I have a new to me RF94. It has an older Aim Mychron install. The RPM on the Aim Reads approx 3% to 4% lower than the mechanical tach. While it does not sound like much When the Mechanical Tach reads 6500 to Aim reads ~6250.
The lead for the Aim is installed on the coil to dist wire and some time the aim acts funny and the rpm justs at low speed.
What should I be looking for? At the last race I shifted at 6500 on the Mech tach as I did not want to over rev the engine.
Also as an aside, my number 4 cylinder is misfiring. The plug is brown/black and you can here the miss fire at speed. Not sure if it is related to the tach issue. I have changed the cap and wires but no difference. I am doing a leak down and compression test tonight.