RPM signal noise

I have been having an issue with my aftermarket tach.(Speed Hut) I am getting noise that is disrupting it, how ever it does it in a very unusual way.

The tach works normally on start up and will work fine unless I rev the engine over about 6k RPM after that the needle will wander and register lower in general for the rest of the ride. I believe the ignition is the source of the problem. I am running a Dyna S, Dyna rev limiter, #3 Ohm green coils, Suppression Wires. and B8ES plugs.

I am wondering if i need to use a resistor type plug but I cannot determine if that will be an issue with these plug wires. It specifically states to not use a resistor cap, but is that true for the plug as well?

I installed a "noise reducer" from Speed Hut and that seemed to help but it didnt fix the problem.

The Dyna S uses a fixed dwell angle somewhere over 300 degrees (crank degrees). So it only lets the coil rest for a short period. That is somewhere under 60 degrees. That means as the RPMs increase, the coil's rest-time decreases. Some tachs require a certain amount of rest time in order to register a signal pulse. If that rest time is too short, it won't register a signal pulse.

After the coil fires a spark, there is a very short period of time where the coil "rings". That is a brief oscillation in the coil signal which dissipates the last remnants of the coil energy after a spark happens. Sometimes, if the rest-time is too short, the entire rest time is taken up with this "ring" oscillation. The tach may not interpret that oscillation as a single pulse, but will see multiple pulses.

So it can be noise, as there is more electrical noise at higher RPMs, or it can be the rest-time of the coil signal is too short where the tach doesn't acknowledge enough pulses, or it can be the "ring" which the tach acknowledges as too many pulses. Or it can be any combination of the problems which can make for a very erratic tach operation.

You can filter the noise and the "ring" using a low-pass filter. That only requires a cap and resistor, and maybe a diode. If the problem is the rest-time is too short, then you need an edge detector. That is a circuit that gives a short pulse when it sees (in this case) a downward going voltage pulse, but will not wait for the upward-going pulse.

I assume this is a 4-cylinder bike. Is the tach signal only going from one coil?

Generally speaking you only need to use one resistor element... resistor plug, OR resistor cap, OR resistor wire. Using more than one will reduce spark energy considerably. (Using one also reduces spark energy a bit, but is usually not noticeable in normal street use.) Using a second resistance element will dampen the coil "ring" a bit and will reduce noise, so maybe give it a try, at least as a troubleshooting tool.

Yes this is for my kz1000. I'm using the same tach on my 750 twin with no problems but that uses a C5 optical ignition. I believe the noise filter Speedhut supplied me with is what you describe, its a small bundle wired in between the tach and the coil. The tach is only sensing from one coil. What is this edge detector you describe?

Had the same trouble like you DoctoRot. I fix my jumping tacho needle with some signal converter.



kzrider.com/forum/4-electrical/607690-de...lectronic-tachometer
www.kzrider.com/forum/4-electrical/61087...age-nightmare#805851
www.zmm-electronic.de/ignition-signal-ch...:44.html?language=en

An edge detector is a circuit that detects the upward swing, or downward swing of a voltage signal. For the purpose of tachometer, it would be used to trigger a pre-determined pulse every time it detects a downward pulse on the ignition's primary voltage. That pre-determined pulse is fed to the tach instead of the primary signal.

From the description of the device (last link in the post) that Scirocco posted , I am guessing it is something like what I describe above. The english language description is not very detailed., but it might be worth a try.