This is quoted from the 084 experimenters Yahoo group.

The engines were tested at TEN thousand feet at 107 degrees that they were
tested at.
> > These engines were run at full load for one thousand five hundred
> > hours, and THEN dynoed to insure that they pulled the rated power at
> > those conditions.
> > AFTER the dyno test, they were taken apart, and the parts were
> > measured for wear. If the sample engines did not BOTH make the rated
> > power, AND pass the wear test, the ENTIRE shipment of engines was sent
> > back.
> > It is safe to say that most of the members of this site know this,
> > but for the one or two absolute "newbies" that we may get in the
> > future, aviation engines are power rated at sea level, onder standard
> > temperature, (79 degrees I think), and pressure. This power falls off
> > as altitude rises. If you reverse extrapolate the power numbers from
> > the conditions that the engines were tested under, you will see that
> > the engines should pull between 40 and 45 horsepower at sea level.

Here is a pdf file of the 4A084A engine manual. You can either click on it a read it or left click and save it to your computer.

http://miniairboatassoc.com/084/4A084a.pdf
http://miniairboatassoc.com/084/4A084b.pdf
 http://miniairboatassoc.com/084/4A084c.pdf
http://miniairboatassoc.com/084/OCR.pdf

a. General.
Engine Model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 41084-2, -3 and -4
Type . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 4 cycle gasoline powered
Number of cylinders . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 4 (horizontally apposed)
Horse power rating . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 20 at 3600 r.p.m.
Fuel pump . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Diaphragm 3.0-4.5 psi outlet pressure
Air cleaner . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Dry models 4A084-2, -3 and -4
Spark plug . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. Shielded
Gap . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28-.033 in. (.071-.083 cm)
Magneto contact
Gap, Fairbanks Morse . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ..015-.019 in. (.030-.48 cm)
Gap, Slick Aircraft . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .008-.012 in. (.020-.030 cm)
Valve tappet clearance cold . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ..007-.009 in. (.017-.027 cm)
(Intake and exhaust)
Governor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Centrifugal
Oil capacity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 quarts (4 1/2 gts. with oil filter change)
Alternator . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40 volts alternating current (ac)
Starter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24 volts direct current (dc)
Regulator-Rectifier . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 28.5 ± .5 volts at 1.5 amps
Firing order . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ..1-4-2-3
b. Dimensions and Weight Models 4A084, -3 and -4.
Length . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 20 in. (50.8 cm)
Width . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28.50 in. (72.4 cm)
Height . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 22.10 in. (56.13 cm)