1938 ac b

Can you hear the impulse coupler snapping over when you hand crank it?

i'd check the points

ya i've seen that be the problem...of course i too the mag off my c and swapped to a distributor form a ca

This will start a fire storm of criticism but I test a mag by pulling all the wires out of the cap, place your palm over where the plug wires go in and slowly crank it over. If you have enough spark to jump the gap and bite you, you'll have enough spark to start and run the engine. Quick and easy.

what shoud i expect or should i say get for compression readings on a good engine?

Probly a weak spring or rounded pawls in the magneto impulse. When hand cranking the spring has to spin the armature fast. I have had rounded pawls that would click but still didn't load the spring enough when they slid by the pin to soon.  

Phasing of the points and point gap is something that is best described when you've got a  magneto apart, on the bench, with the mag service document in front of you.  Sometimes it's covered in service books, but most times it's something explained by a magneto guru who's pointing it out... but I'll give you the best explanation I can, without having said items before me.

A simple type of magneto, is the kind that shows up on... say... an outboard motor... a mid '20's Evinrude.

There's a magnet in the flywheel... and the flywheel is spinning.
There's a pickup coil on a plate benenath the flywheel, and the magnet zips by it.  The whole plate rotates in order to allow the spark timing to be advanced or retarded... because on a 2-stroke outboard motor, timing pretty much determines how fast that motor will spin.

Inside the coil is two windings... a primary (higher current) and secondary (lower current) coil.  They're grounded at one end... the secondary goes to the plug... the primary goes to contact points.

When you place a coil IN a magnetic field quickly, the coil and polepiece have to 'accept' that magnetic field, and for a very short time, there's an imbalance between the coil, and the magnetic field, and as a result, you'd get some voltage appearing across the coil.  If you had some sort of a load between the ends of the coil (or coil and ground, since most coils are grounded at one end), then you'd have current flow.

If you yank that coil OUT of the magnetic field, the exact same phenomena occurs... the field which was 'accepted' into the coil, has been yanked out, so the field around the coil collapses, which induces action in the coil.

Back to the outboard motor... If you had just the secondary, as that flywheel spun by, you'd have a spark... and the spark would be visible at speed, and you'd feel it, but it'd be pretty lame.  Reason is simple-  the magnetic field builds as the flywheel comes closer, and falls off at the same rate at which it arrived... slowly.  Of course, the faster the flywheel cruises by, the faster it rises, and falls, so you get a more aggressive spark.

Now... let's modify the design, and add a second layer of wire to the coil, and call it the PRIMARY.  Let's short both ends of the primary, or leave them open, and spin the flywheel... we get exactly the same action as we saw before... but since there's more wire in that coil, the magnetic field occurs in a greater intensity.  Now, let's put a SWITCH in that secondary (call them 'points')... and just as that flywheel comes through, but just before it starts falling off... let's open the points!

What this does, is build that magnetic field as it normally would, but at a much higher intensity... and then by interrupting the primary, the entire magnetic field (built in both the primary and secondary) must discharge through the secondary AT THE SAME TIME!

This causes the magneto to create a much more intense spark, strong enough to burn oil and fuel residue off the plugs, and ignite not only a wet, but also a very lean mixture.

Now... let's say there's something wrong with alignment in the points... the cam lobe is worn off, or something like that...

The flywheel comes by... the points open about 5 degrees AFTER the magnetic field has passed...

Yeah, there's spark... we can feel it, we can see it... but it's sissy.  She'll run when you pull her, but when you hand-crank it, it won't start.

Also... to keep those points from burning, there's a condenser in there... it's a 'buffer' or a 'cushion' of sorts... it provides protection from the points because that collapsing field will WANT to jump the gap of the points at the moment they open.  Of course, this burns the contact points, but worse yet, it means the points really didn't open... hence, a lousy spark.  As the engine gets going faster, the magnet comes by faster, so she still runs fine... but with that arcing going on, you get an unstable spark, so she may run fine sometimes, and other times, she's lousy.

Now, let's say the point cam is worn... it ALWAYS wears on the 'climbing' side, more than the falling side... particularly if it's full of abrasive dust, wasn't properly setup or lubricated... or just sixty-eight years' run without a rehab.  That cam lobe's face has worn enough so that the actual point OPENING time is not lined up with the magnetic pass's departure... it's LATE in opening.  Lousy spark.

A magneto with an impulser not only gets a really quick spin of it's armature, and after-TDC spark for starting assistance and kickback, it also is less susceptible to hard-starting amidst degradation of the ignition points, point cam, and condenser... since it's whipping it over fast, you could have a well-worked setup that still kicks out a good enough spark to run... and never notice much change.

Hence, I suggest you grab a mag off of another machine that DOES hand-start well, stick it on, and see if your B will hand-start.  If it does, give your magneto a healthy going-over... or if you've got a local guru who's made an investment in the special tools for rebuilding magnetos, throw him a bone, bring a couple of beverages, and observe the maestro at work.

let me ask you this,i watch a video on rebuilding mags.they remagnitized it,when they were done they were able to pick up a ballpeen hammer with it.while i dont have mine completely disassembled,if i lay a screwdriver on it it barely sticks,sound weak?i have nothing to compare this with as its my first tractor.

much thanks dave kamp for all of the great info.loved the detail & the simplicity of your explanations.now i feel i know a little about these old tractors,hope to llearn much more.