UI’m wondering if you could help me with some clarification of an
article that appeared in a July 2009 MAN Engine Clinic. I saved this
particular article when I purchased an O.S. 120 Surpass III pumped
engine around that same time. I’m finally getting around to installing the
new-in-box engine in a project this spring, and I want to be prepared
to adjust the engine if necessary. Your “Tip of the Month” described an
alternate technique to tweak the pump pressure if it ran rich at idle and
through the midrange due to the pump supplying too much pressure.
You described a technique to adjust the full mixture on the 1.20’s carb
by “removing the idle mixture adjustment screw and manually moving
the notched mixture lever slightly leaner.” Here are my questions. Is
“notched mixture lever” in reference to the throttle arm? Does “slightly
leaner” mean in a direction that opens the venturi slightly to allow
more air into the mixture? I’ve occasionally had a similar problem with a
pumped O.S. .91, so when I rediscovered this article packed into the 120’s
box, l considered trying the same adjustment with that engine. Thanks
in advance for any information you can supply.
—Jim McCoy, via email
Answer: Jim, I’m afraid you made a mistake in assuming that the
O.S. 120 III used the same carburetor as the 120 II; it does not. They use
two entirely different carburetors, with different methods of adjusting
the idle and midrange mixtures, as pointed out in the article. The 120 II
uses an eccentric screw that moves a lever with limited movement.
Removing the eccentric screw allows the lever to be manually moved
farther. Clockwise leans the mixture and counterclockwise richens.
Our next letter is actually a two-parter: the writer Bob Davis’s original
letter (and my reply) and Bob’s follow-up.
UI have a new Thunder Tiger 75 four-stroke that’s driving me crazy.
I have it installed upright on a Phaeton II biplane—and it isn’t my first
rodeo with operating four-stroke engines. My problem is a lack of
power. I can only get a reliable high end of 7900rpm using a 13x6 Master
Airscrew prop. A 12x6 only yielded an increase of about 300rpm. I’ve
tried different glow plugs to include an F and Thunderbolt four-stroke
plug, checked and adjusted the valves after break-in, and checked the
valve spring tension. Compression is great: It starts easily, idles reliably,
and runs smoothly. It just won’t “wind up” to the advertised 10,000 or
so rpm. I tried running it on a test stand to ensure this wasn’t a fuel
on Hollywood Boulevard in Hollywood, California, in the mid-1930s.
(The Hollywood Freeway now passes through where the shop once
stood.) Denny decided to add an engine to his line of products and
started looking for someone to manufacture it. One stipulation was
that the engine had to run continuously for 50 hours—something full-size aircraft engine had to do for licensing. Several people submitted
designs, but Walter Righter’s design was chosen and given the name
“Dennymite.” It was later named the “Dennymite Airstream,” due to the
teardrop-shaped cylinder. The engine had a displacement of .57ci and
appeared in 1937. Production continued until 1945, when the company
was sold to Pacific Aeromotive Corp. in Burbank, California. The
displacement was raised to .5588ci, it was given a round cylinder, and
the name was changed to the “Meteor 60.” Then, in 1948, production
rights were sold to Ohlsson & Rice (O&R) who went back to the original
.57-size engine with the teardrop cylinder and “Airstream” name.
Whether O&R did not receive the rights to the Meteor engine and name
I do not know, but I assume it must have received a large inventory of
the original Airstream parts.
During WW II, Walter Righter manufactured a 45.6ci, 20hp,
horizontally opposed twin for RC target drone aircraft. Along with
Reginald Denny, he and a man named Whitley Collins (the main financial
contributor along with some other investors) formed the Radioplane
Co. to build RC drone aircraft. One of their employees was a young lady
named Norma Jean Baker, who later became Marilyn Monroe, the movie
actress. A little side note: In the mid-1950s while making my Lee 45s,
my wife had a friend named Fran Tucker. When Fran found out I was
making model engines, she told my wife that her dad had also made
model engines. Guess who that was? Walter Righter. She also went on
to say that, as a small child, she used to sit on a stool next to her dad
and watch him assemble engines. When she got a little older, she also
started assembling engines until WW II, when model engine production
was put on hold.
The O.S. 120 Surpass II lever-operated idle and midrange mixture control
carburetor was exclusive to the 120S II and was not used on later models.