Magneto fire problems
Printed From: Unofficial Allis
Category: Allis Chalmers
Forum Name: Farm Equipment
Forum Description: everything about Allis-Chalmers farm equipment
URL: https://www.allischalmers.com/forum/forum_posts.asp?TID=101570
Printed Date: 27 Jul 2025 at 11:39pm
Software Version: Web Wiz Forums 11.10 - http://www.webwizforums.com
Topic: Magneto fire problems
Posted By: chieff
Subject: Magneto fire problems
Date Posted: 06 Feb 2015 at 5:34pm
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Tying to give as much info without writing a novel. I purchased a 49' B with electric start from an older man that had let it sit for almost 7 years. He said it ran when he put it up but when he put a fresh battery in and attempted to start the engine it was stuck. I put marvel mystery oil in the cylinders and let it set for 2 weeks. With the spark plugs removed I put put the tractor in 3rd gear and rolled the rear tires by hand. The engine turned over and is now free. I have pulled the gas tank and cleaned it out, cleaned the fuel bowl, and rebuilt the carb. It has the Morse Fairbanks magneto. I pulled the magneto cap and installed new points and condenser, new button cap, new wires and plugs. The coil has been changed out previously and is the "red coil". When I re-installed the cap I made sure that the "C" groove was aligned with the red marked dog ear notch on the magneto. The condenser is on the right side so it is a clockwise rotation so the C mark should be correct. Here is my problem, when the engine is turned over I get good fire on the plug wires for cylinder 1 & 4 but little to no fire on plug wires for cylinders 2 & 3. I have pulled the cap off again and put a wire on the coil and took that to ground and I get good fire across every rotation of the magneto so I have to believe that I am losing fire in the cap. I thought the problem could be in the cap so I purchased a new cap and still have the same issues. The cap is getting fire on 1 and 4 but not 2 and 3. Any ideas?
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Replies:
Posted By: 1939Dodge
Date Posted: 06 Feb 2015 at 6:58pm
Posted By: Chalmersbob
Date Posted: 06 Feb 2015 at 9:24pm
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how about bad wires. or bad plugs You must have solid wire cables not carbon on a Mag. LOL Bob
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4 B's, 1 C's,3 CA's, 2 G's WD, D14, D15, B-1, B10, B12, 712S,
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Posted By: chieff
Date Posted: 06 Feb 2015 at 9:34pm
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I am taking the same wire with the same spark plug and when I put the wire in cylinder 1 or cylinder 4 port on the mag cap I get fire every time. If I take the same wire with the same spark plug and put it in cylinder 2 or cylinder 3 port I have no fire. It is about to drive me crazy.
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Posted By: Bill Long
Date Posted: 06 Feb 2015 at 9:56pm
Get hold of Steve NJ. He is our resident electrical expert and will be glad to be of assistance. If you would need a new mag or a rebuilt one he is your man too.
Thanks for bringing one of my favorites back.
Love to see pictures.
Good Luck!
Bill Long
ps: I would check the cap closely and take a good luck at the rotor. Never know.
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Posted By: Ted J
Date Posted: 07 Feb 2015 at 5:06am
Can the shaft be bent as sliding away from firing?
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"Allis-Express"
19?? WC / 1941 C / 1952 CA / 1956 WD45 / 1957 WD45 / 1958 D-17
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Posted By: DaveKamp
Date Posted: 08 Feb 2015 at 10:08am
I would suspect something in the disassembly/reassembly process makes it so that either the point cam is not properly operating in the 2/3 position, OR, when the rotor is in that position, something is shorting rotor or coil.
There's more to magnetos than timing... they depend on proper phasing in order to generate a hot spark. That means, the rotor angle is spinning rapidly across magnetic lines of force, and the points open at just-the-right time in the magnetic angle, to cause the field to collapse and cause the spark intensity. If they don't open at just the right angle, you'll get basically nothing, the points will just flash over. It's basically the same idea as dwell in a point-coil system, however, there's no primary current generating the field, it's permanent magnets moving.
I've worked on many magnetos, and to this day, I'll only work on one a little bit, usually just to see if it has viable potential for rebuild, or something simple. If I want one to work well and be reliable, I take it to a guy who has a magneto test stand, condenser tester, and experience. Not because I can't do it myself, but because he already has the tools and parts on-hand to set it up right.
IMO, the Magneto is a self-contained device, so it's best worked on, and tested as a stand-alone device.
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Ten Amendments, Ten Commandments, and one Golden Rule solve most every problem. Citrus hand-cleaner with Pumice does the rest.
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