Lycoming starting Procedure

I recently took a refresher coarse and when it came to starting the engine the instructor requested that the CC 180 18 be started on the left mag. How weird I thought. My 94 150 18 clearly stated to turn both mags on and begin the starting procedure. Iasked if it stated this in the owners POH and he said yes. I have never owned any a lycoming that said to start on the left mag only. Who does and why? Please list the source and reference page no's if applicable.

It may have an impulse coupling on the left magneto to retard the timing while starting.

If you have a mag switch with the starter circuit it will cut out the right mag on start up if it is equiped with an impulse on the left mag only. The mags firing at two different times can cause a kick back, broken stater bendix and nose, broken ring gear or worse. The wiring diagram that comes with a new mag/start switch shows this and has jumpers you hook up. Those of us with the old AN switch just have to know. It has been a word of mouth thing as far as I know. I haven't seen it in print.

And, kudos to your instructor for giving you that bit of wisdom. The reasons have been described. Nearly ALL four cylinder Lycomings actually are supposed to start on the left mag only.

Here's the down side if you fail to do so: If you have one of the lightweight starters, ie: skytec, lamar, etc., and you start on "both" position on the mag switch, eventually you'll get a little backfire, due to magneto timing differences. With the lightweight starters, more than likely you'll shear the starter shaft, which means you buy another starter.

The original Lycoming geared starters are a little more durable (the reason the lightweight starters are light is cause there isn't as much metal on critical parts), but they too can suffer this demise.

MTV

starting

The new faster spinning starters can fire the non-impulse mag

Hey SuperCubber...ditto for me on the left mag.

By the way, whats the drill on gas tanks? This discussion on mags made me think of "primary" wing tank. I think I remeber something about always using the LEFT tank for landing a take-off. Anyone have comments on a primary tank? (Maybe I was thinking of a J3/PA-11, header tank?)

I use whatever tank I'm running on, or whatever tank has the most gas in it.

good flying...>Byron
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Byron,

Depends on the particular airplane you have, and how the system has been modified.

The original Cub Special (PA-11) came with a single fuel tank in the left wing-18 gallons. This was also true of the first PA-18's. The right tank was an option.

As such, the left tank has a front and back fuel pickup port, so that if you are climbing or descending, fuel will continue to flow.

Since the right tank was optional, it only had a rear fuel pickup port, for simplicity, and to keep down extra expenses.

As a consequence, in a steep descent, for example, you could unport the fuel pickup on the right tank if you were in a descent long enough to run the fuel out of the header tank. All these airplanes were originally placarded to use the right tank only for straight and level flight, or words to that effect.

So, the "normal" procedure would be to takeoff, land, climb and descend on the left tank, and cruise on the right tank.

I always admonish students not to run the left tank dry enroute for this reason.

Now, many Cubs have been modified in many ways, including having the header tanks removed, and some have even had a second fuel pickup added to the right tank, I believe.

As always, you need to know specifically how your airplane is equipped.

There's a long thread on here on this subject.

MTV