Magneto coil

Mon Apr 28, 2008 7:47 pm

Hello fellow cubbers! A couple of weeks I posted a topic about my Cub not wanting to start. Heres the deal. I kept fiddling with it and got it to run and I have mowed with it. The tractor has new plugs, points, condensor, wires, cap and rotor. Last time I mowed with it, shut it off, went to start it and it didn't want to start. I ended up jumping it off my 65 Chevy pick up which is 12 volt. About 3 cranks, and she fired up.
The battery that was in the tractor was getting a bit weak, so I replaced the battery. It seemed to start great then. But now, she wont start. Tonight I jumped it again with my pick up, and after about 3-4 turns, she fired up and ran great. I would like to know if anyone thinks maybe the magneto coil has gotten weak, not supplying enough fire for it to start? This is the only thing I haven't replaced.
Thanks in advance for your help!
KT

Re: Magneto coil

Mon Apr 28, 2008 7:56 pm

Because the tractor spins over nicely with the new battery but after several cranks it starts slow, are you sure that the Generator is charging? The next suggestion I would have would be the impulse on mag may be getting weak. If you can start it and it runs all day on the mag, I would think that it is NOT the coil that is going bad... The mag and coil doesn't run on the battery. Does the cub quit when it's warm (after working) and not want to restart?

Mike in La Crosse, WI

Re: Magneto coil

Mon Apr 28, 2008 8:25 pm

Hi Mike! Thank you for your reply. Forgive me, I am learning about the magneto system!! Sometimes I wish my dad was still alive. He had plenty of exp. with these old Cubs, and firing problems! Now that I have put in the new battery, it don't seem to slow down trying to start. I think the other battery was getting old. With the new battery, and all new ignition parts this lead me to believe it wasn't getting enough fire to start. So tonight, (new battery in), I jumped it with the pick up, and after a few turns, she fired up. Once it starts, it runs as long as I need to mow with it. And, if it would fire, she would start right back up. I have owned the tractor for 2 years, and she has always ran great. No problems like this. Another thing I did tonight was to put a spark tester into the magneto cap to see if I could tell what kind of fire was coming out. One of my employees loaned this tool to me today. It looks like a screw driver with a plug wire on it. He said it should glow red it there is plent of fire. It only barely lit up.
KT

Re: Magneto coil

Mon Apr 28, 2008 9:05 pm

Well, if you aren't getting a good spark on starting, but you are running well, then I would say that your impulse coupling is weak. When you start the tractor is there a nice "snap" in the mag that goes away when the tractor is running? That snap is the impulse coupling kicking the mag just a little faster at the low RPM of starting...this gets it a good fire. Without that impulse, the spark will be weak.

Mike in La Crosse, WI

Re: Magneto coil

Mon Apr 28, 2008 9:07 pm

Could be a coil.Putting out just enough.Coil getting $$$$ for the mags.Check with the vendors listed.Kevin if you ground the coil wire out of the mag,should be a blue spark ouch don't touch it it will make you jump out of your shoes

Re: Magneto coil

Mon Apr 28, 2008 9:18 pm

Hi Guys, tonight I wanted to make sure I had the points setting on .13, so when I cranked the engine was on TDC, then I pulled off the cap to line the rotor up on #1, and each time I turned it over by hand, I could here the impluse snap.
KT

Re: Magneto coil

Mon Apr 28, 2008 9:25 pm

1) Clean up all connections between the battery and the ground and the battery and the starter.
2) Check the generator output - see if it's charging.
3) Check the starter condition - probably needs to be rebuilt.
4) Take the plug wire from the coil to the distributor (on the mag) and install a spark plug with a nice wide gap. Crank the engine with ignition on. You may have to make up a plug wire to fit the coil - piece of copper electrical wire will do.

The impulse coupling does provide a snap to the magneto during start up. It's primary purpose/function is to retard the spark so that the magneto fires at #1 TDC during engine start up rather than at full advance.

Re: Magneto coil

Mon Apr 28, 2008 9:50 pm

Thank you all. I will try what you have suggested.
KT

Re: Magneto coil

Mon Apr 28, 2008 10:01 pm

KT...

Did you confirm that the tractor is charging? Does the ammeter move to the positive side when you are running? I was leaning toward a charging problem because the tractor spins fast enough to spin it over, but as the battery doesn't get charge, the starter spins slower and doesn't kick the tractor over. When you have a fresh (albeit 12) batter, all of a sudden the tractor kicks over again...

Mike in La Crosse, WI

Re: Magneto coil

Mon Apr 28, 2008 10:44 pm

A magneto is just like in a gas lawn mower or chain saw. A way to generate electricity and jack up the voltage enough to make a spark and have it happen at the right time.
The battery runs the starter on your magneto tractor. A batery is like a bucket of water, pour some out a few times and the bucket is empty = discharged battery = no turn over or slow turn over.
Add a properly wired good regulator and generator and you now can charge the battery to keep starting. Using a battery charger until you fix the gen or reg will do it too but is awfully hard on the battery.

Re: Magneto coil

Tue Apr 29, 2008 4:09 am

Wow! Thank you all for the great advice!! I do have one more question that I didn't ask. If the impulse coupling is weak, what is the fix for that?
Thank you!
KT

Re: Magneto coil

Tue Apr 29, 2008 5:55 am

The impulse coupling causes the magnet to spin faster in reguard to the coil at that instant in time will generate a higher voltage when starting. It also retards the spark at low speed (starting). Retarded spark occurs at top dead center (TDC). As speed is increased it is helpful to have the spark before TDC so that the ignited fuel has time to expand. Don't forget this is happening at 1600 or 1800 times a minute.
By weak impulse I think you mean the spring is weak. All it does is give a slower movement resulting in a lower voltage spark at start up. This of course is just the opposite of what you want. I have no experience with Cub mags but feel the first thing would be to clean and lubricate. If that didn't do it I would send it to Mag Man one of our members and ask him to fix it for I understand that replacement parts are difficult to come by.

Re: Magneto coil

Tue Apr 29, 2008 8:43 am

From experience... a bad condenser can also cause the same problem. It's much cheaper to replace. Enough so that it may be a good idea to try it first. It sure solved the embarassing problem my tricycle was having at a show. :oops: