Posted by ScottyHOMEy on February 20, 2009 at 04:32:31 from (64.222.247.5):
In Reply to: B no spark posted by farmall guy on February 19, 2009 at 13:46:43:
There are arguments in favor and against both mags and battery ignitions.
As far as mags, they have the advantage of running in the absence of a battery, so you can have all manner of problems with your battery or charging system, and the tractor will run just fine. The power curve is a little different on a mag-fired ignition, in that a mag does not have a variable spark advance -- from a stop, it fires at TDC and, once running, has a constant advance (on the H4 mag on the letter series, that's at 35 BTDC). That said, tens of thousands of these tractor have put in millions of hours of service with mags, doing everything that was expected of them. The biggest thing that might be considered a drawback is that most folks find them to be quite mysterious, though mechanically they're quite simple.
One perceived advantage to the distributors on battery ignitions is the variable advance mechanism. In that case the spark advances at a gradual rate that varies with engine speed, so the power curve is a little different. Many thousands of those tractors worked hard and well, too, so better might be a subjective thing. Another advantage is that they are easier to maintain -- points are just as easy to change on either but to change the coil or even the condenser on an H4 mag requires removing it from the tractor. One drawback, especially for us mag-heads, is that the battery system does require a good battery and charging system. If you fry your battery or generator or, more common, that pesky voltage regulator acts up, you're either improvising, limping or just plain dead in the field until you track down the problem and get it fixed.
As far as changing over, it's not difficult. One critical element is to find a distribuor with the Code "A" stamped on it, which will give you the right advance curve for your B. For other parts, you'll need an external coil and the bracket to hold it, and a different ignition switch. (The difference is easy to spot. The switch for the mag will have one pole on the bottom of it -- it grounds out the magneto when pushed down to stop the motor. The switch for the battery ignition has two poles -- when it's up it completes the circuit to send current to the coil, pushing it down breaks that circuit.
The re-wiring is pretty simple. Your operator's manual may show just the mag configuration, but the parts catalog has all the different variations.
For all that, you still have a problem. With the mag off the tractor, test it like the others have suggested, listening especially for that click of the impulse coupler while watching the spark. If that seems okay, then the mag is probably fine. If not, I'd think a call to your rebuilder would be in order.
If it does seem to be working okay on the bench, put it back on and time it up, then turn the motor with the crank instead of the starter. It's a lot easier to hear the click of the impulse coupler that way and as you pull the crank up, it should click with the hand crank at about 11 o'clock. (With the plugs in, you always want to reset the hand crank so that you're pulling up -- DON'T push it down or try to spin it, it can kick back and break your arm! For this test it's a lot easier to turn off the fuel and remove the plugs -- you won't be fighting the engine compression and eliminate the possibility of kickback-- and you can safely turn the crank all the way around and listen for the impulse couple to click with the crank at 5 o'clock as well.) If it doesn't click at those two points consistently, you may have a different problem, like a bad gear somewhere in the timing train.
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