What is the impulse? I've got an old riding mower sitting out in the woods I gave up on because I couldn't get a spark out of the magneto at all.
 
Impulse on a tractor mag is not the same as working on a mag on a lawn mower two different types of mag and not even close to each other unless the engine on the lawn mower is a Wisconsin type. Shoot all B/S engines have mag but no impulse but then they are not even close to the type of mag used on a tractor engine like the old Fairbanks Morse mags
 
The impulse is the "clack-clack-clack" you hear when cranking. It's a spring wound device in the mag that winds up then releases to spin the mag fast enough to spark when cranking.

If you don't hear it, it's not working.
 
Here's the deal: First of all when you say " great spark at idle speed but very little week spark at cranking speed. "

Is this on a farm tractor or some sort of a small engine????????????????

If its on a typical old farm tractor its equipped with a spring wind up and trip "impulse mechanism" so it can fire at slow cranking over speed, otherwise there wouldn't be any spark at all when hand or starter motor cranking.

If its on many (not all) small engines it has no impulse as you can rope pull start it fast enough to still produce a spark

If its a farm tractor with an impulse as you crank the engine over slowly you will hear a loud sharp SNAP each time the spring winds up then trips. If not, the spring may be broken or the wind n trip centrifugal engage dogs are stuck/corroded so the spring never winds then trips. If it has a faint dull clunk NOT a loud sharp SNAP sounds like a working mechanism but a weak spring.

The most typical question is whats wrong if it don't hand crank and start (no fire) but starts instantly if pull started and that answer is a bad impulse mechanism (broke spring or dogs not working).

When you say "idle speed" note that even at a slow idle of around 250 RPM THE IMPULSE ISNT NEEDED OR WORKING THEM. Its more ONLY while cranking over its needed and working.

I don't view the condition is that an impulse "boosts the spark" while cranking, its more that at such slow RPM THERES NO SPARK AT ALL (and no spark that can be boosted). However the spring wind up and trip impulse mechanism makes the mags magnetic rotor spin fast enough by the coil so it can produce a spark. You cant boost what's not there in the first place and at cranking RPM without an impulse there's just no spark at all let alone a spark that can be "boosted"

Hope this helps, best wishes

John T
 
Stephen, good question

Most lawn mowers don't have a mag with an impulse mechanism. You can rope or motor start it fast enough to still create sufficient spark to get it started UNLIKE an old tractor which if hand cranking or even starter motor cranking the RPM is too slow to create a spark THEREFORE a spring wind up n trip impulse mechanism is required. See my post above for further info. Often on a lawn mower engine I have cleaned the rust off the flywheel and/or set the gap to the flywheel and fixed the spark.

John T
 
Self-contained magneto has a rotating magnet that is very small diameter. So when cranking over, it does not spin fast enough
to make spark. So to work - a spring winds up and then snaps loose to make the magnet spin faster then normal cranking. When
the impulse spring stops working - the tractor will still run fine but you might have to tow it to start it.

Lawnmower magnetos work on all the same prinipals. Just happens that the rotating magnet on a engine with a flywheel magneto
is much larger diameter so the magnet moves fast enough with normal cranking and no impulse spring needed.

I have seen many an old self-contained mag "fixed" when the impulse parts were not available - by wiring in a battery coil and
making a distributor out of it.
 
(quoted from post at 20:27:30 06/10/17) I have a magneto with great spark at idle speed but very little week spark at cranking speed. WHY?
Since all of you agreed my problem was the impulse I started working with great confidence . WRONG both impulse operated just fine.
One is a J.D. B with a winco magneto. The other M.M. R with a fairbanks moris magneto.
Both are on 12 volt. Do you suppose they are cranking to fast for the impulse to work but not fast enough to fire?
 
Hey . . Try to be accurate here. I certainly never "agreed" the impulse was your problem. I just explained it as a
"potential" problem based on your description of events. There is a difference. If you want accuracy from the "free help"
section, try being accurate yourself.

I doubt you have a "Winco' magneto. "Wico", yes. No "Fairbainks Moris" either."Fairbanks Morses, yes.

What do you mean . . "both are on 12 volt?" If they are magnetos and not distributors, hooking up 12 volts to them will burn
them up. Or do you have distributors and not mags? Note that Wico and Fairbanks Morse both made distributors that look like
magnetos to replace magnetos with battery ignition.
 
Here is a Wico model XB battery-distributor from a Deere two-cylinder tractor. You don't have this, do you?
a162896.jpg
 
Self-contained magneto works like any other magneto, other then the impulse. Note the impulse has two jobs that make it a
little different from a flywheel mag #1 is the spring wind-up and snap-loose action to make the magnet go faster. #2 is to
give timing retard at cranking.

Your mags have condensors, points, and coils - just as a lawnmower or outboard motor does. The magnet cannot go "too fast" to
lose spark. I'd check the basics. Points and condensor. Coils also go bad but when they do - often the engine starts and
runs fine, but starts to crap out after it gets good and hot.
 
If these were originally 6 volt cranking tractors and you are using 12 volts? Yes, they might crank fast enough so the
centrifugal flyweights collapse during cranking the impulse stops working. In theory though, once spinning that fast - they
are rotating quick enough to make spark without the impulse. Maybe the mags just need "tune ups." That assuming the magnets
aren't weak.
 
Now that you added more info, an update is in order.

1) Some (not all) John Deere B's used a Wico XB Battery Powered Distributor which is NOT a Magneto,,,,,Has no Impulse assembly,,,,,,,and requires battery power to operate.

2) Some (not all) John Deere B's used a Wico C or Wico XH (or other brands like FM and others) MAGNETO that has and requires an impulse but if you applied battery power to them you could burn up the coil.

You asked about cranking speed and impulse operation. On many Wico mags the impulse stopped functioning around 250 RPM but it would take a strong battery and starter motor to electric crank them so fast the impulse never operated, but at hand crank RPM that wasn't an issue IE impulse worked then. I have seen where a 6 volt tractor was converted to 12 volts and the 6 volt starter was retained and they can crank very fast where there could be an impulse and timing problem.

NOTE on some Wico Mags the start impulse timing was retarded maybe 25 or so degrees behind the run time advance timing. They could impulse fire at TDC but once up to RPM they could fire at 25 BTDC

If I knew what mag or distributor I might be able to help more, but just be aware of the difference in a Mag versus a battery powered coil distributor ignition and that only Mags (for many typical old farm tractors) have impulse assemblys and Mags (unlike distributors) dont require external battery power to operate.

Hope this helps

John T
 
Thanks for the info. Indeed if they are Mags (NOT battery powered ignitions like a Wico XB that looks like a Mag) on old tractors they use an impulse mechanism. What it does is wind a spring which then releases to spin the magnetic rotor faster past the coil to produce a spark. Hope this helps.

John T
 

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