I know some of you guy have gotten a cub to run well. how do you do it.
I tried 5 and 10 % nitro all castor on the two engines I have and one runs so so and the other runs hot.
I took my time breaking them in and I used a cox 5 1/2 X 3 prop.
its a brodak plane that weighs 5.2 ounces.
30 ft  .008 lines and they can't get out of their own way.
I KNOW I had one as a kid that worked much better.
can I drill out the intake a little?

no bind, they flip over nice
and the first thing I did was check for air leaks, didn't find any.
 I'll try a few more break in tanks and see if they get better.

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!

I want to thank everyone for the suggestions and if I can get some time this weekend I am going try a few out

The normal curve distribution for OK Cub engines is very broad, given the more casual quality control practices vs, say, Cox. The guys from Philly that used to come to the VA Musciano Meet indeed had very good Cubs, but they had to choose from a big sample to come up with one or two that ran well. Also, they did some "fitting', moving the shaft from Cub engine #1 to Cub #6, and fitting the piston from #2 to the cylinder of #11. That sort of thing.

Goozgog, I am also scratch building the F-51 for this year. This is the 23" early version; they later cut it back to 21" to save a penny or so. This is the second one I have built. The first had a Wen Mac on it, because the rules in those days gave you bonus points for the "iffy" engines such as WM. Only flew it a couple of times, so it never had a chance with one of those (kind of underpowered) engines. This time, it will have a Black Widow, and I don't worry too much about the CG being a bit more forward vs the WM, which had a Perfect stunt tank within the fuselage.

As luck would have it, I stumbled on an actual kit F-51 with the 23" span at a local swap meet. Of course, it had a flat foil bottom, where my scratch ones have some semblance of symmetricality.  I use the overall shape of the kit model to help me build the scratch one.

Currell

Hi Goozgog: A couple of comments on your last post...

The Musciano Rules are silent on the subject of vertical leadout readjustment, and you certainly are not "stretching the rules" by lining up your leads with the vertical CG, or changing them from what is shown on the plans. I've done maybe 10 Muscianos (Va and Mi), and no one worries about this. What is important: outlines, shapes, solid wing, solid fuselage (although you can build in laminates here). You can adjust leadouts as specifically stated in the rules (here, the meaning is adjustable in the plan view), but you cannot bury them in the wing, by the way.

Rules also are reasonable re LG and cowls. You don't necessarily have to fly with them, even though you may have them for Concours, which of course makes sense from a judging standpoint.

As far as airfoils are concerned, the rules do in fact specifically allow an airfoil design at the builders discretion. As far as thickness is concerned, you cannot increase the size. However, if you are building a 50s Scientific with the typical 3/16"+ foil, and wanted to slim it to 1/8" for Speed, I doubt if anyone would object. No one, to my knowledge, as done this, and of course there are around 50 or so models eligible with the 1/8" foil anyway, so why bother. This could be a gray area, but, again, I don't know of anyone who would test it. The thicker foils usually were used with the larger (than the typical 18") spans, so they are less suited to Speed, anyway.

Agree that lightness is the key. My first F-51 was light, but powered by the wrong motor, a Wen Mac. This next scratch F-51 (and the first one, too) now will be powered by a Black Widow.

It was kind of interesting that the Bel Air, with a hollowed, clamshell wing and buried leads, was allowed for a number of years. But, no longer. The Bel Air makes a nice Unlimited, however, and it would be neat to scale it up 2:1.

Currell