Won't start with 6v battery, but will with 12v

[capecodcub]

OK, here is the deal. I have 49 Cub that has a rebuilt engine, rebuilt mag(timed perfectly), good starter, new battery cables, with good connections, good compression, original IH carb, good starter, fresh 6volt battery, etc etc. So here is the situation. When I try to start it with the 6v setup, it turns once or twice, engine pops and starter disengages. And it happens over and over. I hook up a 12v jump battery, it spins quickly and starts. I have looked at everything, checked everything twice and I still can't get it to go with the 6volt. Hand crank is not any better.FYI, once I started it this weekend, it ran fine, I plowed/disced my garden, and it would start fine with the 6v once warm. I am interested in everyones thoughts. Thanks
David

[Dell]

First thing I would do is have the 6v battery load tested.

Are you hooking the 12v into the system of just jumping it to starter.

If all the connections are bright and tight then I would lean towards something in the mag.

There are as many answers as there are components.

Dell

[capecodcub]

Thank you Dell. I had doubts about the Mag, hence having it rebuilt. It has great spark and all new components. And, when I substituted the Mag for a distributor, it started fine. So, obviously I have tried to make the mag work, since it should.

[Rob in NH]

you said it fires, but could the impulse not be working?

[Barnyard]

has a rebuilt engine, rebuilt mag(timed perfectly), good starter, new battery cables, with good connections,

What size battery cables do you have? For a 6v they need to be at least a 2 awg, but a 1 awg is better. If you have off the shelf cables that are any lighter, you may have found your problem. A 6V battery needs heavier cables then a 12v.

[Eugene]

1) Magneto impulse coupling not working. Do you hear the magneto click when cranking over the engine?
2) Magneto not correctly timed to engine. Close, just not close enough.

[capecodcub]

Barnyard, Eugene, Thanks for the replies. Battery cables are from T/M and the Negative is at least a #2 if not a #1. I do hear the impulse clicking. Especially as I was timing it, again, last weekend. Wish I had another mag to test with. I am sure it is something simple, and I am bound and determined to find out what it is!

[Eugene]

Static timed the magneto to engine?

[epinswla]

You say it started and ran fine and would restart ok after tractor was warm? Could possibly be to much for starter and #2 wire.May consider rechecking all connections and/or going to 1/0 or better yet 2/0 wire.Just my 2 cents.(Usually worth half that)

[28plymouthman]

I had the same problem on my 28 plymouth 4 cyl till this old guy told me to go to the farm store and buy 0 or 1 ott diameter battery cables for the 6 volt battery. It has cranked great ever since. Hope this helps.

[Jason (IL)]

Sounds to me like it might be trying to fire to early. When you use your hand crank does it kick back on you a bit? I have run into a similar problem and narrowed it down to a timing issue.

[capecodcub]

Follow up. I ended up finding another old mag I had laying around. For grins I swapped coils, points, timed it and the Cub is working as it should! Someone mentioned the impulse springs being weak, and I suspect that was the problem. I have been chasing this problem for years and now finally it is up and running as it always should have! Just wanted to thank everyone for their input and help.

[Rob in NH]

did you find out what was causing your problem or did you just change everything?

[capecodcub]

did you find out what was causing your problem or did you just change everything?

Rob, I can only assume it is the impulse springs since there really isn't anything else in there that I wasn't able to replace. Hope this helps

[Rob in NH]

it does help to know the cause as i have 2 with mags, thanks