Hello Bill. Have you tried hotter plugs?
I don't want to claim any particular expertise
with OK Cubs, but I seem to have a different
approach to them. I just want to tell you
why I like these engines.
As my previous post show, I am using
a 1953 "B" to pull a 4.2 oz Musciano
F-51 Mustang. I'm trying for good Stunt
performance. I am running 35% nitro
(no nitro restrictions here) and 25% castor.
On most stunt engines, it is normal to
sacrifice pure power for steady runs by
lowering the compression. The Cub naturally
has low compression and gentle port timing.
By matching the size and weight of the
airframe to the engine, the Cub is a
decent stunt performer. It is consistant
and that is important for stunt.
(As we all know.)
As a stunt engine, the Cubs have some
advantages over the Cox's, mostly because
of the tank.
A Cox tank places the engine weight
further forward than is optimum for a
Musciano design. Shortening the log by
1/2" is stretching the rules and who
likes to add tail weight?
Also, being stuck with only two choices
of Cox tank sizes really limits your options.
Using a balloon tank for the Cub,
lets me place the weight of the fuel directly
over the CG, and as it burns, the CG/balance
doesn't change.
I've watched You-tube movies of Cox
engine/Musciano flights, so I know that
it is possible to get excellent flights from
Cox engines, but I find reed valve engines
less predictable and when they do run
badly, they are much more difficult to fix
than venturi engines.
Don't get me wrong, I love Cox engines
and I have eleven of them but I think
the Cub is a more manageable engine when
maximum power isn't the only objective.
Cheers!