"Swing" mag or fixed timing

**Mark75H** · 06-01-2007

It took me a long time to learn the reason some guys use fixed ignition timing on modified Mercs ... I'm very certain that it came from copying what they saw on alky deflectors ... copied on the theory that if it is good for padded alkie motors, it must be good for gas mod motors, too.

The reason a lot of the padded alky deflectors used fixed timing is that they were running the timing so close to top dead center there was no room to retard the spark. If you are only running 3 or 4 degrees BTDC, there is no point in letting the spark occur after TDC for any reason.

On the other hand ... on a near stock deflector ... running 30 or 40 degrees of spark advance at WOT ... it is a lot less likely to pop back and "bite" you if you can set the spark back to 10 or 15 degrees while pulling the rope.

Some racers have tried to justify running fixed timing as a real improvement by saying keeping it at a constant setting improved accelleration. If you look at the way the Merc linkage works you will see that there isn't much time that an old Merc spends on the race course with the timing other than at full advance: as you close the throttle from full to pretty much closed the timing is still against the full advance stop.

**JohnsonM50** · 06-01-2007

Ive gottem the impression that [speaking of 2 cyl - MK25,20 or OMC 22s etc.] That to be sure the points would be 180 degrees apart [when right] it needed to be locked to ensure they stayed right. Thats not a problem with CDI because of the pick up coil tho.

**Skoontz** · 06-01-2007

With any 2 cylinder OMC, you place a factory timing fixture on the crank, disconnect the ground for the coil, put one lead of a test light on the points, and the other at ground. When you move the crank pointer into the hash marks on the mag plate, the light will go off because the points just broke. If you want to advance timing, you drill a hole in the brass strap that pulls the mag plate ahead of the one the factory set. We used to make straps with incremental holes.

**Fast Fred** · 06-01-2007

some race motors (other than merc) came with fixed right out of the box, witch
the factory made special parts just to set it up fixed, thay could have more easily used the swing, why would you suggest thay went to the trouble.

No, i have not worked at that factory.

**mercury400m** · 06-01-2007

the omc's mag adv. are fairly equal to the carb but 95% of the idle is mag advancmentment and the top end mostly the carb

the brass peices are adjustable arn't they? just have to use a 5/16 socket to loosen both very small nuts and you can advance a decent amount but still having the motor to be able to run or are you talking about a different brass piec b/c i know there are a few. also what hp roughly are you mentioning some were a little bit different.

**Skoontz** · 06-02-2007

the brass peices are adjustable arn't they? just have to use a 5/16 socket to loosen both very small nuts and you can advance a decent amount but still having the motor to be able to run or are you talking about a different brass piec b/c i know there are a few. also what hp roughly are you mentioning some were a little bit different.[/QUOTE]

The early V-4 50, and 75's were adjustable, and that trickled into the 40 twin. By the time the 40 got that ridiculous compression release added (which was the first thing you tossed away in a tune up) the brass strap had two drilled hols that were not movable with out drilling more hols. 40 and under had a pressed in pin which went through the strap on the bottom of the flywheel plate and a stainless cover snapped over the pin then locked with a couple tabs that were formed into the piece. Nothing under 40 HP ever had an adjustable slide, except maybe, just maybe the pre 1956 36 cubic inch 25's At least if my blonde hair is not over riding my common sense memory.

**jeff55vDSH** · 06-02-2007

Originally Posted by Mark75H

It took me a long time to learn the reason some guys use fixed ignition timing on modified Mercs ...

Interesting theory Sam.
I never saw any reason to "lock" the mag on my 4cylinder mod Merc. Although, it would have made the throttle setup a lot easier. The LAST thing I wanted was that thing to kick back while roping it over. Once I got overzealous with my Mod that wouldn't start. I opened the throttle nearly all the way, and that in turn, advanced the timing all the way. The thing kicked back, ripped the rope from my hands and shot it 50 yards out into the river! OUCH!

**JohnsonM50** · 06-02-2007

The 31.8 ci OMC s I mess with have 3in. pistons,, they can hurt ya when advanced to start.

**mercury400m** · 06-02-2007

lol jeff i bet you still felling that one today =p=p (so far mines been nice to me ...keyword SO FAR)

ok i understand now what brass piece your talking about now. didn't most
40's have a bent up clock spring that went through a hole and other side hooked on to a pin???? if im totally wrong let me know

**RichardKCMo** · 06-06-2007

Both of those can hurt a 10 yr. old if advanced past the start on the dial.
I don't know the degrees , but i know how much shorter
it makes your arm feel, next bay.
rfkcmo