A65 Timing and Bendix Scintilla Questions

[zipperdu]

I'm a new owner of an 7AC with a question about mag timing... logs state the timing was set to 28btdc, the TCDS states for this A65 (as you all know) it should be 30, am I missing something or did the last mechanic incorrectly time the mags??

My second question relates to the Bendix Scintilla Mags installed on this engine.
Having never flown an airplane without impulse couplings, I am trying to get some infomation on how these mags retard the spark for start.

Thanks, I'm enjoying the forum, and happy to be a member!

Jon

[Al Hatz]

I don't know why anyone would write in a logbook he set the timing at something other then what the specs call out for, just setting himself up for trouble. A degree or two isn't going to make that much difference but you don't write those kinds of things in a logbook. I'd just get it as close to 30 as you can, sometimes the lunchbox Bendix won't let you get it perfect.
There is no retard for starting on those mags, that's why you need a good coil and give it a good pull to get it started.

[zipperdu]

Thanks Al, I would agree 2 degrees wouldn't make that much difference with this smaller engine. I'll reset to 30.
What does surprise me is the lack of any spark retard for starting on the Bendix mag. I've not seen any kickback, perhaps if I set timing to 30 degrees BTC I just might! Thank you for your reply.

Jon

[Bo Grave]

Your engine was timed per the continental engine manual for an A 75 which is timed 28 btc on one mag and 30 btc on the other. If you go to 30 on both you will see a small rpm loss on your static run, maybe 100 rpm. remember rpm is HP.

regards,

Bo Grave
N1663E

[Bo Grave]

Here is the timing for the A75 from the TCDS (Ignition timing, °BTC Right 29, Left 32) I was off a bit. I think they run better with this timing.

regards

Bo Grave
N1663E

[N86250]

I suspect the previous owner was getting old and tired and had problems with kickback when handpropping. Or it was a young guy with limp wrists and no arm strength. This was done to help him avoid the kickbacks. If you see the old guys who grew up with these engines and non-impulse mags hand prop one, take notice of their technique. Good footing, good form, good communication, good body movement away from the prop arc, and a swing so strong you will expect the prop to go around an extra blade or two. jrh

[zipperdu]

Hey , thanks to all-
jrh- your comments for safe proping are right on the mark- I do find however that if I prop the engine less agressively I have a better chance of a start. I've not seen a kickback yet, but the airplane is still new to me.

Jon

[David Nielsen]

I may be way off base here. I'm sure you guys know more then I do. However, I was under the impression the timing on the A-65 was 28 Deg when the mags did not have an inpulse coupling. I am thinking here of the old SF4's.

Dave

[flyer fixer]

A good place to start this discussion is go to the Teledyne Continental Motors web site www.tcmlink.com technical support and print out MSB94-8C, this is the current S.B. for timing, it may give you more than you wish to know. Here is some food for thought, today, the point of a magneto firing is the opening of the points in normal direction of rotation as indicated by the light on a vibrator/electronic alternating current induced test box. When the A series engines were produced the magnetos were timed by being able to pull a piece of cigarette package cellophane out of a set of closed magneto points and a timerite indicator! How accurate was this for timing? With todays testing equipment if your timing is +/- 2 degrees you're better than an engine timed in 1946. If you get zapped by an old SF4 you know you have been enlightened!