Please explain to me.......

Just how a starter motor designed for 6 volt is going to spin an engine over so fast on 12 volt that a magnito will not function when the engine at full throttle will run higher RPM's than the starter will ever turn the engine as some claimed on another post???????

Inquiring minds wnat to know.

Rick

The problem with too fast cranking RPM (12 volts on 6 volt starter) is NOT running performance ITS THE STARTING THAT CAN BE PROBLEMATIC.

A Mag has ONLY TWO timings (Start and Run) unlike a distributor with variable mechanical RPM based or vacuum advance

1) The initial start timing is retarted by perhaps 15 to 25 degrees what is is once RPM reaches maybe 200 or so + (depends on Mag). The typical start timing is right at TDC BECAUSE IF IT FIRES TOO SOON BEFORE TDC WHEN SLOWLY CRANKING SHE KICKS BACK.....

2) The run time advance may be like 15 to 25 BTDC which in a mag ONLY kicks in when she reaches around 200 RPM + .....

THEREFORE if the starter spins it too fast shes might kick into the run time advance timing of 15 to 25 BTDC and thats baddddddddd when starting

Got it??????????

John T

(quoted from post at 08:46:38 05/03/12) Just how a starter motor designed for 6 volt is going to spin an engine over so fast on 12 volt that a magnito will not function when the engine at full throttle will run higher RPM's than the starter will ever turn the engine as some claimed on another post???????

Inquiring minds wnat to know.

Rick

Click to expand...

It has to do with the "impulse" on the magneto. The impulse is what kicks in to spin the magneto faster so it can generate some electricity and make a spark. Once the engine starts, the impulse kicks back out so the magneto will run at normal speed. 12 volts on a 6 volt system will spin the engine fast enough that the impulse will not kick in, or it may kick in, but then immediately kick back out, but still not fast enough that the impulse is not needed.

OK thanks. My only experance with a mag on a tractor was in the early 70's when my dad had an AC CA that he switched over to 12 volt. Never had a problem starting it.......unless we forgot to plug in the engine heater when it was -30F. It would start on it's own at -15F without plugging it in. It was used daily untill he retired from farming in 83.

Rick

It's a possible problem, not a definite problem.

On one of my Ms, the impulse doesnt work, and it fires off after 3 or 4 revs on 12 volt. I would fix it, but I hate mags!!!

gosh -- the things you learn. guess i had better quit using one of my h's. it has a mag with 12v alternator. better get it "fixed" back like it was. only been on 12v since 1990. used close to 75 hours a year. always starts- runs perfect. i am beginning to decide that some peole have waaaay too much time on their hand.

My opinion is that 12 volts to turn a mag will be too fast to start it or possibly damage it, is a rumor. Several years ago I bought a mag equipped H, and removed the inside gears without thinking what I was doing, these were not marked or else I didn't find the marks, so I guessed how it went back together. I cranked that tractor hundreds or maybe thousands of times in my spare time during the next few days. Finally got them back right and it started. It was still starting 14 years later when I sold it last Feb. Takes awhile to start when cold if it has set a long time, as most tractors do. When warm it starts fine. I wouldn't worry about it.

John am I missing something? I have never seen vacuum advance on any old Farmall I have ever worked on. Timing is hard set.

i believe someone's theories have gotten crossed up, if what is being explained is correct how did the old timers leave them (parked on starter as they called it)then let it roll down the hill pop the clutch to start it ,the principle behind the impulse coupling is to snap the magnets in the magneto past the windings fast enough to create at spark at low speed,once the speed range of the impulse coupling is exceeded the magnets are spinning fast enough to produce enough current to fire the coil,if you want to test this turn the flywheel of a small engine in a slow continuous motion passing the magnets under the coil no spark,then rock it past in a snapping motion it will fire.

not vacuum, its weights with springs in the dist that advances the timing by centrifugal force with engine speed.

The mag should fire great. When i put new points and condenser in with the mag fastened in the vice i run them with a battery variable speed drill to check the fire and they fire great at full speed with the drill. Have you cleaned the impulse spring on the driving end it mite be loaded with dirt,crud and so forth. Just put that end in some solvent or even diesel giving it a good cleaning that should make the impulse work fine turning the engine with 12v shouldnt make any difference

They are talking Magneto not a dist.

What type of mag are you talking about? The E4A and F-4 do NOT have any automatic timing adjustment. The impulse mechanism on these mags makes a hotter spark but does not adjust the timing!

(quoted from post at 11:46:38 05/03/12) Just how a starter motor designed for 6 volt is going to spin an engine over so fast on 12 volt that a magnito will not function when the engine at full throttle will run higher RPM's than the starter will ever turn the engine as some claimed on another post???????

Inquiring minds wnat to know.

Rick

Click to expand...

Easy answer is, it's not a problem at all.

The H4 mag impulse retards the timing by 35 degrees when starting, which if you have your mag static timed correctly, draws it back to TDC, or 0 advance. If the mag is in good shape, it'll fire no matter how fast the engine is spun by the starter. The impulse only activates below 150 RPM, above this, it is turning fast enough on its own to spark just fine, and the timing is advanced to 35 degrees BTDC. The retard function is only for hand crank purposes, to avoid a kick back and a possible broken arm or worse! Kick back has never been a problem when using a starter however. The engine will crank over and start using a 6 volt starter turned by a 12 volt battery even with the starting timing set to 45 degrees advance, because the starter is strong enough to turn the engine right on through any possible kick back it might try to do. The only symptom you "might" run into would be slow cranking due to too much initial static timing, the old familiar "wroo ... wroo .." situation, when the engine is already hot. The cure for this is to retard the static timing a little until it starts fine when hot too. It usually takes over 50 degrees initial advance to cause this issue when hot, but boy will she ever start easy like this when cold!

Yes it does. It's what is called the "lag angle" It's where the notch is set to trip the impulse. In most tractors it's 23 to 35 degrees retarded. Engines start better slightly retarded. (besides being safer for hand cranking)

Exactly John. And engines start better slightly retarded. So depending how you have your mag timed you may have trouble starting if the impulse is not functioning. (ie 12 V spinning it too fast)

(quoted from post at 15:02:47 05/03/12) gosh -- the things you learn. guess i had better quit using one of my h's. it has a mag with 12v alternator. better get it "fixed" back like it was. only been on 12v since 1990. used close to 75 hours a year. always starts- runs perfect. i am beginning to decide that some peole have waaaay too much time on their hand.

Click to expand...

Get a grip, man!

People are only saying it CAN happen. Not that it will!!

If you convert to 12V and your magneto tractor won't start anymore... THAT'S WHY!

You are RIGHT, Cornhusker Cowman!
This is what I like about this site -- the willingness to share and explain.
Thank you for correcting my error.