Messing Around With Magnetos

THE SPORTPLANE BUILDER By Antoni (Tony) Bingelis

EAA Designee Program Advisor 8509 Greenflint Lane Austin, Texas 78759

-L HE MAGNETO I G N I T I O N system used with small aircraft engines iVW's too) is a Spartan installation which ordinarily performs as expected. However, as we homebuilders ruefully discover during the early

stages of construction, the term 'ordinarily' ordinarily describes a condition or state applicable to someone else's project, not our own. When one's own project is involved, it is just as well to assume that "Murphy's Law" prevails . . . anything that can go wrong, will. In deference to said law, the following remarks will apply to practical applications and not to theoretical enlightenment. When operating properly, the magneto generates a high voltage which forces a spark to jump across the spark plug gap at the precise moment the piston is on the proper stroke (compression), and at a specified number of c r a n k s h a f t degrees before top dead center (BTDC piston position). Well now, if that is all there is to this shocking device, why are so many of us perplexed by the little black boxes? That may be, indubitably, attributed to our lack of familiarity with magnetos. After all, not many of us work on engines and it is only when our project approaches the engine installation stage that we realize how unsure we are about timing procedures. Mechanically simple, magnetos are reliable, efficient and when installed on an engine . . . potentially deadly! The installed magneto might be likened to a rattlesnake ready to strike. It won't hurt you if you are alert and retain control of the situation but it is, without doubt as deadly as the rattler. If there are latent fuel vapors in the engine cylinders together with a faulty ground or switch connection present . . . look out! Don't move that prop even the slightest amount! Why! Blame it on the impulse coupling of that innocent looking magneto bolted to the engine. In view of the foregoing, I'm surprised the government has not required us to post a public notice on the engine cowlings in 2" letters: "WARNING": Touching or moving this propeller may be injurious to your health". Although no such warning is evident, the need for

caution around propellers merits frequent repetition . . . to you and to me, and particularly to all the experienced mechanics, pilots and lookers. You know the old saw, "Familiarity breeds contempt" . . . or, at least casual indifference. THE IMPULSE COUPLING

Contrary to what many people believe; to start a magneto equipped engine, it is not necessary to grasp the propeller with both hands and snap it through smartly with one leg gyrating through the air ala Hollywood. Not at all. Not with an impulse equipped magneto. 20 FEBRUARY 1976

How then, can you know the engine has an impulse coupling? You can bet on it. Without an i m p u l s e coupling (or booster mag, or vibrator), the starter won't crank the engine fast enough to get the magneto up to sparking speed. A good hot spark is not produced until the magnet is turned above a certain number of revolutions, let's say, more than 100 rpm. This is known as the "coming in speed" of the magneto. Because of this characteristic, it is a standard industry practice to utilize an impulse coupling to generate a better spark for starting. It does this by causing the magneto to spin faster than the engine cranking speed and, at the same time, automatically retarding the spark for starting. If you tried to start the engine with its normal advanced (early) spark timing, it would kick backwards like a mule and maybe break an appendage or worse, the starter. Therefore the retarding or late spark capability provided by the impulse coupling for starting is essential. (Figure 3). After the engine starts running, the impulse coupling performs as a drive coupling for the magneto allowing it to function in the full advance spark position. (Fig. 2). VW engine buffs frequently encounter a u n i q u e problem affecting the impulse coupling's action. If the homemade adapter for attaching the magneto to the engine does not provide the proper spacing between the engine and the magneto, when bolted in place, the impulse coupling binds and cannot function. You can recognize the presence of this condition

when you turn the propeller and do not hear the familiar "clack" noise of the coupling. In addition, while turning the prop, the crankshaft will feel unusually "tight" with a noticeable absence of end play. The remedy? Shim the magneto away from its mounting as necessary, to free the impulse coupling. One scary thing about magnetos. You can never be sure they are "OFF" when the switch is in the OFF position. Very few light aircraft have an IDLE-CUT OFF feature with their mixture control and these engines are normally stopped by turning the magneto switch to the OFF position. If the engine stops, you know the switch is functioning, and the magneto, at that time, is safe (electrically grounded). An ignition switch for a magneto system functions opposite to ordinary switches. In the OFF position, the switch is closed causing the breaker points to be electrically grounded. The magneto, therefore, cannot operate. If, on the other hand, the engine does not cease firing in the OFF position, the magneto ground lead (also referred to as the "P" lead) is open and the trouble must be corrected.

The external magneto wiring circuit is simple. A short wire from the magneto case runs to some point on the engine that serves as a ground. The only other wire is the magneto's primary ground wire (P lead) which is connected to the ignition switch. A shielded magneto such as the Scintilla has a builtin safety device that automatically grounds the magneto whenever the switch wire is disconnected. Unfortunately, this requires a rather fussy connector. You can't

just twist the switch wire around a terminal and snub it down as in some unshielded installations. It is necessary to obtain a Terminal Assembly Kit for the magneto if one is not already installed and dangling from the magneto (most unlikely). The kit consists of a

CO COUPLING

NUT

SHIELDED WIRE^ FERRULE- OUTER (GROUND WIRE)

SLEEVES INSULATING (GROUND

WIRE)

WASHER GROUND WIRE CONTACT

SOLDER

WASHER

TO END OF

BARE

WIRE

contact washer, insulators, ferrules, and a union or

connector n u t . Figure 1 shows how the switch wire is assembled into a unit of the exact length required

FIGURE

to make proper contact, inside the magneto, with the

GROUND

breaker grounding spring. Also required is some shielded switch wire. In addition, a radio noise suppressor is ordinarily installed in

TERMINAL

I.

ASSEMBLY

the magneto switch wire circuit to eliminate electrical

interference picked up from the switch wire. Don't assume a minute that if the primary ground

RUNNING SPANK

lead ("P" lead) is disconnected the magneto is rendered

25' TO SO'

harmless. It may not be: Many magnetos (Scintillas) do have a special breaker grounding spring on the magneto to short circuit the

BEFORE TOP DEAD CENTER (ADVANCE)

primary . . . when the switch wire is not installed but, other magnetos are NOT provided automatic grounding springs at the switch wire terminals. After months or years of flying, it is easy to forget

whether or not your magneto(s) has the built-in safety feature of a grounding spring, so always be careful when

FIG. 2

messing around with the propeller or engine.

Play it safe. Whenever you do any magneto maintenance requiring the removal of the "P" lead from the

magneto, remove the top spark plugs or, at least disconnect all spark plug cables from the installed spark

plugs. Often recommended but not as safe, of course, is to make sure the magneto is off and that there is a completed connection from the switch to the engine and from the magneto to the switch.

When installing the magneto "P" lead connection, insert and secure the switch wire terminal assembly before replacing the breaker cover plate so you can visually ascertain that the end of the switch wire terminal

IMPULSE COUPLING RETARDS STARTING

SPARK

FIRES (OR

AT

NEAR)

TOP DEAD

CENTER

is making positive contact with the breaker contact

spring. If not, reposition the breaker grounding spring as necessary. Otherwise, the magneto might remain

grounded even when the switch is moved to the "on" position. If you can't get a sparking in your plugs from

FIG.

3.

the magneto, your difficulty may be as described. This problem is most likely to occur after an engine rebuild

or magneto overhaul.

TIMING THE MAGNETO TO THE ENGINE

There are many types of magnetos used by homebuilders but most of them are timed to the engine using

the same general procedures. Realize, of course, that there may be many minor variations and theatrics that

can be engaged in during the magneto-to-engine timing

ritual. Most of the methods in use are good-to-excellent and any one may be preferred by the individual mechanic

(builder) simply because he does have the particular gadget, feeler gage, buzzer, tool, piece of cellophane,

or light required. Use whatever you have. If you don't have anything . . . make something or borrow something. You will be able to do a more precise job of timing. Why such a fuss about timing? A bad timing job can cause all sorts of problems, failure of all cylinders to

fire, irregular operation of all cylinders, low horsepower output, loud exhaust, overheating of the engine, detonation, backfiring, and difficult starting. As if that's not enough, improper ignition timing, especially advanced timing, can seriously damage an engine in a very short time. Advanced timing can lead to preignition, resulting in piston b u r n i n g , stuck and broken rings, and scored

cylinders. The best guidance for timing an aircraft engine will

be found in the maintenance manual for that engine. Its instructions should be followed faithfully. Your only

alternative data may be the information obtained from the aircraft engine data plate.

SPORT AVIATION 21

VW engines do not have s t a n d a r d aircraft-use manuals nor data plates for guidance and most builders grope around on their own for the best results.

DIAL G R A D U A T I O N S "FOR E A S Y REFERENCE . B R A Z E / W E L D OR TO SHELL

ONE MAN'S TIMING PROCEDURE

1. First of a l l , remove all upper spark plugs for safety! 2. Determine the number of degrees before top dead center (BTDC) you want the engine to fire. As previously mentioned, refer to your engine data plate or engine manual. Most engines are timed to fire from 25° to 30° BTDC. For example, the C85-12 is usually timed at 28" BTDC for the left mag and at 30" BTDC for the right mag. The last VW aircraft conversion I worked with was timed at 27'i>" BTDC. 3. Locate top dead center (TDC) for the No. 1 cylinder. This means, of course, that you will have the No. 1 piston on the compression stroke and both valves are closed. To find the compression stroke, you can press your finger or thumb over the spark plug hole while turning the crankshaft in its normal direction of rotation. The compressed air in the cylinder will try to escape past your finger with a "pfuph" sound. Be sure that you have one plug installed in the opposite side of the cylinder (dual ignition engines), or else you'll be turning that prop all day waiting for the "pfuph". At this point you might ask how TDC is determined. Most aircraft engines have engraved marks on the propeller hub which are aligned with the centerline split of the crackcase. Other engines and converted auto engines might have no such references to use and the TDC will have to be established from scratch. A "screw driver and hammer" mechanic would probably poke a stick or a wire through the spark plug hole and feel for the piston's rise in the cylinder. When the rising sensation ceases, the piston is somewhere around top center . . . not TDC maybe but something close to it. With the piston at or near TDC, there is very little motion of the piston in relation to the crankshaft rotation and, as a result, it is not easy to establish the exact TDC of the piston. You might be off as much as 5° on different tries. An exact TDC determination using a "Piston Position Indicator" is possible. There are store bought types, but you can make your own piston position or top dead center i n d i c a t o r from a piece of wire and an old shielded spark plug with its guts punched out. Mechanics have been using such a device for decades. The hollow spark plug shell screws into the cylinder and serves as a very convenient support for the inserted indicator wire. Minute movements of the wire indicator are observed more accurately from this steady base. (Figure 4.) 4. Set the crankshaft at the proper position for firing No. 1 Cylinder. After you have found the TDC position, back off the crankshaft 30 to 40" and then begin to come back slowly in the direction of rotation until you reach the prescribed number of degrees ahead of top dead center (BTDCHas specified by engine manufacturer). When this position is reached, you are ready to mate the magneto to the engine. 5. Rotate the magneto shaft to the position where the No. 1 distributor block lead is ready to spark. Different magnetos have different features built-in to assist you in establishing the point where the magneto breaker points should be just beginning to open (ready to fire). On some magnetos look for a marked tooth on the distributor gear which shows up in a small window in the cover of the magneto at the drive end. or There may be an opening in the distributor cap through which the position of the distributor finger may be seen . . . 22 FEBRUARY 1976

SILVER

REMOVABLE

OLD

SPARK

(SHIELDED KNOCK

1/8*

PLUG

TYPE)

OUT

CENTER

WIRE COR

INDICATOR

LENSTH

r FIG.

PIN

SOLDER

s'

TO

SUIT

4

4.

PISTON

TOP DEAD CENTER INDICATOR

or In some cases there may be matching lines on the distributor gear and magneto housing. or The magneto may have a step cut on the timing col-

lar secured to the cam.

or If all else fails, install a spark plug wire in the No. 1 distributor block opening and cause the magneto shaft to be rotated until that lead sparks when held close to a metal ground. (This could be a shocking experience if you are not careful.) Now, with both the crankshaft and magneto positioned to fire No. 1, install the magneto on the engine and draw the bolts up snug, not tight. Rotate the magneto in both directions as far as the elongated mounting holes will allow. During this rotation, check the breaker points to see that they open and close. If not, the magneto will have to be removed and its drive shaft turned slightly before reinstalling for another check. As a final check rotate the magneto in the direction opposite to the normal rotation until the breaker points are just opening. The exact moment of opening may be determined by using a feeler gage (.0015") or a thin piece of cellophane. ( D a d d y , what's cellophane?) Insert the cellophane between the breaker points and rotate the magneto case slowly by tapping it while maintaining a slight pull on the feeler gage or cellophane strip. It will begin to slip as the points are just beginning to open. At this point, stop and permanently tighten the mounting bolts, as your timing is complete. The other magneto should be timed in the same manner but using different firing position if applicable. Next, hook up all the other spark plug leads. The number 2 distributor lead of the magneto goes to the second cylinder to fire and not to No. 2 cylinder, etc. If both magnetos are timed to fire at the same crankshaft position, you can check their synchronization with the cellophane strip as described before, by inserting a strip in each magneto. Rotate the engine crankshaft backwards about 45° and bring it back in the normal direction slowly. Both sets of breaker points should open simultaneously. Don't be surprised if the engine starts easily and runs good . . . you probably expected otherwise but Murphy's Law once again proves you're wrong.