Does the SPE have a fully-shielded plug cap like the ZDZs and many other gassers intended for model use?

If not, I suspect that at least some of the noise is coming from the conductor that passes through the plug insulator, since this will be radiating spark noise like an aerial.

If ignition noise is a problem then don't mount your CDI battery any further back than is absolutely necessary -- it too will be radiating noise.

And don't get too confident about the PCM filtering out the noise -- it does, but onlyup to a certain point. After that, it goes into failsafe and you have zero control.

The reason you're not seeing any glitching with PCM is that the bits of data that are clobbered by the engine noise are being disgarded and only the unaffected bits of signal are being used by the receiver. This doesn't mean the noise is less or has gone away, just that your receiver is picking what it can out of the noise/data and ignoring the rest.

This can lead to complacency and leave you very disappointed (with a busted plane) when you discover that, at an inopportune momement, there simply wasn't enough good bits getting through to provide adequate control.

Do your very *best* to eliminate as much noise as you can, don't just rely on the magic of PCM.

A shielded plug cap will help some.