Mag Troubles

[Dward]

Bruce, you might want to be carfull about shutting the mag switch off during run up. I did that accidentally a couple of times on my old Cherokee 235 and the resulting backfires cost me a new muffler at the next annual. It is also within the realm of possibility that my mechanic heard them and made himself a note to sell me a new muffler . Ya had to know the guy!

[GAHorn]

I'm sure Bruce does it only at low idle, when it's not likely to cause a problem. This is actually a recommended procedure by many mfr's. Don't do it at any engine speed except LOW idle.

A backfire is ALWAYS reason to investigate. Backfires can cause many types of damage, including dropping flame-cones within mufflers (which reduce power) and causing swollen mufflers (the reason mufflers should have their shrouds removed and inspected for swollen mufflers...which indicate backfires.) Backfires thru the carb can damage air boxes and carb heat butterfly valves.

Anytime you experience a backfire, inspect both exhausts and intakes.

[blueldr]

Only Avgas backfires!

[Bruce Fenstermacher]

Only Avgas backfires!

Well BL i hate to disagree with you but you must not have spent much time around the campfire with some of our members.

Actually I turn both mags off at 1700 rpm (1500 on the Cub) and back on so fast that the engine barely misses a beat but I can tell it's gone dead. Never had a back fire on my C-145 or my C-85. If did I'd change my procedure. I do this mostly with my Cub because the mag drop on either mag is difficult to perceive and I want o make sure both P-leads are intact and working. I won't argue that doing it at an idle would be better procedure.

I can't stress enough how important checking your mufflers is after a back fire. A good friend had the back of his muffler blow out on departure after a back fire in hiss Belanca Super Viking. The heat started the firewall blanket on the inside of the firewall smoking and filled his cockpit. He did an abbreviated 360 degree pattern at about 300 ft at Myrtle Beach and landed on the runway. He and his wife barely got out and the insurance company totaled the plane. He did shut down after the back fire and do a cursory inspection of the tail pipe security and a visual inspection as much as one can do without removing the cowl and all seemed well.

This scenario has since happened to several Super Vikings and is now a subject of an AD.

[GAHorn]

Bruce (and others) I'd like to suggest you check the mags in the off position ONLY at very low idle, not just for backfire reasons.
When the engine operates at higher speeds, even at 1700 rpm or so, when the mags are switched OFF and then ON... the counterweights on the crankshaft sling violently forward then back, and it can mark them and their bushings for stress.

[W.J.Langholz]

Did I miss a thread Here??!!???

Did we get the new switch on yet?????

[Bruce Fenstermacher]

Bruce (and others) I'd like to suggest you check the mags in the off position ONLY at very low idle, not just for backfire reasons.
When the engine operates at higher speeds, even at 1700 rpm or so, when the mags are switched OFF and then ON... the counterweights on the crankshaft sling violently forward then back, and it can mark them and their bushings for stress.

Well George, I won't have to worry about that since neither my C-145 or my C85 have counterweights.

[GAHorn]

Well George, I won't have to worry about that since neither my C-145 or my C85 have counterweights.

For some reason I thought you had the dampered crankshaft on your C-145. (Don't you have the 35A gen?)

[blueldr]

Bruce,

Believe me, I know that a ham handed pilot can backfire an engine on mogas. I was merely being facetious for "Av Gas" Georges comment.
As a matter of fact, I was taught,in primary flying school in the Army in 1942, to switch the mags to off and then quickly back on,at idle, to make sure that both P leads were grounded to preclude a hot mag that might injure someone pulling the prop through by hand on a preflight.

[Dward]

Who uses Avgas?

Hammy

[Robert Eilers]

The hot mag check is part of our shut down procedure. I recommend it.

[Bruce Fenstermacher]

George I have a 25 amp generator because I don't have a dampened crank.

BL I was attempting humor myself pointing out that besides AvGas our members back fire as well. Besides not becoming an English Teacher I should stay away from becoming a comedian as well.

[Showboatsix]

There is a far easier and safer way to check for a hot mag, when your engine is warm and shut off, I simply leave the key (mags) in the off position and crank the engine over, if a mag is hot, the engine starts, no backfire, no pops, easy to do and no $1000.00 mufflers to replace!

Dean

[GAHorn]

There is a far easier and safer way to check for a hot mag, when your engine is warm and shut off, I simply leave the key (mags) in the off position and crank the engine over, if a mag is hot, the engine starts, no backfire, no pops, easy to do and no $1000.00 mufflers to replace!

Dean

You'd think that'd be a reliable method but unfortunately the "coming in" speed of some magnetos may subvert that particular method, and many hot engines are more difficult to start therefore may not give a satisfactory response if only one magneto is weak and/or not grounded. ( I suppose it could also be argued that such engines are unlikely to start by someone leaning on a propeller, as well, but I don't buy it. I once wore myself out hand-propping a hot engine that refused to start. After standing around visiting with others and catching my breath...and the engine had a chance to cool as we discussed various topics for about a half-hour.... I absent mindedly gave the prop another spin as I walked away giving up on flying it that day. To my utter surprise it started right up and nearly killed me, since not expecting such a response I was nonchalantly standing within the propeller-arc.)

[Showboatsix]

Hey "G",

When you "absent mindedly gave the prop another spin", had you turned off the mag switch from the last attempt, or did you leave it on?

If you left it on, then the problem was yours, and not an "ungrounded mag" problem we are talking about in here.

Also if we are talking about a mag so weak that it can not start an engine, I would assume that you would also see that in a mag drop check during runup also.

However I have heard mag checks at UOA where the engine will hardly run on single mag during runup, and the guy takes off anyway! (maybe it will fix it'self in the air)

However a periodic switch check at idle is also an accepted way to check your switch/mags.