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Discussion Starter #1 (Edited)
About magneto timing.

Many posts on whose method is best or most correct. I do not wish to argue their merits, but I would like to review a few basics and let users decide which method to use.

The first settings in Gravely manuals stated that spark should occur at about 30 degrees BTDC, some say 27 degrees, and some manuals say 5/16 of an inch BTDC and others say 3/8 of an inch BTDC; but what always left out was, “of piston travel", it's not from the top of the cylinder, and there is a big difference. These settings were always 'run' timing, not start timing.

Next we need to understand what an impulse mechanism does in a magneto. At cranking speeds, hand cranking, and the first few spins of an electric starter, the impulse mechanism retards the spark so that it occurs with the piston at TDC of compression. This is accomplished by the impulse arm and a spring in the rear drive cup of the magneto. When the impulse arm engages a 'trip post' then the spring winds until it has enough force to propel the arm past the trip point.

When the spring mechanism trips, it spins the rotor very quickly to generate spark. Without that spring unwinding the rotor would not turn fast enough to generate spark at cranking speeds.

That mechanism must come back into play on every compression stroke until the engine is running. When the engine comes up to run speed, centrifugal force holds the impulse arm out so that it no longer engages the spring and retards spark.

The difference in timing between the impulse arm working and being held out, is called ‘lag angle’, or start timing and run timing. The rotor assembly’s lag angle is factory set by the manufacture with some very special equipment to a spec set by Gravely. The lag angle spec Gravely chose was for the best operating range of the T head engine, which in the old days was about 1600 to 2800 rpms where the engine developed max torque.

When Gravely mechanics start taking apart magnetos, I hope they mark the rotor settings; if not the lag angle setting is lost. Removing the rotor assembly also weakens the magnets which are used to generate spark. Another words, magnetos are sort of high tech to play with, there is no problem changing points and condensers; but removing rotors or altering lag angle settings, is not back-yard mechanic work.

Now we can cover some of the simple problems with Wico magnetos at least. With time and use, the impulse arm can become fouled with dirt and then it begins to ‘stick’ in the run position. You just cannot turn a Gravely engine over fast enough to start it without the help of the impulse arm mechanism. Removing the rear cover cleaning/blowing out debris and placing a ‘drop’ of 3-in-1 oil on the impulse arm pivot pin should help. Do not slop oil in that area.

With a great deal of use some impulse arms become magnetized and stick in the operating position, but it may not stick in the run position until the engine has been spun over once or twice. That is just one of the delights of owning a Gravely, you cuss them and praise them in the same breath.

If start timing occurs BTDC of piston travel, for whatever reason, then ignition wants to drive the piston backwards, breaking fingers, dislocating shoulders, making electric starts cantankerous. I guess that is why I believe in timing with "impulse snap at TDC while hand cranking". Run timing has a little more 'give' in it. I hope nobody tried to 'work' on your magneto before you.

This was the introductory course, or magnetos 101; there is even more for later, but only about Wicos, I never got to really work on the other magnetos.

Roger.
 

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I Love All Color Tractors
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Thank you Roger. No matter how much or how long we play with these machines, this writeup proves that there is always something more to learn about them.

This thread is now a sticky.
 

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The best way to sit up run timing is to remove the head and belt drive the engine off another power source. Touch the piston with the coil wire and watch the spark.
This make any engine easy to time. once you try this you wont never want to do it any other way. I make engines from scratch and I alway time my engines this way.
 

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Discussion Starter #4 (Edited)
And as Carl Sagan once said, "Imagine if you will"

That the coil was partially shorted out.
That the points were worn or out of spec, which changes timing.
That the plug wire was faulty.
That the spark plug was bad.
That the condenser was growing weaker.
That the magneto front cover had a short in it.
That somebody messed with the magneto before you.

And what if the drive cup of the magneto had a notch worn into the drive edge. I don't have a photo right now but I do have a drive cup with the drive edge worn back almost 1/8 of an inch. What would that do to magneto timing, both on start up and run?

We all want hard black and white answers, but life has a way of messing with us and so do old Gravelys.

We have plenty to learn, Roger
 

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It can be tough, its best to get a understanding of just what it is your working with.


as a youngster I'd had a hard time timing a replacement magneto to a Farmall F20

http://www.youtube.com/watch?v=uxKlaGkR_Vg

http://www.youtube.com/watch?v=eYZH9Jku7oM

Light aircraft engines (90%) use a pair of either Bendix and Slick brand magnetos wither its a 1946 Taylorcraft or a 1980's era Cessna
the magneto buzz box timers help setting your mag's 'E-gap' and proper time to engine.

http://www.sacskyranch.com/faqslickmagneto.htm
http://www.avweb.com/news/maint/184370-1.html
http://www.chiefaircraft.com/aircraft/tools/magneto-tools.html
http://www.aircraftspruce.com/catalog/topages/magneto_timlt.php



http://www.youtube.com/watch?v=XiCMAxPeChs

http://www.youtube.com/watch?v=8LgHZ04xKXA

http://www.youtube.com/watch?v=1V4vIkV4I0U
 

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Magneto rebuilder trades for Cores

My 2013 issue of The Best of Farm Show, on page 87
Magneto rebuilder trades for Cores.

He charges $45 plus parts for basic rebuild.
he also rebuilds carburators

Rudy Calin
8431 Scandia trail N
Forrest Lake, MN. 55025

ph #651 233-6655

email:[email protected]

website:wwwrebuiltmags.com
 

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The Gill story and plot is for timing advance, as I understand it. It is not total timing. Car engines, and other engines, have base timing set above tdc for the most part. The plot is advance in addition to base timing. A engine of any type will not produce decent power set to the numbers in the plot.
 

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Discussion Starter #11
wrecher, or is that wrencher,

If a Wico magneto has not had its factory lag angle altered, a lag angle of 10 degrees of magneto rotor rotation, which equates to twenty degrees of crankshaft or flywheel assembly rotation; then where would you place run timing for an early Gravely engine running leaded gasoline, in an engine which develops max torque and useful horsepower at about 2700 rpm? The first time a home mechanic alters the factory lag angle setting, all results, run and start timing change.

First let's all remember, there is no timing advance mechanism in a Wico magneto, it works at either start timing during cranking speeds, or run timing with the impulse arm out of the circuit.

Many Gravely fans believe that start timing should occur at TDC or slightly past; that makes a pull start tractor easier on the fingers and shoulders. Then, if we look at Gravely’s choice of lag angle, doubled for flywheel assembly rotation, it just sounds as if an old T head engine, running at about 2700 rpm should be set to twenty degrees BTDC run time, or about .100” of piston travel BTDC.

If you wish to believe the 1951 Gravely manual timing settings, be my guest, and do not hold onto that starter strap too tightly.

Roger,
 

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How does your L run set at 1/16" btdc? Thats 1/4" less than the manual. All the wico L's and C's that I have set at 5/16" btdc have had the impulse snap at tdc using the marks on the mag. If you set for .070 or .100 You probably will have to readjust the lag angle because impulse will occur way after tdc.
 

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Discussion Starter #13 (Edited)
wrecher,

Actually, all my tractors run better than ever before; easier first hit of the starter button or first pull of the starter strap; and no, I did not mess with the lag angle settings.

Spark at or slightly after TDC during starting helps drive the piston down on the power stroke, not drive it backwards on the compression stroke.

I have not run any of my tractors for years to see if there is any more carbon fouling with my timing adjustment. Maybe you are correct, and all the Gill studies are just so much hype, but I like my results so far. I still have to study the under sides of valves after running an engine like this for a while. I am not happy with what I see on engines that were "supposedly" timed to Gravelys 5/16" BTDC specs.

Using a degree wheel and a dial indicator, I found that .312" of piston travel BTDC showed 33° BTDC on a degree wheel zeroed at TDC. Now if we back out the 20° of lag angle, where would that leave start timing while start cranking?

Roger,
 

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I agree 100%, timing needs to be tdc or a hair after tdc for starting. If lag angle is set correctly this will happen when the counterclockwise mark on the mag lines up. If i read you correctly the next mark on the mag should correspond to about 20 crankshaft degrees, not 30 that was said to be. Because there is no spark advance on these mags timing is a compromise. At idle or slightly above idle the engine will not like much more timing, but above 1500rpm or so it might like alittle more. Good work with the degree wheel and dial indicator
 
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