Really hard starting cub

Pete P.

Member
Have a '47 Cub that is stumping me...

She's always been a bit hard to start especially when hot but now it won't start at all. I can get a few puts but no hard fires and definitely not two in a row to get the engine to take off.

Carb. is a Zenith; Was running lean so I tore it down and found some gunk -- cleaned out and reassembled. I'm now getting good fuel (after cranking on choke I get the usual Cub dribble of gas out the intake.)

Spark was questionable so went through the mag -- new points, new condensor, cleaned rotor and cap contacts. burnished new points with emory cloth since they had been on the shelf a while and then cleaned them off with contact cleaner just to be sure of a good surface with no oil. Coil is only about two years old (since installation anyway.)

I'm still only getting a marginal spark at the plugs -- faint, blue but not very fat.

Time for another new coil? I've heard that J4 magnets can get weak -- is this true? Is there a fix? Am I missing something?

Thanks in advance,

Pete P.
Harborcreek, Penna.
 
I would be looking for another condenser. Last mag I worked on I had 2 new condensers
that didn't give a spark I was looking for. Put in an old used one that tested good &
got a big fat spark. Those magnets don't lose much over many years. If you need help,
I work on them as a hobby.
 
You are confusing. One place you say you went through the mag and then you talk about a coil. Which is it. If a mag you don't need the coil if not then you need the coil. But not both. Now if you don't have a good spark check your connections from the battery to the distributor if a mag then you need to look at connection or what ever in the mag. Since I've never had a mag apart. I will go with the distributor check voltage at switch then at coil then check if the points open and close correctly and gap is correct at lobe on shaft under point arm. If correct then check that you don't have a wire or end touching the side of the case going through the side of the distributor.
 
The Cub magneto does have a coil. It is located directly under the coil cover (of all places). I assume the OP is actually dealing with a magneto.

Forget all this stuff about connections from the battery to the ignition system. I agree that the condenser is suspect in this case. Although coils fail, they usually don't in a couple years. According to reports from a lot of people, recently available condensers are bad right out of the box.
 
Is the impulse coupling snapping when it is cranking? they can become fouled over time and also have a bad spring, or release trigger. This
device is located in the Mag drive and provides starting spark. If inoperative or fouled, the spark will be very weak. If you can pull start it
safely (know how and have experience, or a friend that does) the impulse coupling is not used if the engine speed is above cranking speed. If it
runs when pull started, the impulse is suspect. I think Teddy52food can mend them. Jim
 
I dont know about weak magneto magnets, Same process alnico magnet last a lifetime in h4 . Got to test it , small fish scale ,how many
pounds is release. .

I got 5 h4 no magnets issues .

Get the blue ribbon magneto repair guide . Need to start at the beginning and rebuild it per manual .
 
have you adjusted your valves in a while. Also, how is the compression. I know you are looking for a spark issue, but timing and compression are just as important. Turn crank by hand and see if the impulse trips on the timing mark. If you get to the mark and not trip. Leave on mark, loosen and rotate distributor top away from engine slowly until it trips, then lock it down and recheck.

Good luck,
Larry
 
I had trouble on my 'A' when the replacement cap did not match the rotor. If you put the original mag rotor and an aftermarket rotor on the table side by side, you may see one is maybe a millimeter or two higher than the other. Too high a rotor with too low a cap means the rotor will strike the cap. The other way around and the spark has too far to jump to make it to the plug wires.

I've also had problem with the new points. My new points were just a touch taller than the originals, so when I put the points cover back on it disrupted them and messed up the gap just enough. I discovered this by bench testing with the points cover off and then again with it on. This is on the H4 magneto, but the J4 is not too different.

By the way, I'm about 10 minutes from you in NY. Just up I-90. Small world.

Good luck, Dave
 
If you have blue spark you are pretty much there.
Thin spark suggests to me that you have resistor
plugs or wires that are not copper or steel core.
Take a piece of copper wire (any size) and run it
directly from the coil nipple to the nearest ground,
and see what is really coming out of the mag.
 
DOES the rotor line up with the tower on the cap when it impulses as the
rotor can be off a tooth becuase someone didnt get it correct.
 
Thanks all for the ideas. Let me try to unpack stuff one at a time:

Coil -- Was just pointing out that I replaced the coil in the mag about two years ago. I suppose it's possible but I agree that a decent coil should not go bad in 2 yrs.

Valves -- No, I have not gapped the valves in a while. Nor, come to think of it, have I measured compression in a long time. Was 90 PSI ish with only a couple pounds of variance across all four but
that's got to be 8 or 9 years ago now. Feels the same on the hand crank but definitely a good idea to recheck as my hand isn't a well calibrated instrument.

Condensor (and for that matter points) -- good point, no pun intended. Lots of bad out of the box stuff. I'm going to pull the mag and go back through it just to set my mind at ease that I didn't just
mess something up. I can swap the condensor with another just for assurance.

Resistor plugs, wires, etc. -- Using the Autolite non-resistor cross for a D-16 Used to use Champion exclusively but had a bunch of bad luck with them and switched to AL maybe 20 years back. Have
Champions come back to their former quality? I don't have the AL number handy but I believe it's a 3116. Wires are solid core (assembled myself from bulk
wire stock.) Bench testing the mag with a some AWG12 Cu wire directly into the coil discharge bent so the free end is near the mag chassis I can get it to jump only about 1/4. My other mags do a much
fatter spark at 3/8 or even a little more.

Well, off to go through the mag again and see what I can find out. Also time to measure compression and see how far the valves are off. Please let me know your thoughts on better plugs.

Thanks

Pete P.
Harborcreek, Penna.
 
So just to complete the story:

Swapped the J4 condensor into a battery system it happened to fit into and lo and behold crappy spark. Replaced & much better. Retimed mag again (don't think it was off but just to be doubly sure) and she runs just like she should again.

Hard to find good ignition parts and hard to test a condensor.

Thanks for all the help.

Pete P.
Harborcreek, Penna.
 

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