On Oct 10, 9:38*am, wrote:

* * * * Good thought, although I'm not sure that I buy into the propwash making pulsating
ram air. *In any event, we re-indexed the prop awhile back and it made no effect on either
vibration or power output. *The irritating thing is that the PA28 service manual specifies
the prop indexing in an ambiguous way. *It says to put it on at 2 o-clock and 8 o-clock
at TDC. *That's not an option though... the 6-holes line up at either the 12-6 or 10-4
position. *Either position is halfway incorrect (but we've done both to no effect). *Even
if it did have an effect, it would show up as an increase in manifold pressure.

Prop blast can have an effect on the carb, but usually only on
installations with poor cowling designs or open intakes. Older 182s
and some 172s, for instance, will run roughly with the cowl off, and
run fine after it's installed. With it off, the air pulses off the
prop will strike the airbox inlet and upset the pressures in the
venturi, upsetting fuel flow from the nozzle and creating uneven
mixtures to various cylinders. With the cowl on, the air flows PAST
the intake filter rather than AGAINST it and the pressures aren't
disturbed so much.
Some homebuilders have had problems with this and turning the
intake away from the front fixes it.

Has anyone looked at your mufflers for loose baffling that might
be blocking the oulets?

Have the mags been checked? Electrical problems are the biggest
single source of trouble.

If the carb is a Marvel Schebler/Precision Airmotive unit, have
the ADs been done with regard to the two-piece venturi and fuel
nozzle?

Dan