I recently changed my engine from a Blade 954 to a CBR1000RR and am now having issues with erratic tacho reading on my ETB Digidash 2. This previously worked perfectly with the old engine.

The dash takes a feed direct from the ecu and the revs are accurate when the rectifier is not plugged in. With the rectifier plugged in, they seem generally accurate at idle but then jump all over the place (even up to 20k+) when above idle. The engine runs perfectly so believe that the feed from the ECU is correct.

I have swapped the rectifier and still got the same result. I have also moved the rectifier output feed from the permanent live busbar in the fuse box to the battery positive which may have made a slight difference.

Could this be interference from some wiring or the digidash picking up noise in the ECU signal? Any help appreciated!

yes it can be 'interference'
the most common reason for this is the systems using different grounding points, so you get weird 'spikes'
you need an explicit ecu ground circuit.

I was having trouble getting the rev counter to read properly on my R1 Locost. Not a digi dash, just a Smiths electronic rev counter but when connected to the ECU dedicated tacho out signal wire it was highly erratic. For a simple solution the night before a track day I just connected the rev counter signal input wire to one of the coil primaries and it has worked fine since with a solid readout. I had to set the rev counter to the correct pulse count as first off it was showing half the revs the ECU was showing (on the laptop) but the Smiths has a way of telling it how many cylinders and changing that from four to two (wasted spark coil pack) brought it back.

I agree that the problem is probably interference as the dedicated tacho out is probably a low output signal and the wire wasn't shielded but the coil primary is a much stronger signal so harder to mess up with interference?

I don’t really want to have to hack into the engine loom if I can at all avoid it.

The main fuse box feed does run quite close to tacho feed under the scuttle so may try relocating that and some shielding.

I fitted a 1N4007 diode into the feed to the dash which seems to have resolved the problem.