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Discussion Forum

ACB rotor pinion / distributor gear + moisture

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Chris Cramer

02-21-2002 12:59:39

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I continue to have problems with moisture accumulation inside the distributor cap of my 1938 ACB. I can crank it until my arm falls off...but if there is just a little moisture in the distributor cap, it will not fire until I remove the cap and spray a little WD40 and allow it to dry for a couple hours. The distributor cap is only a few years old, and I seal it onto the magneto housing with silicone. Is there anything else I can do to prevent this moisture problem? It seems to be worse in hot, humid weather when I mow .

Also, to reassemble the distributor cap to the magneto housing, my owners manual says the “marked” tooth of the rotor pinion must mesh between the two marked teeth of the distributor gear at a point marked “C”. Is the “marked” tooth of the rotor pinion the dog-ear tooth?

My distributor gear actually has one tooth marked “A” and the adjacent tooth marked “C”, so I attempt to get the dog-ear tooth of the rotor pinion to mesh between tooth “A” and tooth “C” of the distributor gear. Is this the correct position? Problem is, as you place the distributor cap onto the magneto housing, you lose sight of the distributor gear and rotor pinion...so I am never quite sure if I have meshed them properly. Does anyone have any to tips to make this process easier for an amateur like me?

I really appreciate your help.

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Scott ks

02-25-2002 17:07:00

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 Re: ACB rotor pinion / distributor gear + moisture in reply to Chris Cramer, 02-21-2002 12:59:39  
In response to your moisture problem. On some distributors there is a vent to outside air so the distributor can "breath" so the temp can be about the same on inside and outside. Make sure you do not stop the hole up. Kinda like the balloon in the sunshine.


Scott ks

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Rod (NH)

02-21-2002 13:46:30

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 Re: ACB rotor pinion / distributor gear + moisture in reply to Chris Cramer, 02-21-2002 12:59:39  
Hi Chris,

You should find (look carefully) that the "A" and "C" marks on the distributor gear are actually BETWEEN two teeth and not ON a tooth. There should be a faint line extending from the letter to the valley between two adjacent teeth. The "A" and "C" locations are there to use depending on rotation of the magneto. For the AC-B you should use the "C" location as your manual says. The "marked" tooth of the pinion is the dog-ear one and should be located between the teeth at "C" and NOT between the "A" and "C" marks.

I haven’t had mine apart in years and have forgotten how I lined 'em up the last time...with care I guess :o).

No particular thoughts on your moisture problem except to make sure there are no cracks in either the distributor cap or the cover. I prefer to use new gaskets in lieu of silicone except where absolutely necessary...but admit that silicone should do the trick.

I have a Fairbanks-Morse manual for the FM-J magneto that has an illustrated parts breakdown and service instructions, including timing to the engine. The markings indicated above are also shown in pictorial in this manual. It is a pdf file of just under 1MB. I would be glad to email you a copy at no cost if you wish. Just let me know.


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Chris Cramer

02-21-2002 13:54:20

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 Re: Re: ACB rotor pinion / distributor gear + moisture in reply to Rod (NH), 02-21-2002 13:46:30  
Rod -

Thank you for the great advice, and yes, I would really appreciate that FM mageneto .pdf file.

Chris Cramer

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