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dougknight

Well Known Member
I have a lyc O-320-D2A that was converted from an O-320-E2D by a reputable engine builder with 370 hrs TT on it.

160 hp w/ 8.5:1 compression ratio
carburated
Slick mags 4370 (right-0 lag) and 4371 (left-impulse-20*lag)
RV-9A
Sensenitch WOOD prop
PC-680 battery showing 12.5-13.4 volts

Data plate on the engine and the lycoming manual both say 25* BTDC for the advance timing of magnetos to crank, which it is. However I just broke my second skytec flyweight starter due to kickbacks and I'm thinking it is because the lag on the impulse magneto is only retardeing at 20* allowing it to fire at 5* BTDC on start-up.

My question is if I change this timing from 25* to 20* what if any are the adverse effects. Loss of HP? if so how much. Am I increasing my chances of detonation? Increase/decrease CHT/EGT?

If I were to change left mag to 20* should I also change the right mag to 20* or leave it at 25* BTDC.


I would love to change the left mag (4371) out for a 4373 which has the 25* lag built in but I just don't have the money for that right now and think this timing change would keep from breaking starters untill I can afford the swap out.

Any advise from the engine guru's?

thanks in advance
doug
 
Last edited:
Try it.

Set the left to 20 degrees, leave the right side at 25 degrees. Might be best to make the 25 degree one fire all the top plugs, and the 20 degree one fire all the bottom plugs.

You will see an increase in EGT, probably a small drop in CHT, small reduction in power (at the same fuel flow).

It will be less likely to detonate.
It will also idle slower, so you will need to readjust your idle, or just keep the throttle cracked a bit.
All these changes are going to be small, so I think you could try it for awhile, until you can afford to buy a mag with a 25-degree impulse coupling. Or buy a P-mag. or maybe a shop can change your impulse coupler on the mag you have?
 
There are an awful lot of setups out there just like yours that aren't breaking starters.

Does you engine crank like it supposed to (very important)?
Are you sure your RT mag is grounded during start?
 
scsmith said:
This is really important too! first thing to check.
Click to expand...

More specifically, if you are using a standard keyed ign. switch, be sure there is a ground jumper between the two terminal lugs that are very close together on the edge of the ign. switch.

If you have individual switches for the left and right mag., be sure you are only turning on the left for start.

Regardless of which system you have, make sure when configured for start that the P lead wire going to the right mag is connected to ground (sometimes the connections get reversed).

Your wood prop is a worst case situation, because it doesn't have the rotational inertia that metal props have.
 
steve: thanks! good info....what is the reasoning for the 20* to fire all bottom plugs? As is my left mag fires top plugs on #2,#4 and bottom plugs on #1,#3.


Walt, rvbuilder 2002: Right mag is disabled and ACS switch is correctly grounded. On the first broken starter I posted here and rvbuilder2002 enlightend me on the possibilites of the switch being the issue, even if it is wired correctly. I checked it out and it was indeed wired correctly (i'm electrician) but just to make sure I installed a manual switch in series with the ACS switch to keep the right mag grounded through start and allowed to come on line only after a good engine start on the left mag only. This would keep the right mag (@25* BTDC) from firing if the spring return happend to be released from "start" to "both" at just the wrong time.

doug
 
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