There could be several problems. 

First, make sure the engine is not running lean at idle.  Many folks don't realize that it is harder to get the correct mixture at idle than it is at wide open throttle.  One of the side effects of being lean is the engine builds up heat and may not recover when the throttle is opened.  A good idle mixture setting is indicated when you can pinch the fuel line and the engine picks up a couple of hundred rpm and then dies after a few more seconds. 

Second, if the engine is too rich at idle, fuel pools inside the engine and when the throttle is advanced the engine can't clear the liquid fuel charge in the case and the charge may interfere with the flow of the incoming atomized charge.  If this is the problem, when the throttle is wide open the intake sound (you can hear it because the exhaust sound hasn't come up yet) has a kind of "hollow" sound to it.  If the mixture is really rich, sometimes pinching the fuel line won't stop the engine for a very long time.  It might not even speed up any.  The engine can end up with a real rich idle mixture when chasing a low idle speed.  Be sure that the idle speed you are looking for is reasonable.  The idle speed only needs to be low enough that the model will not maintain level flight, well maybe a little lower so you have a controlled decent.  For a 35 size Carrier model (say 2.5 pounds) and 350 sq inches this rpm is around 3500 rpm with a 6" pitch prop.  You may find that the idle rpm only needs to be around 5000 rpm on a 15 size model to be flyable.

Basically you have to find a lean idle mixture then work back to a slightly rich mixture setting.  Always set the idle before moving to the mid-range (if there is a mid-range adjustment) and then on to the high speed mixture.

Third, many carbs have a lean mid-range.  This manifests itself by poor throttle response in the mid range.  Various Perry style carbs are known to have this problem.  If you remove the idle disk you will notice that there is a round hole with a "v" shape on both sides of the hole.  The shape (size) of the "v" sets the mixture at idle and the mid-range.  The hole (with the high speed needle) sets the high speed mixture.   If the engine is really going lean in the mid-range, opening the "v" slightly will enrichen the mixture.  Be careful, a little change here is a lot of mixture change.  A couple of spare idle disks is probably a good idea if you want to try filing the "v".  By the way, pay attention to the o-ring on the idle mixture disk.  If it is bad, nicked, worn, or just plain old it will leak air and cause a lean mixture.  A symptom of a bad o-ring is a changing idle mixture even when you don't touch the idle disk.

Good luck,

Dave

Thanks for he help.  You answers have triggered a few more questions.

These Perry carbs are unused units that came with the engines in the 1978-1983 era.

Fuel, I wasn't sure if I could use funny fuel at the Brodak, I'll try more than 10%.

How do you remove the idle disk?  It looked like a factory press fit to me. 

Do people actually get these Perry carbs to work on Cox Conquests, or do they change to something else?

Another issue:  The spring that's supposed to keep the needle set doesn't get it done.  Is there any fix beyond merely stretching it out a little?

If you got an engine for less than $40 you got a deal!
The guy that answers the phone at Perry is the owner I believe. I don't know his name but he is very helpful and will work with you to make any size you want.
The needle with the spring is an older design, the new one uses an o ring inside and is not interchangeable.
The mixture disc is held in place with a veeery small c clip. The small o ring on the disc also helps retain the disc by its tight fit. Remove the c clip and pull and twist the disc until it pops out. The o ring has most likely hardened and taken a set. Replace it with a new one ( if you can) or a new disc which is easier.
RJL (MECOA) makes parts for these engines if you need any engine parts.     Mike

these are the .15 carrier planes published in the model magazines;

A-7E Corsair mk.II *1994 Model Aviation Ag p43  *34.00*206.00*.15    *Baardsen, Steve
AD Skyraider       *1989 Model Aviation Ap p71  *30.00*151.40*.15    *Lidberg, Al A
Curtiss XF-13C             *1990 Model Aviation Jn p77  *28.00*168.00*.15    *Schuette, Melvin
OS2U Kingfisher    *1994 Model Aviation Jn p44  *35.00*202.00*.15    *Baardsen, Steve
SB2C-1 Helldiver   *1971 American Aircraft Modeler Jl p26 *30.50*162.00*.15    *Schaaf, Paul H Jr
SBD Dauntless      *1984 Model Aviation Nv p73  *28.75*161.00*.15    *Deneau, Gerold
SO3C Seagull       *1993 Flying Models My p46  *30.00*198.50*.15    *Dinerman, Steve

Tom Wilk
www.cpinternet.com/~tawilk36  Stir the pot