The Cessna 120-140 Association Forum

'47 C120 (C-90)

Our tach died, and we had it replaced (both the cable and the tach itself). On the first post-mx flight, we noticed that:

- It now takes full power to bring the RPM's to 2,000rpm on the ground, for a mag check (didn't use to).
- The mag check drops are barely perceptible (the drops used to be much more obvious)

At altitude, however, full power, level pitch, indicates 2,600 RPM (red line), which feels normal, but for some reason the RPM readings on the ground seem low.

Any advice for next steps in troubleshooting?

v

First thing I'd do is verify I'm really doing 2000 RPM with a Tru-Tach or similar.

1700 RPM is customary for the mag check. 2000 seems a little low for full throttle static, unless you have a wooden prop. 2450 is redline unless you have a Beech-Roby adjustable prop.

Forgot to mention ... I used a PROP TACH (visual RPM counter placed on the glare shield) and it showed 1,715 when the RPM indicated 1,500.

Do tachs require any adjustments? Could it be installation error?

Who made the tach? I'd contact them. I am not aware of different calibrations for the tach itself, only for the hour meter.

You said you could turn 2600 in cruise on the tach. What does the Prop Tach say then?

I used to sell Mitchel tachs. I found the product solid and the customer service very good. I don't get anything from them for saying this...

Although perhaps a little late for this, I have been routinely replacing mechanical tachs with EI R-1 electronic tachs when an analogue cable or gage fails. Does remove the "all needle" look but works very well. Plus your tach time can be set to TTAF if you order through EI. Nothing goes through firewall.