Large RPM drop during runup
Hi folks, yet another "large RPM drop" post.
The 912ULS in my RV-12 worked great until I replaced all the spark plugs at a recent oil change. Since then, it's been failing mag checks when I turn off ignition B. Most recently I replaced all the spark plugs again with new DCPR8E. I do my mag checks at 4000 rpm, and currently get a 1000 rpm drop when turning off B. Despite the 1000 rpm drop, the engine remains quite smooth at 3000 rpm. Things I've tried:
- Swapping the 6-pin connectors (new-style ignition modules). Swapping either connector makes it fail when turning off "A" instead. I think that means the problem is "downstream" of the CDI modules, pointing to coils/wire/spark plugs.
- Checked resistance of primary side of all 4 ignition coils. All read 0.1-0.2 ohms on my crappy meter (manual says it should be 0.25-0.35, but I suspect my meter just isn't accurate that far down). I wiggled the wires a bit to look for an intermittent open but it seems solid.
- Checked resistance from plug cap to plug cap of opposite spark plugs (e.g. 1T-3T, 2T-4T). All are 15k-17k, as expected.
Anything else I should be looking at?
