Why set max spark advance @ 32* BTDC to attain 30*BTDC? Why use 33* BTDC w/ starter?

Fishsqueezer

Cadet
Joined
Apr 5, 2012
Messages
28
I?m working on a Mariner 50 hp, 4-cyl, 2 stroke, electric start, serial # 6542853

Here is a page from the Mercury service manual regarding timing for my engine. The table at top shows specifications.

Mercury Shop Manual Timing pg1.jpg

Can anyone explain what the electronic timing characteristics are of this ignition system that require setting max spark advance at 32* BTDC to attain 30* BTDC spark advance?

Can anyone explain why the service manual would specifies setting Throttle Secondary Pickup at 33* maximum spark ?cranking engine with starter??

I?m confused as to why there would be any difference between cranking by starter or by combustion. Why not just scribe the flywheel appropriately for the correct timing? It seems somewhat convoluted to specify one number but say that setting should read another to attain specs. Seems like it should be simple enough, but it confuses me a bit. Only thing I can think of is that the flywheel was used in several models with different timing specs.

Any experts out there know why the manual specifies these things?
 

Faztbullet

Supreme Mariner
Joined
Mar 2, 2008
Messages
15,589
Re: Why set max spark advance @ 32* BTDC to attain 30*BTDC? Why use 33* BTDC w/ star

Thats a misprint it should have read 32?... this ignition will retard as rpm increases where a OMC will advance with rpm. Just follow the book and use 32?.
 

Fishsqueezer

Cadet
Joined
Apr 5, 2012
Messages
28
Re: Why set max spark advance @ 32* BTDC to attain 30*BTDC? Why use 33* BTDC w/ star

Thank you!
 

Texasmark

Supreme Mariner
Joined
Dec 20, 2005
Messages
14,544
Re: Why set max spark advance @ 32* BTDC to attain 30*BTDC? Why use 33* BTDC w/ star

Thats a misprint it should have read 32?... this ignition will retard as rpm increases where a OMC will advance with rpm. Just follow the book and use 32?.

Well sir, thanks for clearing that up. I too was curious while checking my timing, several months ago when I was having problems and noticed the same sort of notation in my manual. But since cranking speed is like 200 rpm, and idle rpm is spec'd at 675 +/- 25 it makes sense.

Mark
 
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