Sporadic RPM and GPH

[Mark Spickard]

I have an AF2500 (updated from ACS2002) in my RV-8/IO-390/Dual Lightspeed Plasma III with your Tach Interface Board. Lately the rpm starts to take a DC offset anywhere from 20-100 rpm and also fluctuates about 20-30 rpm. The engine itself is steady as is proven by a secondary tach (Lightspeed's rpm/MP/Deg meter). At the same time the GPH meter starts to read about 2 GPM high. Every once in awhile the problem goes away and everything reads steady and correct. I have tried turning a bunch of other electronic items on/off in flight to see if it has an effect but it doesn't. I replaced the main interface cable (from analog board to screen) recently due to other problems (see my other post) and everything has worked great until recently. I also recently rerouted my two Lightspeed power lines from the "always hot" buss straight back to the battery + terminal......shouldn't have made any difference. Any ideas?

[Shawn McGinnis]

It sounds like your RPM problems are being caused by additional noise. I would recommend checking your alternator ground, engine ground, or spark plug wires. One of the simplest tests is to take jumper cables and grab the engine / alternator and the airframe. To check the spark plug wires insert a multimeter set to continuity mode into both ends of the spark plug wire then gently pull on both ends to see if the meter stops beeping.