i believe the Renix system doesn't engage fully until 300 RPM to prevent dry starts. computer was designed by an aircraft manufacturer so they wanted to prevent damage to the engine.
once you get the hang of it you can let off the key half a second before it fires and save a few starter turns. almost an art form in an of itself.
thanks for posting the fix! many people just run off and forget about us XJ lovers and leave us wondering what happened.
'87 XJ Pioneer with 32" KM2s, etc.