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1948 cub ignition

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36farmallf20
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1948 cub ignition

Postby 36farmallf20 » Mon Dec 16, 2019 4:03 pm

Have recently acquired this cub for mowing duties around our new place, It fired up fine yesterday but today all it would do is once in a while hit cough a little and backfire like the timing was all of a sudden way out of wack. After per forming a little trouble shooting I discovered that the rotor had stripped and the drive gear on the mag was badly worn. When cranking it the rotor would mostly sit still but occasionally turn at random so I'm pretty sure I found my problem. That being said the mag parts are too high for what they are so I was thinking about putting a distributor on it instead for cheaper tuneup cost down the road. But all of the distributors that I have are from an M with what should be a 40 degree advance, also all of the new distributors I have found are also advertised for 40 degrees and claim to fit almost everything. According to my cub manual they ran 16 degrees advance. Have any of you guys ran a 40 degree distributor on these engines successfully or do I need to bite the bullet and just repair the mag?

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Eugene
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Re: 1948 cub ignition

Postby Eugene » Mon Dec 16, 2019 6:08 pm

Contact the advertisers on this site or place an ad in The Vine for a complete battery powered ignition system. Good used ones are available.

16 degree advance is what you need.

If you need any more Cub parts, a part Cub may be the route to go.
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36farmallf20
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Re: 1948 cub ignition

Postby 36farmallf20 » Mon Dec 16, 2019 8:22 pm

I understand that a 16 degree distributor would be ideal but I am curious what would happen if someone ran a 40 degree distributor since parts suppliers like stiener are selling these saying they are applicable to the cub. Surely someone has tried one of these units and knows how they run.

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Re: 1948 cub ignition

Postby MiCarl » Mon Dec 16, 2019 10:03 pm

You do know that at cranking speed the rotor doesn't move continuously but will jump 90 degrees every 1/2 revolution of the crank? That's by design so the magneto can fire at essentially 0 RPM.
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Re: 1948 cub ignition

Postby inairam » Mon Dec 16, 2019 10:24 pm

You have to be careful about tech support or tech details of parts from Steiner. I have got a lot of wrong info from them. Post on the vine here on this site or look at eBay for a distributor.

A rebuilt 100% great mag http://www.rebuiltmags.com/ is about $255 with your old mag in trade. I have gotten two from them worked great. They also sell mag parts. Hot of the tractor distributors on ebay are going about $100-120, (add $20-30 for cap and tune-up kit plus 30 for a coil and $10-12 for the ignition switch) aftermarket about the same (personally do not like ebay for new parts) and rebuilt about $225.00 plus coil and switch.

Personally I would get the rebuilt mag. I think they work great and my mag cubs are my most reliable starters. Plus all in I think it will not be that much different in the money.
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Re: 1948 cub ignition

Postby Glen » Mon Dec 16, 2019 11:24 pm

Hi,
Below are listings at TM Tractor for the rotor, and rotor drive gear, if you didn't see them.

http://www.tmtractor.com/new/el/294fp.htm

http://www.tmtractor.com/new/el/615fp.htm

I would replace the points and condenser too.
Below is a listing for those.

http://www.tmtractor.com/new/el/367fp.htm

The coil in a Cub magneto can quit, from being old, then that needs replacing, costing more money too.
You can fix the magneto, or put on a Battery Ignition unit, whichever you want.
J P Tractor Salvage, at the bottom of the page, has used Cub parts.

I have not used a Battery Ignition unit with 40 degrees advance on a Cub.

Below is a pic of the timing marks for the rotor and rotor drive gear, they should be aligned.
Clean the old grease out of the area under the cover, and grease the gears also, like the Cub owner's manual says to do. :)
Attachments
Cub magneto 4.jpg
Cub magneto 4.jpg (25.13 KiB) Viewed 890 times

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Re: 1948 cub ignition

Postby Gary Dotson » Tue Dec 17, 2019 8:40 am

Our dyno testing has shown that Cubs, in general, will start to lose power with any more than about 18-19 deg. advance. At 40 deg. it would likely be a real slug, I haven't tried it. Any supplier selling 40 deg. distributors, and saying they are proper for a Cub, is just plain wrong.

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Re: 1948 cub ignition

Postby Rick Spivey » Tue Dec 17, 2019 5:44 pm

You would never want to hand crank a cub that had 40 degrees of advance in it. I know of NO good reason to try a 40 degree distributor in a cub, just a recipe for something bad to happen.
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Re: 1948 cub ignition

Postby Barnyard » Tue Dec 17, 2019 11:23 pm

Gary Dotson wrote:Our dyno testing has shown that Cubs, in general, will start to lose power with any more than about 18-19 deg. advance. At 40 deg. it would likely be a real slug, I haven't tried it. Any supplier selling 40 deg. distributors, and saying they are proper for a Cub, is just plain wrong.

Rick Spivey wrote:You would never want to hand crank a cub that had 40 degrees of advance in it. I know of NO good reason to try a 40 degree distributor in a cub, just a recipe for something bad to happen.

I am not that knowledgeable about mags, but when these two guys talk, I listen.
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Re: 1948 cub ignition

Postby John *.?-!.* cub owner » Thu Dec 19, 2019 1:12 pm

I have never had personal experience trying, but 40 degree advance on a cub would likely shorten the time till a rebuild was needed.
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Re: 1948 cub ignition

Postby Bill Hudson » Thu Dec 19, 2019 1:51 pm

Barnyard wrote:
Gary Dotson wrote:Our dyno testing has shown that Cubs, in general, will start to lose power with any more than about 18-19 deg. advance. At 40 deg. it would likely be a real slug, I haven't tried it. Any supplier selling 40 deg. distributors, and saying they are proper for a Cub, is just plain wrong.

Rick Spivey wrote:You would never want to hand crank a cub that had 40 degrees of advance in it. I know of NO good reason to try a 40 degree distributor in a cub, just a recipe for something bad to happen.

I am not that knowledgeable about mags, but when these two guys talk, I listen.


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Jim Becker
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Re: 1948 cub ignition

Postby Jim Becker » Thu Dec 19, 2019 2:46 pm

The lost power that Gary mentions will mostly be used up by the engine fighting with itself and turning fuel into excess heat.

36farmallf20
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Re: 1948 cub ignition

Postby 36farmallf20 » Sat Dec 28, 2019 9:10 am

All of you guys have confirmed what I thought about the distributor timing so to make life easier on me, or so I thought I came up with a rotor and pinion from another mag. When timing the mag I proceeded as follows
1. engine at tdc number 1 compression
2. lined the timing marks on the rotor and pinion
3. I noticed the lugs on the engine were not at the 2-8 o-clock positions, closer to 3 and 9 from what I remember but installed the mag anyways with it tipped as close to the engine as possible.
4. rolled the engine over 1 turn and rotated the mag away from the engine to get an impulse click, never got one.
5. when the engine is rotated 1 turn the impulse trips once and the second trip is just after the timing mark on the pulley passes the pointer. from what I understand the second trip should happen as the pointer lines up?
6. The engine starts good but has a miss if accelerated quickly or loaded in any fashion.
I know I could have a fuel system problem cause this but I am concerned that the governor may be installed incorrectly do the symptoms of this mag install give indication of that or do you guys think I need to start over.
sorry for being long winded thanks for the help.

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Re: 1948 cub ignition

Postby Jim Becker » Sat Dec 28, 2019 10:54 am

It looks to me like you followed the mag installation and timing perfectly but the governor is installed one or two teeth out of time. The only thing NOT noted in your procedure is in step 3. You need to make sure the rotor is pointed towards the plug wire number 1 position before installing it, rotating the mag backwards as needed to position the rotor. I assume you did that.
36farmallf20 wrote:. . .
3. I noticed the lugs on the engine were not at the 2-8 o-clock positions, closer to 3 and 9 from what I remember but installed the mag anyways with it tipped as close to the engine as possible.
4. rolled the engine over 1 turn and rotated the mag away from the engine to get an impulse click, never got one.
. . .

The clues about governor timing are in your steps 3 and 4. The drive lugs should be at about 2-8 o'clock, more precisely 35 degrees away from horizontal. The governor gear has 18 teeth. Dividing that into 360 degrees says that each tooth off in the governor installation changes the lug position by 20 degrees. That is 35-20 = 15 degrees, which is half way between 2 and 3 o'clock. If the governor is 2 teeth off, 35-20-20 = -5 degrees, just beyond 3 o'clock. So depending on exactly where your drive lugs are at TDC, it looks like you are 1 or 2 teeth off with the governor.

I don't know just what the range of timing adjustment is for the magneto. It is limited in retard by either the top of the magneto contacting the block of by the left end of the slot at the lower mounting bolt. Advance is limited by the right end of the slot. It would have to allow for at least 20 degrees of advance to be able to compensate for the governor being one tooth off. I don't believe it has that range.

36farmallf20
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Re: 1948 cub ignition

Postby 36farmallf20 » Sat Dec 28, 2019 3:22 pm

I was afraid of that, well back to the shop. Thanks for the help


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