Testing a magneto?
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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=36494
Printed Date: 22 Jun 2025 at 6:26am
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Topic: Testing a magneto?
Posted By: SC Dan K.
Subject: Testing a magneto?
Date Posted: 31 Aug 2011 at 6:25pm
| My 38 B had been sitting outside in several hard blowing rain storms. I went out to start it. After a little attention it started and ran for 20 second s. It cut off like the fuel was shut off. The fuel cutoff is open and I am getting plenty of fuel to the carburetor. I think there is no spark. I can't see anything wrong. What do I look for on a magneto? Thank you for any help. |
Replies:
Posted By: Leon B MO
Date Posted: 31 Aug 2011 at 7:21pm
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To test for spark, I take out # 1 plug, put the plug wire back on the plug, ground the plug to the motor. I take the wires off the rest of the plugs to make sure it dont start, you can even take plugs out to crank it over easy. Hand crank until you feel compression on front plug hole, keep cranking slowly, the next click of the mag should give you fire at the grounded plug.If you have fire, put it back together. Before you install the plugs put a small squirt of fuel right in top of each piston. After putting the plugs back in, try starting it. If I let the U sit all winter that is the only way I can get it to start. Good luck Leon B MO------------- Uncle always said "Fill the back of the shovel and the front will take care of itself". |
Posted By: jccleav
Date Posted: 31 Aug 2011 at 9:44pm
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Did it die after you took the choke off? I guessing you do not have water in the gas since you said there is plenty of fuel to the carb. |
Posted By: Unit3
Date Posted: 31 Aug 2011 at 9:58pm
| Years ago dad told me to hold onto the end of the wire on a small B & S engine as he slowly pulled the rope. I was young and didn't know. Somethings you never forget. FYI Not a good way to check for spark, but it worked. He knew right away the little engine had plenty. |
Posted By: wkpoor
Date Posted: 31 Aug 2011 at 11:36pm
| Mags can get water in them and the only fix is to take off the covers and blow out or let dry. Couple of yrs ago I towed a farmall Cub to a show in pouring rain. After I got there it wouldn't start. Discovered wet cap. Dried it out and it started right up. Went to plow and it started acting up again. Pulled off cap and it was wet again. After 3rd time I went a little deeper and found the case and rotor full of water. It just kept evaporating up and condensing on the cap. So basically getter dried out and go from there. |
Posted By: Steve in NJ
Date Posted: 01 Sep 2011 at 7:40am
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Moisture plays havoc with any Mag. A little spritz of Brakekleen on the points and some compressed air will dry the inside of the Mag out, if its a "no spark" situation from moisture. Make sure the gasket(s) on the cover is in good shape. If its old and not sealing well, moisture will get in there and oxidize the points everytime. Sometimes the condensor fails which will cause' a no spark situation also... HTH
mailto:Steve@B&B - Steve@B&B
|
Posted By: SC Dan K.
Date Posted: 01 Sep 2011 at 8:00pm
| Thanks everybody. Definitely no spark. Good friend farm all enthusiast stopped by and took a look. We sprayed everything including inside magneto with WD-40. That's always displaced any water if I had a wet ignition system. Still no spark. I'm going to call a magneto expert after the weekend tractor show. Don't guess this one will be at the show. If anybody is at dacusville,SC this weekend I hope to find one tractor that will run |
Posted By: SC Dan K.
Date Posted: 01 Sep 2011 at 8:04pm
| No one will volunteer to hold a spark plug wire :) |
Posted By: beeman
Date Posted: 01 Sep 2011 at 8:05pm
|
Old Harley Davidson Sportsters [XLCH models ] had Fairbanks Morse magnetos up till 1969. They were prone to getting moisture in them especially down in the humid South where I live. An old mechanic showed me how to cut little pieces of a sanitary napkin and stuff them down into the case of the "mag' to absord the dampness. Change them out with fresh once in awhile... Worked great!! [tough part was explaining why you kept one of those things in your toolbox .......] ------------- 1949 B 3930 Ford- Have owned other Orange ,green,red,yellow,dark green tractors and equipment. |
Posted By: wkpoor
Date Posted: 02 Sep 2011 at 12:09am
I like the brake clean but WD40 no way. Any residuals left over could create a secondary path to ground and thus no current to plug. I would do as Steve said and brake clean everything you sprayed with oil especially the cap. And look closely for cracks. High voltage will travel in air much easier than rubber or Bakelite. If that don't do it Ohm out the coil or if its an old gooey black one change it anyhow. Unless you've got dead magnets they aren't hard to bring back. Better yet send to Steve and problem solved. |
SC Dan K. wrote:Posted By: Gerald J.
Date Posted: 02 Sep 2011 at 1:25am
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Actually Water Displacer number 40 was developed for this water removal task from aircraft magnetos. Its use as a pseudo penetration oil and temporary lubricant came later. Gerald J. |
Posted By: wkpoor
Date Posted: 02 Sep 2011 at 3:46pm
| Never seen WD40 in aviation. Reason I'm told is to be used in Avaition it must have a mil spec and to do that they must devulge the formula. WD40 has no mil spec. I've been a tech for 26yrs. |
Posted By: CTuckerNWIL
Date Posted: 02 Sep 2011 at 3:53pm
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Bil, go to this link and read the story. http://www.wd40.com/about-us/history/ - http://www.wd40.com/about-us/history/
------------- http://www.ae-ta.com" rel="nofollow - http://www.ae-ta.com Lena 1935 WC12xxx, Willie 1951 CA6xx Dad bought new, 1954WD45 PS, 1960 D17 NF |