O-470U suddenly idling 150 RPM high

[George Mohr]

Seemingly overnight. No other indications of any problems. Thoughts?

Thanks!
G

 

[Dan Thomas]

Seemingly overnight. No other indications of any problems. Thoughts?

Thanks!
G

1. Sticky tachometer. Needs calibrating, maybe replacing. The oil in the tiny bearing in it between the input shaft and the indicator needle gets dired out and sticky and the needle gets dragged to a higher reading.

2. Creeping idle mixture needle, creeping from a much-too-rich position to start with. Not likely.

3. Leaking induction manifold.

4. Loose, shifted magneto. Less likely.

5. MOST LIKELY: the throttle cable housing clamp has let the housing slip and the throttle control is no longer closing the throttle to the idle stop screw. Or the rod end on the control is loose and the wire shaft is backing out of it. Or the lever on the carb is loose.

Lots of possibilities. You need a mechanic to chase it.

 

[George Mohr]

Thanks,

1) EDM 900, so unlikely
2) Will do the mixture pull / raising RPM test next time im at the airport
3) All the EGT's look pretty normal, so hoping its not this
4) Same, as above, in flight everything is spot on. I doubt it's timing
5) So yeah, I agree its something with the linkage.

TY!!
G

 

[Clip4]

My vote is a induction leak.

 

[Stewartb]

I’d check the throttle arm and mixture arms very closely. It those are good I’d check the intake rubbers at the Y pipe. Leaks aren’t easy to spot. If your manifold pressure wandered a little on the last flight or two? Induction leak.

 

[George Mohr]

And the winner is..... induction leak! Two of the hose clamps on the silicone couplers in the intake manifold were very loose. Tightened and all is good. TY for the help, folks.

 

[Dan Thomas]

A 150 RPM rise indicates that the idle mix is likely set too rich. Consult the airframe manufacturer's service manual for the correct setting.