Home> Continental & Lycoming TroubleShoot
Hunting or wavering tachometer. Tach needle wavers
1. Hunting is a cyclic or constant variation of engine speed, above and
below that desired. It will be shown by constant wavering of the tachometer
needle. Surging is a wide or excessive increase or decrease in engine speed
followed by return to the established speed after one or more oscillations. Hunting is seldom caused by the propeller itself. The problems is
usually caused by the tachometer, governor, uneven fuel flow, flat hydraulic
lifters, broken valve springs, etc. This may be best checked in the air
by setting up full throttle (if your operating manual allows), full high
rpm settings, and then gradually reducing manifold pressure several inches
until rpm drops slightly. This will positively put the propeller on the
low pitch stop and will not be controlling. After the throttle is reduced
and the speed stabilized, the needle can be observed for the wavering or
hunting condition. Hunting can be caused by the tachometer. Mechanical
tachometers get old and stiff. They are seldom if ever lubricated. Slight
binding causes the needle to waver. Kinks or tight bends in the tachometer
cable can cause the needle to waver.
Leaking tachometer drive seal. Oil works its way up the tach cable into the tach
caused intermittent binding shown as large fluctuations
|
