Hail Bruvvers!
AoA posed a salient and worthwhile topic of discussion for all 'tiny piston' drivers - reespecccct. Lets not get too personal (AutoB and Rolling R!) just coz sum of the Bruvvers is confuzed!
If you look real careful at all this stuff written above then the answer will come bonk you on the nog.
Rolling C gave the correct answer which Watford, ChickenT (and me) agree with. 2R added some bits and DeadH made a good point about the terminology being confusing - it sure is.
All the jazz on alternator belts and mixture is fluff. Let me try and get a grip.
Old aerodonk has two sparky things in each cylinder. Each sparky thing made to spark by its own magneto, so that when one mag goes futt, cylinder still has a sparky thing that is working. This is the dual ignition redundancy thing we know and love.
Magneto is a simple generator and when roted will always make electric. Snagette - don't always want electric (such as when shut down!) so must be able to send electric away from sparkeys when not wanted. Light bulb - send it to earth (airframe/engine mount) by piddly little copper wire called p-lead. Good. But monkey in cockpit (me!) must be able to CONTROL when electric goes to earth and when it goes to sparkeys. Light bulb - provide a switch - only one would do!
Snagette - monkey not only needs to be able to CONTROL magnetos on/off but be sure them is workin fine. See, sparkeys must go fizz together at same time in squeeze-bang bit of suck-squeeze-bang-blow...ie, monkey must be able to check PERFORMANCE of ignition.
Light bulb - give monkey two switches, let him control (switch on/off) each magneto. Lotz of extra added bonuses at no extra cost now:
- monkey can check that each mag will go off (electric to earth thru p-lead and not to sparkey) so will be safe when stopped(remember, mag ALWAYS makes electric). CONTROL.
- monkey can check that sparkeys go fizz at correct time at representative advance angle in squeeze-bang cycle by switching on/off at representative rpms (usually around 1700-2000). Small drop in rpms expected, but big drop might mean timing duff. PERFORMANCE.
Fuzz - the switch. Easy solution, 2 of, one for each mag. Complication - Mr Ezekial J Abbernachher III wants keys to his plane like he has keys to his Caddy. Clever Mr Cessna/Piper etc do this by having one 4-way switch: Off-L-R-Both (could have 5-way with 'start' pos, but stik with 4 for now). Snag - now need to be able to test that switch works in OFF posn as well as L-R-B; can safest to this from cockpit with engine running at LOW rpm (small fuel/air charge so little bang, spark retarded & moment of inertia 'I' of rotating system of pistons & crankshaft dominate ) by switching ign temp to 'off' posn.
More fuzz - Bulldog drivers - I think that their airlordfurhers didn't believe the young shiitehawks should have to carry keys to their mount so paid doubly extra to have the same 4-way switch modded to do away mit da key (similar philosophy to not physically checking the fuel qty wot woz ably put in by groundie - just a difference in ops, thats all ).
Back to the point. Putting it into praccy.
Start engine. May have to start on L if impulse coupling on L-mag but this is fuzz to question asked. When donk going want to check that both mags work fine as a) no point going mit just one and b) certainly don't want to switch entire system off & on again (as wot would happen with duff mag during run-up) at hi pwr with spark advanced all that way back and lotz of bang about to happen. Could be v.costly. So at opportune time after start me just go B-L-R-B. You is lookin for drop in rpm from B to R, that rpm stays dropped between R & L (subtle), then goes back up when back to B. At no time should donk stop coz this is bad (duff mag) and we stays put. Call this check whatever you like - 1st ign check, drop-no-stop, Ivan the check, whatever; must comprendo whats happening. Note that holy tradition passed down from our forefathers suggests that we don't go to the OFF posn here - but sky wouldn't fall in if we did.
Bruvvers, we have just tested that we have control of our ign system.
Run-up/Test Engine. Now want to test PERFORMANCE of ign system (ie, it workin' cool). Now me go B to R, note amount of drop, then back to B. Next me go B to L, note amount of drop, then back to B. Me now compare to drops with each other & maxi allowed from book of words. Me confident that no BIG BANG due whole system go futt coz mags checked ok at startup.
DH - sorry blokey, but theres no point hovr'n over the Mix incase Bloggs (or 'Bruce the Lesser') accidental go to OFF during run-up...damage will already have been well & truly done.
Shut-down. All being well ign still ok after arduous aviation committed (ie still have CONTROL of ign) but want to be sure donk don't start when wife accidental hangs handbag on fan-blade. So test OFF posn. Now me go B-R-L-OFF-L-R-B bein sure not to hang out at OFF for too long. Engine should show signs of wanting to sleep at OFF, but get extra bonus for free - can also test control of ign again! Whizzo.
Hope you not bored like fart in breeze.
Enjoy.
PS. YMML - beware book of words with Cessna/Piper etc on cover when it comes to donks. ALWAYS a good instinct to go for book of words for advice, but donk luvvingly nurtured from birth by Mr Lycoming/Continental etc. Donks in planes get changed, but motor-maker is always authority. How many Mr Lyc/Mr Cont books you seen floatin at your flyclub?