I think it’s the mag AC-B
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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=151330
Printed Date: 04 Jun 2025 at 12:43pm
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Topic: I think it’s the mag AC-B
Posted By: HoughMade
Subject: I think it’s the mag AC-B
Date Posted: 16 Jun 2018 at 2:56pm
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Long story short, my B has been very hard to start lately (6 volt, FMJ magneto). There is no hand crank (someone replaced the front pulley- no provision for hand cranking), but the starter spins the engine over fine. It will eventually fire and sometimes start, but never easily. I’ll spare you everything that I’ve been through so far, but let’s work on the assumption that the carb is clean and flowing correctly and the timing is set correctly. Have tried 3 sets of plugs. So, I think I have narrowed it down to the mag. The impulse snaps over properly. I tested the mag off the tractor. With a wire connected to the coil output and held 1/4” form the mag body, no spark. However, at 1/8”, maybe close to 3/16”, I get a bright blue, but very thin spark. I read somewhere that it is preferred to have the spark jump 10x the plug gap, which in this case would be .3”. I don’t get spark anywhere close to that. The coil tests perfectly in the resistance range from coil output to the wire to the points and from the points wire to the laminations. There are new points (polished) and set to .20”. I got the same results on an old condenser and a new one. Here’s the thing, when I put a screwdriver on the magnets, I can just barely tell there is magnetism. It doesn’t pull on the screwdriver hardly at all when I pull it away from the magnets on the rotor. What say you? Do the magnets need to be remagnetized? Something else? Anything I should try? ------------- 1951 B |
Replies:
Posted By: PaulB
Date Posted: 16 Jun 2018 at 3:24pm
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I have a good used one on a tractor that is currenty running that I will sell for $125 plus the shipping. ------------- If it was fun to pull in LOW gear, I could have a John Deere. Real pullers don't have speed limits. If you can't make it GO... make it SHINY |
Posted By: DougS
Date Posted: 16 Jun 2018 at 4:33pm
| It seems there was a discussion here a couple of years ago about how to remagnetize the magnet. Maybe SteveNJ can help. |
Posted By: HoughMade
Date Posted: 16 Jun 2018 at 5:51pm
Thanks for the offer. Right now, I want to see if I can get my present mag up to snuff. ------------- 1951 B |
PaulB wrote:Posted By: Gerald J.
Date Posted: 16 Jun 2018 at 6:57pm
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I have a FMJ magneto shop manual at: http://geraldj.networkiowa.com/Trees/FM-Xmagneto.pdf" rel="nofollow - http://geraldj.networkiowa.com/Trees/FM-Xmagneto.pdf An ohmmeter test of the coil won't show shorted turns and shorted turns really kill the magneto voltage. Other things that are critical are that the distributor cover hasn't converted to carbon and that the rotor points actually point at a wire post at the time the points open. That's set by the two gears in the magneto. Gerald J. |
Posted By: Dick L
Date Posted: 16 Jun 2018 at 8:47pm
| You found the problem. I had one a couple years back that had the problem. I swapped out parts from other magnetos that I had bought for parts. |
Posted By: chaskaduo
Date Posted: 17 Jun 2018 at 8:34am
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Found a couple forum discussions on Yesterday's Tractor about this. http://www.yesterdaystractors.com/cgi-bin/viewit.cgi?bd=jd&th=168611" rel="nofollow - http://www.yesterdaystractors.com/cgi-bin/viewit.cgi?bd=jd&th=168611 Sounds Expensive. Good Luck ------------- 1938 B, 79 Dynamark 11/36 6spd, 95 Weed-Eater 16hp, 2010 Bolens 14hp |
Posted By: HoughMade
Date Posted: 17 Jun 2018 at 9:05am
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Anyone ever used “Kevin’s Magneto Service” in Manito, IL? In doing some internet searching, it’s about as close to me in Northwest Indiana as I can find...and if you know somewhere closer to Northwest Indiana, feel free to share. Chaskaduo- Yes, I looked into doing it myself, but to do it right, it seems like more than I would like to do. I have seem some references to a more, shall we say “simple” way to do it, but it seems a little sketchy to me and I would hate to put the time and money into something I would use once only to have it not work. At this point, as you can see above, I am looking into having someone recharge the magnets who knows what they are doing. I’ve heard that’s not too expensive. ------------- 1951 B |
Posted By: Dick L
Date Posted: 17 Jun 2018 at 9:35am
| Good way to go! |
Posted By: HoughMade
Date Posted: 12 Jul 2018 at 7:21am
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Now for the “rest of the story”. I will also reference a thread I started a while ago that dealt with spark plugs: http://www.allischalmers.com/forum/b-spark-plugs-reality-voodoo-or-coincidence_topic144231.html?KW=Coincidence" rel="nofollow - http://www.allischalmers.com/forum/b-spark-plugs-reality-voodoo-or-coincidence_topic144231.html?KW=Coincidence I talked to Kevin McWhorter at Kevin’s Magneto Service. He obviously is very knowledgeable. We talked about what may have been going on with my mag. I didn’t just want to randomly change parts and hope for the best, especially when I suspected weak magnets in addition to the coil. He could take my mag and diagnose and only fix what needed to be fixed, but he has a long backlog of that work. When you are good, that’s what happens. I opted to send him my mag and have exchange it for a rebuilt one. He shipped out the rebuilt mag the day he got mine and called me to let me know it was on the way. The mag comes set, with tape across the splines, to be installed on TDC #1. He sent a flange to governor gasket with it. It came with a nice spring steel grounding lever on the body, but I will be adding a kill switch wire. I bolted it down in less than 5 minutes. It looks great. I pulled the starter rod and it fired up with just 1 or 2 seconds of cranking. It runs smoother and sounds better than it has since I got the tractor. Obviously the mag was the problem and now its solved. Thanks to Kevin: http://www.magneto-repair.com/" rel="nofollow - http://www.magneto-repair.com/ So this brings me back to the spark plug voodoo issue. I think that the mag, being weak, would work better with some spark plugs and not a good with others, but never very good. I don’t know why. Maybe the Champions had slightly different resistance. Don’t know. I know what I observed and I couldn’t get it started with Autolites before. Well, with a strong, rebuilt mag, it’s running on Autolites now. I endorse Kevin’s magnetos completely. ------------- 1951 B |
Posted By: HoughMade
Date Posted: 12 Jul 2018 at 8:59pm
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Just finished mowing the front yard (about an acre) and then ran it around and up hills to try it under load. It runs great! It’s amazing what a difference a good magneto makes. ------------- 1951 B |
Posted By: chaskaduo
Date Posted: 12 Jul 2018 at 11:27pm
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Good to hear. ------------- 1938 B, 79 Dynamark 11/36 6spd, 95 Weed-Eater 16hp, 2010 Bolens 14hp |
Posted By: B26240
Date Posted: 13 Jul 2018 at 6:18am
| Glad to hear your problem is solved. I had a B that would start ok when cold but was hard starting when warmed up. Changed mags and it was like I had overhauled the motor. A good mag was the difference. |
Posted By: HoughMade
Date Posted: 13 Jul 2018 at 8:55am
That's exactly what mine was doing. I hauled it home from my Dad's place last October and after freeing up a couple of stuck pistons, I wasn't able to get to starting it until November. It started OK, but not great (as compared to now). When the weather got warm this year, it got progressively worse to start and with temps in the 90s, when I could eventually get it started, it would run OK for a while, but when the engine got warm, it would start to miss. The rebuilt mag solved all of that. ------------- 1951 B |