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Magneto question

M

Machine249

Member
I'm trying to get a Farmall A running and I'm having trouble getting spark to the plugs. I have good spark coming off of the coil but nothing coming off the cap. I checked it by hooking a plug and wire straight to the coil, then holding the plug to the block, I spin the crank and I get a good spark. Then I hook everything up as it should with the #1 plug to the block and spin the crank, no spark at all. I have a new cap with the center brush, rotor and wires. Is it possible that I'm getting good enough spark to lite it up straight from the coil, but not enough after traveling through the cap and rotor? This is my first time even seeing a mag, so I certainly need some help. Thanks Justin
 
99% chance the rotor is not in time with the drive gears in Mag. There are marked teeth to align. An angled "nick" on one tooth and marks on others. If the spark is made when the roter is pointing between plug towers, it will not fire. Jim
 
Sounds like your rotor is not "pointing at" the distributor terminal when the spark is ocurring.
Your mag has gears which time the rotor to deliver the spark to each distributor terminal at the proper time--- when each cylinder is supposed to fire. Take the dist. cap off & bring the #1 cyl. up to TDC. The rotor should be very close to where the #1 plug wire attaches on the cap. Could be close to #4 terminal if timing is half off.
If rotor is off by much, take the 2 screws off the gear cover under the rotor & set the gears into time. One tooth will have a mark or flat & there will be 2 teeth on the other gear. These need to be in register.
 
Not being close dont get it it has to be lined up with the tower they just wont start hardly at all if the rotor isnt lined up with the tower/
 
Pull No1 sparkplug near the radiator and hold your thumb over the plug hole while a helper hand cranks the engine slowly until you feel pressure then drop a long plastic straw on top of the piston then you watch the straw rise while your helper slowly cranks the engine. When your straw quits rising your No1 piston should be at TDC on the compression stroke. Remove the cap from the mag and see where your rotor is pointing. If it is not pointing to your No1 plug tower you need to pull the mag and rotate the rotor so it is at the No1 plug tower and reinstall the mag. Your plug wires need to be in this firing order on the cap. Hal

2-1
4-3 As the rotor rotates CW.
 
It takes an extra 2000 volts to jump across the rotor to the pole inside the cap if it's new and clean. So if the spark is not as strong as it should be, you can get it out of the coil but not through the cap.

Easy test is to get a big spark plug and break the ground electrode off of it. This way the spark has to jump a 1/4" gap. If that works from coil with a 1/4" blue spark, you know you have enough to got through the cap.

A clean plug gapped at .025" takes around 8,000 volts to fire. If run through a distributor cap and rotor it takes 10,000-11,000 volts.

Gap a plug at 1/4" and it will take 20,000 volts which any good battery coil can make.

Also, points timing must be correct. If you points cause spark when the rotor is inbetween distributor poles, you can lose spark. If points are timed right, the rotor will point at a pole when spark occurs.
 
I just realized you've got a mag and not battery ignition. What I stated still applies but mags have less max voltage. Usually 16,000-20,000 max.

An .025" plug gap needs 8000 volts to fire when clean - directly from the coil. If through the cap and rotor it takes 10,000-11,000 volts.

An .035" plug gap needs 11,000 volts to fire directly from the coil. If through the cap it takes 13,000-14,000 volts.

Some mags at best can only make 17,000 volts and if worn (weak magnets, bad impulse, etc.) much less.
 
You dont want it ahead you want the rotor pointing to the cap where the wire goes to the plug. Before you take the cap off mark on the body of the mag where the towers are. Now take the cap off and turn the engine over and you can see where the rotor stops before it snaps and you want the rotor pointing at that mark not befopre or after rite on the mark and the engine will fire on the first turn. I have several and have sold many and thats the way to do it.
 
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