Some of us early in the RV-12 program have engines that were furnished with green label ignition modules and these preceded the newer yellow label modules that are sometimes referred as "soft start". I was curious how soft start actually works and so looked it up in section 74 of the heavy maintenance manual.
Once started the 912ULS engine runs with 26º Before TDC. It could not be started at this advance so the modules combined with cams on the flywheel determine retarded timing advance for starting. The engine ignition timing is actually controlled by the edge of trigger cams on the flywheel and the modules can time when those trigger points are used. Note that there were 2 flywheels used on the 912ULS. At startup the early flywheel will time at 4º Before TDC with or without the soft start modules installed. The later flywheel will time, at start, 3º After TDC either with or without the soft start modules. So, it is the flywheel cams that provide the soft start. The soft start ignition modules compliment this by delaying the change to full advance by 3-8 seconds.
So the early engine starts with 4º BTDC and then advances to the full 26º at 650-1000 RPM and that must happen very quickly. With soft-start modules on this engine with the early flywheel the advance from 4º BTDC to 26º BTDC is delayed by 3-8 seconds. This provides a "semi?" soft-start.
With the later 966872 flywheel the engine starts at 3º ATDC and switches to full advance after 3-8 seconds aiding the soft-start.
If you have an early non soft-start engine you would need to have the new modules and the new flywheel to get the full effects of soft-start.
Here is the data table:
Once started the 912ULS engine runs with 26º Before TDC. It could not be started at this advance so the modules combined with cams on the flywheel determine retarded timing advance for starting. The engine ignition timing is actually controlled by the edge of trigger cams on the flywheel and the modules can time when those trigger points are used. Note that there were 2 flywheels used on the 912ULS. At startup the early flywheel will time at 4º Before TDC with or without the soft start modules installed. The later flywheel will time, at start, 3º After TDC either with or without the soft start modules. So, it is the flywheel cams that provide the soft start. The soft start ignition modules compliment this by delaying the change to full advance by 3-8 seconds.
So the early engine starts with 4º BTDC and then advances to the full 26º at 650-1000 RPM and that must happen very quickly. With soft-start modules on this engine with the early flywheel the advance from 4º BTDC to 26º BTDC is delayed by 3-8 seconds. This provides a "semi?" soft-start.
With the later 966872 flywheel the engine starts at 3º ATDC and switches to full advance after 3-8 seconds aiding the soft-start.
If you have an early non soft-start engine you would need to have the new modules and the new flywheel to get the full effects of soft-start.
Here is the data table:
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