The Boating Forum - Checking RPM's with photo tach
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11-21-2011, 01:20 PM
I am about to check the RPM's of my 6LP Yanmar with a photo tach to verify my gauge is accurate. Where should I place the reflective tape? I am assuming on the harmonic dampener?
Any help would be appreciated
11-21-2011, 01:21 PM
Yep, that's correct.
11-21-2011, 01:25 PM
Main flywheel. My tach reads about 150 rpm's too high across the full range of rpm's.
11-21-2011, 01:41 PM
I had similar variances on my Yanmars too.
It's an informative exercise to perform (not trusting the tachs) especially, when surveying a new ride, to ensure propped correctly, engine performing well, etc.
11-21-2011, 02:15 PM
On my Yanmar 200HPs.....The port was 100 off and the starboard was 150 off. I was told almost all Yanmars are 100 off but my ears told me something wasn't quite right and the photo tach proved it. Sticker on the harmonic ballancer............
11-21-2011, 02:17 PM
According to Yanmar they actually say that their engines should run WOT under a load about 150-200 RPM's higher than what the manual says. My engine calls for 3800 max WOT and my gauge is saying 3750 so before I have my prop worked on I want to make sure the tach is accurate.
11-21-2011, 03:47 PM
Cheap and accurate method..shine a 120 volt fluorescent light on the flywheel..at 3600 rpms/1800 rpms (60 cycles per second) the flywheel will appear to stand still.