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small tach fluctuations

P

prkaye

Well Known Member
I've recently started noticing that my tach (Vans mechanical transducer into my D-180) has started fluctuating within a range of plus/minus 10 to 20 RPM, regardless of power setting). I can't say for sure how new this behaviour is, but I've only recently really started noticing it.
A couple of things - I very recently reamed the jet in my carb up two sizes (up to #40) to improve fuel flow (only worked marginally, increased by about 1GPH). Also, when investgating the tach issue I found the fitting on the tach transducer had come loose from vibration, so I tightened that down.
I don't notice any fluctiations in engine sound or power. Performance is normal. Fuel flow readings are fairly steady.
Questions:

1) is it possible that reaming my main jet has caused fuel to flow in way that is causing these small rapid fluctuations in my RPM?

2) I've read on other threads about people having the mechanical transducers fail, but in all cases I read about the initial symptom was a significant RPM fluctuation, much greater than a 20RPM change. Could my small fluctuations be early warning signs of a failing transducer?

3) Is there any other potential engine issue that I should be concerned about that could be causing this? Or is it almost certainly an instrumentation problem?
 
Oscillating Prop (if constant speed)

If it is a constant speed prop it might be slack in your prop control cable. Does it continue oscillating after you pull the prop back a little? If not, tighten the cable a couple turns.
 
Well, I replaced the Mechanical Tach Transducer (vans' unit). Still see those fluctuations. I can't hear or feel any power fluctuations, so I figure it must be an indication problem. I'm at a loss... I guess if I care that much I could use the Magneto to feed the tach instead of the mechanical transducer. Not sure I care that much though if it's just an instrumentation problem. The fluctuations don't pose a problem for me in flight.
 
Cable

The cables tend to wear on mechanical tachs. First, I would borrow an optical tach and read it through the windshield when you go fly. If you're not hearing it then it's probably not really fluctuating.

Vic
 
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