[JonnyS]

Currently got an LAA inspector looking into this, but I wanted to see if anybody has any wisdom they can send our way.

Situation:
RV-6 with an O-320, used to start in two turns of the prop. Recently become much harder. Mag on bottom plugs and Lightspeed Plasma 1 on the top plugs.

Plenty of fuel is getting into the equation (electric pump and throttle driven accelerator pump in the carb are working).

Inspected all magneto driven plugs during cranking and they spark, but quite a weak spark so may struggle to start on those alone (this might be what is actually happening when it does eventually start). Impulse coupling is also engaging at the right time.

Checked all Plasma plugs during cranking and no visible spark... The crank sensors and cables appear to be fine.

The engine starts eventually and once running it is OK, isolate left and right on the key switch and no rough running, just the usual mag drop.

Working theory is the Lightspeed box isn't timing the spark properly during start, meaning we are trying to start the engine on the magneto driven plugs alone (hence why it is hard to start), but when turning above starting RPMs the Lightspeed box is performing as it should.

Has anyone experienced a similar problem with the LSE Plasma 1 box?

[T67M]

I'd look at the voltage getting to the Plasma unit from the battery. It will probably be a lot less than the nominal 12V when the starter is drawing 200A, and may be insufficient to allow the Plasma to operate. I know some aircraft with fancy ignition systems have a second (small) battery that directly powers the ignition during start up, but is otherwise not used.

A second thought, does the magneto switch ground out the Plasma during start - most (but not all) aircraft only run one "mag" while cranking. In that case, only the mag will be trying to start the engine, and a weak spark suggests that it may need some maintenance attention.