Hi, I have limited knowledge in this field, so looking for advice on the following issue.

I have a Countax C300H ride-on mower with a Honda GXV530 V-twin engine. It has recently been serviced and has been running well until a few days ago when it started to display inconsistent RPM for the engine speed on the dashboard.

The engine appears to be running fine for instance no noticeable loss of power and with an untrained ear sounds normal to me. The engine starts first time with no issues, but the dashboard displays the engine speed around 1600RPM and then approximately 30 seconds later goes to around 3200RPM (this is the RPM I used to expect) then within a minute it will drop again and then increase again.

With the dashboard showing either low or high RPM's I don't notice a difference in sound and can't see an obvious decrease in the flywheel to reflect the reduced RPM. This cycle continues to happen even with the ride-on mower is in a stationary position and without have the cutter or sweeper engaged.

I'm wondering if this is perhaps linked to a faulty sensor and the RPM value displayed is incorrect, but despite several searches in Google on this issue I'm at a bit of a dead end and hoping for someone to point me in the right direction on how to diagnose this fault?

Note: In addition to the recent service the carburettor was also cleaned as the mower was backfiring.

Hi bails100, and welcome to TGMF.

Sounds like your PCB is playing up, it reads the rpm from the ignition system so if it's running ok with noticeable difference in revs then I would say t's the main board in the dash.  You could try reprograming it but I doubt it will make a difference as this seems like a intermittent fault.

https://help.gardentractorspares.co.uk/help/cts/ser/wlde-pcb-program.pdf