. when cranking (electric or hand) the magnet is not moving fast enough to make crisp spark. The coupling latches the rotation of the mag internal parts for several degrees of rotation, winding up a spring, then releases them so they travel fast enough to make spark big enough to jump the plug's gap. if you are uncomfortable with trepairing or analysis of the mag. put the engine on TDC compression #1 cylinder, and take the mag off. Several companies and even people on this site (Teddy52food is one) repair mags. Jim

adambeck3475 · Jun 11, 2021

I had the mag apart earlier today. I had the impulse out and the dogs were snapping back, not sticking at all. When I rotate the engine over I do hear a really crisp clicking noise. Could a new set of points and condenser give it more spark to jump the spark plug gap?

old · Jun 11, 2021

New points maybe if the old ones are getting bad. As for a new condenser you stand a 50/50 chance that it would be bad right out of the box. I haven't replaced a condenser in over a decade due to that problem

Janicholson · Jun 11, 2021

The magnet in a Mag can have the magnetism fade. They were not very high tech magnets. They can be recharged by a mag repair place. A distributor
can be used in place of the mag but requires some rewiring. Jim

tractorguy2 · Jun 11, 2021

Weak spark can be caused by,

A weak coil
Dirty, burnt, or improper gapped points. New points must be cleaned before installing.

Suppression plug wires. Mags need copper core wires.

Rotor gears not timed correctly.

I have bought and sold H4 mags for over 15 years. Haven't had one yet with a magnet so weak it wouldn't work. Fairbanks Morse mags a different story.

teddy52food · Jun 12, 2021

The magnet in the H4 mags don't lose their strength. I have more than one from the 40's that is still good & never been charged

Dave41A · Jun 12, 2021

(quoted from post at 14:35:23 06/12/21) The magnet in the H4 mags don't lose their strength. I have more than one from the 40's that is still good & never been charged

Yes. Teddy is correct. The magnet will not "wear out."

Some thoughts: check the plugs. You want NON-RESISTOR plugs. I suggest Autolite 3116's, but everyone has their favorite brand. You also want COPPER wires. I got surprised on this with my '41H when I converted from coil to mag--it is an easy mistake to make (easy for me, anyway). It is also helpful to have the rotor and cap be a "matched set." By this I mean the height of the rotor (as measured setting on a table) varies a little between different aftermarket brands. Some are "too low" for the cap, while I have had others where the rotor actually hit the cap on the inside while turning. Try putting the originals back on. I think you are very close. Dave

Farmall model A, no spark at plugs

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