Primer plus/minus?

[Glen Geller]

"We are taught to start on the LEFT mag because that mag has an Impulse Coupler that advances the timing to allow easier starting at the low rpm provided by the starter motor"


I believe the impulse coupling actually retards the timing until the engine rpm accelerates to an idle speed.
If the timing were advance at starting rpm, it ignites the gas BTDC an can actually force the rotation backwards
when ignition takes place.

I believe you are correct about retard vs advance, I stand corrected Sir.

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[Jeff J]

Glen is correct the left mag has the impulse coupler for starting. Mags are connected to either the upper or lower spark plugs. You were doing the correct thing, not for the correct reason.

A mag is sometimes connected to upper on one side and lower on the other. Best to check the manual for the installation to see what is “correct” for the installation in question.

[Steve Pierce]

A mag is sometimes connected to upper on one side and lower on the other. Best to check the manual for the installation to see what is “correct” for the installation in question.

I think most mags fire the top plugs on one side and bottom on the other. The only thing I have worked on that was different has been the small Continentals which one mag fires the top plugs and the other the bottom.

When I worked at a flight scholl many moons ago we had several carburetor fires in winter when students pumped the throttle. Probably safer and actually better to use the primer so that the fuel is injected directly into the cylinder.

[59pacer]

I just had a look at the O-320 manual, and it confirms that the Left magneto should be wired to the top left plugs (Right mag vv), with an optional wiring of Left mag to bottom left plugs (Right mag vv). My Continental O-200 has one mag to the top cylinders, and the other mag to the bottom. I don't have that manual handy to say which goes where.

I thought that the primer sprays a stream of fuel into the intake next to the intake valve rather than into the cylinder??

When I used to use the primer, I felt that the pump 'load' could be determined by how fast you exercise the pump. Doing it too fast seems to introduce air, and you can get a variable pump load. There could be some vaporisation due to the suction through the 1/8" lines, but when you push the pump back in, it seems to easily go in so far before it starts pushing fuel, so I thought air may be getting in on a fast pull. Thoughts?

[Jeff J]

I think most mags fire the top plugs on one side and bottom on the other. The only thing I have worked on that was different has been the small Continentals which one mag fires the top plugs and the other the bottom.

I think the biggest I have seen is the O-300. I am pretty sure every Lycoming I have encountered had the mags split between top and bottom. I thought it was worth mentioning not all ignition systems are set up the same.

[59pacer]

I just had a look at the O-320 manual, and it confirms that the Left magneto should be wired to the top left plugs (Right mag vv), with an optional wiring of Left mag to bottom left plugs (Right mag vv). My Continental O-200 has one mag to the top cylinders, and the other mag to the bottom. I don't have that manual handy to say which goes where.

I thought that the primer sprays a stream of fuel into the intake next to the intake valve rather than into the cylinder??

When I used to use the primer, I felt that the pump 'load' could be determined by how fast you exercise the pump. Doing it too fast seems to introduce air, and you can get a variable pump load. There could be some vaporisation due to the suction through the 1/8" lines, but when you push the pump back in, it seems to easily go in so far before it starts pushing fuel, so I thought air may be getting in on a fast pull. Thoughts?

Just to clarify, the Left magneto should be wired to the top left and bottom right plugs (Right mag vv), with the optional wiring Left Magneto to bottom left and top right plugs (Right mag vv).
Note to self: forget the laundry and concentrate.

[rideandfly]

Just bought the 48 PA-17 Vagabond with C-85 and metal prop last March. Have not flown below 50F so far in southern NC. No electrical system.

Have not used the primer yet, pull the prop through 7 to 8 times, switch both mags on, normally starts first hand prop. When engine is hot, normally starts first hand prop. Engine has new Slick Mags.