Engine Runs

Even tho I am new at this site I have been reading almost 99.9% of it. Especially regarding engine runs. I have wound up with the old reliable Fox 35 in my ARF Nobler. I am getting fairly decent runs with it when the uniflow vent stays put. The guys in Tulsa probably remember I quit in the second round of classic as the engine went super rich. Since the Tulsa contest I have played with it. What happens is when the uniflow comes loose from the fuselage the engine goes rich. Can't lean it out on the ground with it loose and pointing to the rear of the plane. When it stays in place everything is okay. I now have it mounted to where it won't come loose. But, my thought, is when vent is to the rear shouldn't it cause a vacuum? Later, DOC:-(

Are you meaning the plug on the uniflow tube? On any uniflow system, the tank is operating under a partial vacuum. Thus the needle needs to be opened to a slightly greater extent than if one were using standard venting. If the plug on the uniflow vent fails, the tank will revert to atmospheric pressure and the engine will go rich.

Leonard writes:

Is the uniflow vent to be plugged?
I plug the overflow vent and have the Uniflow vent open...
Am i totally mistaken here?
Norvald Olsvold

You are correct. Leonard was refereing to the overflow vent--it must be plugged and NOT leak for the uniflow to work.

Doc,

First, the way I mount the uniflo tube thru the fuselage is to solder a washer onto it to brace the outside (forward facing) portion to the fuse side. Where it pierces the fuse side and doubler, I slide another snug fitting washer tight against the (usually) ply doubler surface and hold it there with JB Weld slow. Before pushing the straight inner tube section through, I rough the surface by rolling it between 2 sanding blocks with 60 grit aboard. MUCH greater 'tooth' for the epoxy! Clear epoxy, used sparingly, on the outside washer; JB Slow on the inner, used a bit more generously...

Second, the only airflow I would rely on is ram air onto a forward facing tube's opening. At any other angle, we are in a different time/space dimension and have no idea what makes the most influence.

I also sleeve the forward facing tip down to 1/32 ID to reduce upwind/downwind variation in the ram air pressure effects. Sorta works...

Yes..if the uniflow vent is facing aft, it would cause a drop in pressure--this will cause the engine to go leaner.

The whole point of uniflow it to maintain constant pressure in the tank. So I don't care to use ram air which varies. I use a one-way valve on the uniflow to prevent siphoning and to insulate the tank from changes in outside air pressure. Failing that, the tiny hole idea is next best.

One way valve??.....What kind? where? why?

The problem is that the engine does the reverse. It goes rich, in fact, so rich I can not even do a loop. Also, in answer to some of the other responses, I do have a cap on the overflow. DOC

Back when I was flying Foxes with muffler pressure, I could tell when the uniflow outlet would come out of the fuel near the end of the flight because the engine would richen up. Then as it ran out of fuel and started sucking air it would go lean and die. Not sure what relation that has to your problem.