Piper WARRIOR III PA-28-161 Pilot Operating Handbook page 50

SECTION 3
EMERGENCY PROCEDURES
3.31 ENGINE ROUGHNESS
Engine roughness is usually due to carburetor icing which is indicated by
a drop in RPM, and may be accompanied by a slight loss of airspeed or
altitude. If too much ice is allowed to accumulate, restoration of full power
may not be possible; therefore, prompt action is required.
Turn carburetor heat on (see Note). RPM will decrease slightly and
roughness will increase. Wait for a decrease in engine roughness or an
increase in RPM, indicating ice removal. If there is no change in approx-
imately one minute, return the carburetor heat to OFF.
If the engine is still rough, adjust the mixture for maximum smoothness.
The engine will run rough if the mixture is too rich or too lean. The electric
fuel pump should be switched to ON and the fuel selector switched to the
other tank to see if fuel contamination is the problem. Check the engine
gauges for abnormal readings. If any gauge readings are abnormal, proceed
accordingly. Move the magneto switch to L then to R, then back to BOTH.
If operation is satisfactory on either magneto, proceed on that magneto, at
reduced power, with mixture full RICH, to a landing at the first available
airport.
If roughness persists, prepare for a precautionary landing at pilot's
discretion.
Partial carburetor heat may be worse than no
heat at all, since it may melt part of the ice
which will refreeze in the intake system. There-
fore when using carburetor heat always use full
heat; and, when ice is removed, return the
control to the full cold position.
REPORT: VB-1565
3-16
PIPER AIRCRAFT CORPORATION
PA-28-161, WARRIOR III
NOTE
ISSUED: JULY 1, 1994