Ok John I'll take a crack at this. I don't know what type of instrument you are using as a tachometer, but I can relate an experience of mine that might help. I use a Rocky Mountain Instruments Engine monitor and connect the tack input directly to the Mazda coil tach output wire. When I first started running my engine the RPM reading was fine up to about 4500 and then as the engine RPM increased above this, the tach would read about half what it should and would bounce all over the place.
Luckily I had a schematic of the Monitor and was able to trace the problem. It turned out to be a glitch suppression circuit that was tuned to cut off pulses above those that would be produced at about 4500 RPM. This engine monitor was targeted to the aircraft engine population and so the designer did not consider higher RPM's. A simple change in a resistor raised the glitch suppression up to about
the 8000 RPM range. Problem solved. I mentioned all this just to say that it could be spurious signals (glitches) on the line causing the RPM's to jump around. If there is not a suppression circuit this could show up as unstable RPM readings. Luckily for you it only happens on the ground. You can check the signal to the tach with an oscilloscope to see what it looks like. It should be a clean pulse train of equal sized pulses corresponding to the RPM. (The pulses get closer together as the RPM increases) Make sure that the tach signal wire is not routed along side of any other wires, especially those that have pulses on them like the injector or ignitor wires. Signal wires such as this should also be shielded with the shield grounded at one end. Hope this helps. Jim
John Slade
<sladerj@sbcglobal.net> wrote:
I've noticed something that might be a problem, or might be leading to
one, and was wondering if anyone had any ideas on the cause....
During high power run-up my rpm reading on the EC2 sometimes "bounces
around". Rpm reading is always steady at 3500, and at 5500 or whatever
in flight. Sometimes, when I increase throttle during runup it'll get to
around 4500, then it'll start changing reading rapidly (2 or 3 times a
second) between 4500 and somewhere above 5000. Other times it'll sit
still at 5300 or so. I don't see any performance change. I know the rpm
is powered from injector pulses. What could make the injector pulses
bounce like that?
John Slade
Turbo rotary Cozy
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