Est ignition modules and other questions on timing

Hi again, I've read that a module could be going bad and still allow engine to run. But may limit the engine to no more than 2500 RPM's. That could be my problem as that seems to be what's going on with my engine. Haven't read of that possibility anywhere until I visited a marine ignition module online site. Also read that it would mess up the timing to adjust the idle inlet screw with out being in base timing. Haven't heard that either. I assume they are referring to the air/fuel screw and not the idle speed screw.How does the tach influence the timing or advance ect.? Lastly, I noticed that engines with serial numbers #0L096999 and below, base timing is set at 1 degree BTDC. My 1993 3.0LX is #0F067528 so I think that is mine. But it also states for engines with serial numbers of 0L097000 to 0L0340999 are set at 1 degree ATDC. And numbers 0L341000 and above are set at 2 degrees ATDC.. So I guess it would be important to know your engine's number and set it's timing accordingly. Thanks to everyone out there for being there for us when we need guidance, and are lost.

The EST = electronic spark timing.

Keep in mind that there is no mechanical advancing with an EST system!
The module controls the progressive advance and the total advance.
BASE advance is a result of where the housing is indexed while in BASE mode.

Once out of base mode, the system controls the progressive advance as with any system.


Since we cannot create an electrical event any earlier than initiated, the indexing of the distributor includes all advance...... Base and progressive and TA.
The module then delays the spark event as per engine RPM via the prescribed advance algorithm.

So yes....... a module can fail and prevent good performance!
I'm thinking that you should be able to strobe your timing marks and view the advance as rpm is increased.

The Merc pros will hopefully chime in and share their experience as for what to look for and how to test.
They will also be able to explain the low speed circuit adjustment procedure.

As for the tachometer influencing ignition timing, I don't see how it could.



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1. Start engine and allow to reach operating temperature.

2. With engine running, install a jumper wire across the two WHITE leads on the distributor using (91-818812A1), or fabricate one using a 6 in. (150
mm) section of 16 gauge wire with two male bullet terminal ends connected.

3. Bypass the shift interrupt switch by disconnecting wires at shift interrupt switch. Temporarily join the engine harness wires together.

IMPORTANT: Do not fail to reconnect these two wires to the shift interrupt switch when timing procedures are complete.

4. With timing light connected, check timing. Timing should be 1° BTDC.

5. If required, loosen distributor hold down clamp and rotate distributor to obtain specified timing.

6. Secure distributor hold down clamp, and recheck timing as above.

7. Reconnect the two wires to the shift interrupt switch. Remove jumper wire at distributor white leads.

IMPORTANT: Be sure to remove jumper wire before returning engine to service, otherwise timing will not advance.


With timing light still connected, jumper removed from setting base timing and engine at
IDLE, verify that timing did advance to 12° BTDC, (plus or minus 2°).

At 2400-2800 RPM maximum (total) advance is obtained and should be 27°
BTDC (plus or minus 2°).