Reenie, forgive me if you've already said and I've forgotten, but which of their engines are you presently running in?

Furthermore; you mentioned dialing down the pump as much as possible; about which pump are you speaking?

Anyway, I've gotten busy with work once again, which means my project is sidelined. Thus, I am living vicariously through your experience. As a consequence, my engine sits - unloved - within its box upon a shelf in my shop following my unboxing to judge the value of removing the existing dummy off a model I had hopes would be a suitable candidate. And yes, this means I purchased the engine before giving consideration to what aircraft it should be fitted. Oh well, not the first time my being queer for engines has resulted in the model coming after the fact (and unlikely to be the last).

So this is my engine besides the Zirolli PT-17 Stearman (1/5th scale) I subsequently purchased in hopes it would be suitable. It's a near perfect substitution size-wise for the Williams Brothers dummy radial with which it is fitted. And yes, the exposed valve train is wrong for the aircraft (and in all honesty, I sincerely wish UMS engineers would trouble themselves redesigning the valvetrain to fully enclose it), but nevertheless, wouldn't it look magnificent exposed this way? Especially because this would make it easier to maintain. Sadly, I don't think I can make this model balance with the engine.

I got myself an UMS 5-100. And the pump I was referring to is a RCEXL smoke / fuel pump. This has a constant but adjustable output and is of the membrane style. The moment I switch on the ignition the pump starts running. This pump cycles the gasoline in a small circuit and the carburetor is connected to this circuit via a T-piece. The flow is set as low as possible so that the pump still runs stable, even if I choke the flow.

My UMS is meant for a Blackhorse Corsair. Balancing it will also be a challenge. I would go for the PT17 if I were you; gorgeous combination!

These 2 1/5 stearmans require 1 lb each to balance with 6 pound radials. Balance is possible. Tail weight is not ideal, but not a deal breaker. Your Stearman will love that radial. And the struts will be fine with it. Mount and fly!

On my 180cm Phoenix Corsair with FG33R3 I had to attach the battery to a long stick and put it as far backwards in the tail as possible to get it to balance. I am afraid something like this also needs to be done with the Blackhorse - UMS combination. I hope I don't need to add lead to the tail. Will see how the balance ends up.

I've done that also. Much rather have working ballast than cold lead. But it sure is easy to add to the tail end lol. My Phoenix 60 Waco flys the UMS 125. Balances with the redundant batteries on the CG with no other adjustments. But my 12.5 pound airframes with 6 pound radials definately need a full pound aft. (the 72 in. Great Planes Stearmans). They fly well. Landing must be scale to prevent bent legs. Touch them down as the training films of the original Army flight classes used, and your golden.