Mag timing

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ahmebane
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Mag timing

Post by ahmebane »

I have continued to struggle with keeping the CHTs down below 400. Recently in AOPA magazine, Mike Busch mentioned a Lycoming noncompulsory service bulletin for the IO-360-M1A engine that recommended retarding the timing from 25 degrees BTDC to 20 to help with CHT. I have been unable to locate the bulletin (I always have a difficult time finding these things!). What timing are people using these days? 20 or 25?

TIA.
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Rich
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Re: Mag timing

Post by Rich »

Another element in this article is that setting timing is +0 and -1 degrees. So making sure you are no more than 25 degrees BTDC would be important.

Assuming this SB is legitimate, how about something like 23 BTDC?

Another thing to think about is just how precisely the mag maintains the timing. It is controlled by primitive mechanical points subject to wear of things like the cam follower (the one inside the mag) and pitting of the point contacts. These internal components are similar to what one would find in a 1940 Oldsmobile (my parents' first car).
Last edited by Rich on Fri Oct 08, 2021 11:54 pm, edited 2 times in total.
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Steve
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Re: Mag timing

Post by Steve »

I've always set mine at spec, 25 degrees BTDC. No problems with CHTs (350-380 degrees F in south TX).
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Re: Mag timing

Post by Karl »

ahmebane wrote: Fri Oct 08, 2021 9:10 pm I have continued to struggle with keeping the CHTs down below 400. Recently in AOPA magazine, Mike Busch mentioned a Lycoming noncompulsory service bulletin for the IO-360-M1A engine that recommended retarding the timing from 25 degrees BTDC to 20 to help with CHT. I have been unable to locate the bulletin (I always have a difficult time finding these things!). What timing are people using these days? 20 or 25?

TIA.
I would be checking all the baffles for a good fit and baffle material for good condition. It should be flexible, not brittle. It's surprising how a small air gap around a baffle can affect engine temps.
Next check the induction system for leaks, especially the induction gaskets where they attach to the head. They often look fine but are very brittle once removed. Small leaks in the induction system will give a lean mixture and increased temperatures. If you haven't replaced the gaskets in the last 2 years they will almost certainly need to be changed.
Once that is done, check and adjust mixture settings, they may have been (incorrectly) adjusted previously due to leaking manifold gaskets.
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Karl
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Re: Mag timing

Post by Karl »

ahmebane wrote: Fri Oct 08, 2021 9:10 pm I have continued to struggle with keeping the CHTs down below 400. Recently in AOPA magazine, Mike Busch mentioned a Lycoming noncompulsory service bulletin for the IO-360-M1A engine that recommended retarding the timing from 25 degrees BTDC to 20 to help with CHT. I have been unable to locate the bulletin (I always have a difficult time finding these things!). What timing are people using these days? 20 or 25?

TIA.
From the Maintenance Manual.
IO-360.JPG
Pbwears1
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Re: Mag timing

Post by Pbwears1 »

Following the mag thread....need your wisdom.
2012 XLS, CHT's not an issue...they are fine.
But, I'm getting a Right mag reading of 1790-1815 and 1850 + on the Left at run up.
All eight plugs are new and timing set...
Getting 2650+ rpm's at full power on take off.
Seems like an issue with the right mag but what else should I be looking at?

Many thanks, Peter
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