Tachometer Problem

[Basstoon]

2007 ETEC 200 HO
E200DHLSUA 05177395

I replaced a blown OMC 2000/200 FICHT with the motor above. I'm utilizing all the original gauges including the original System-Check Tach. It is black with black trim ring, 0-8000rpms and 4 horizontal system check lights across the bottom.

Approximately 20 hours prior to the Ficht blowing I noticed the tach would jump sporadically, but only occasionally. It would jump from idling at 500rpms to 3000rpms and back to 500 without the motor sound changing. This happened at higher rpms when running as well. I had the Etec hung on the back and the tach continues to act up. Last week I took the rig in for the engine temp update and the motor checks out great with 63hrs. I asked for the rpm data and was surprised to find out that the max rpms I have turned with the Etec is 5000 for 4.2% of the total hours. I have no hours above that because I'm obviously over-propped with a 28p Raker II on a TR20. That is because my tach is showing about 5800 at WOT and sometimes 6100 at WOT. The Tech did some sort of check on the tach that involved him matching his gauge with the tach at 2000rpms and he said my tach is dead on.

I assume it is a gauge problem since it occurred with both engines. All connections are tight and corrosion free. The dial indicator on the back of the tach is set on "6P".

I cannot reprop until I find the bug in this tach. I know many of you deep pocket guys will say just to trouble shoot by replacing, but I'd like to make sure I'm not overlooking something simple. Any thoughts?

One last thing for you go-fast guys. I'm GPSing 74mph at only 5000 rpms on a 2000 TR-20 with a 10" plate. I'm also running 14psi H20 at those numbers. I can't wait to unleash another 800rpms.

[Sportwin]

Use a multi-meter set to HZ. Hook the red lead to the gray lead on the tach (sys check gauge) & black lead to the black wire on the tach. When you run the engine the multimeter will indicate RPM. You have to mentally move the decimal point one space to the right (i.e. the multimeter reads 550.0 = 5500 RPM).

[jimh]

Try this procedure:

--note the setting of the calibration control;
--using a screwdriver of the appropriately (small) size, move the calibration control of the tachometer back and forth throughout its range of adjustment;
--repeat several times;
--restore the calibration control to its initial setting.

This will wipe the contacts of the calibration control and may remove corrosion or intermittent contacts which could be affecting the operation of the tachmeter.

I have seen this procedure cure the problem with tachometer readings which you have described.

Also, sometimes a squirt of WD-40 can help to clean the contacts.

[Basstoon]

Thanks Sportwin and Jimh, I'll try these this next week and see what I can figure out. They both sound like good place to start.