: 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.

I'd say this later-model PA28 cowling is a "good" one. In any event,
I'd think prop-wash-induced boost air would show up as a manifold pressure
increase, and uneven fuel distribution as dissimilar EGT. Neither is present.

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

Yes... each year for annual. Brand new mufflers last year for other
reasons... No change.

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

We've put new points and condensers in both, including internally (and
of course externally) timing them. No change. I've done full-power mag checks
and misfires would be apparent in that case. New plugs make no change, either.

: 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?

Venturi AD's checked and CW. Restriction or mixture would show up as low
MP or abnormal EGTs anyway, no? Neither is the present.

Thanks,
-Cory

--

************************************************** ***********************
* Cory Papenfuss, Ph.D. Electrical Engineering, PPSEL-IA *
* Research Associate, Vibrations and Acoustics Laboratory *
* Mechanical Engineering *
* Virginia Polytechnic Institute and State University *
************************************************** ***********************