Guys,
Recently I completed the build on an El Diablo for OTS. The ship uses a Fox 35 but I am having some run problems that I think are related to the tank venting. The tank is a 4 oz round plastic set up for uniflow with double clunks. The tank is air tight and center on center for the height. The plumbing exits the tank compartment in back of the engine. The uniflow vent is directly in back of the cylinder pointing straight up. With this location my thought was to keep the vent out of the draft and eliminate the up wind rich / down wind lean run cycle. What happen was the first few runs were a bit fat and slow but held rpm from launch to cutout, pretty much a perfect run just slow (10x6 prop, 8900 launch rpm, 7% fuel, 28% oil, Enya No4 plug). I did get to check upright and inverted lap times were the same and motor sounded the same.

When I started to bring the needle in to get to about 9200 rpm all seemed fine but once launched it broke lean and we were off to the races. Although the El D liked going fast it was strange because just when you got to the point were the uniflow vent uncovered it broke back rich and the 10 laps later cut. I tried muffler pressure and this held rpm to the point were the uniflow uncovered then broke lean.

Since the run has gone both ways I am pretty sure this is a tank problem. For the open uniflow I think the vent being located directly behind the cylinder put it in a dead spot and once the ship got to a certain speed it was in a negative pressure zone that made it harder for the engine to pull fuel causing it to go lean till the vent uncovered then it pull all it wanted and went rich, ah I think?

For the muffler pressure there may be two things working against each other. I used a ST NVA which reduces the venturi size a bit from the normal Fox spraybar. With the muffler pressure it required leaning the needle to get to a launch rpm (around 9000), now when the uniflow vent uncovered the tank acted like a conventional venting and the engine went lean (very lean). My flying buds use the muffler pressure into this same tank layout on a double star 54, 60 and it goes slightly lean at the end for about two laps. Could the high suction venturi caused by the ST NVA be setting up a situation were the needle is leaner than it would be with a larger venturi area and causing it to go way over at the end? I seem to remember that when we were first using crankcase pressure in the combat ships you had to remove the venturi inserts in the K&B 35's to get them to run right cause the normal suction would make the needle very difficult to impossible to set right, could this be the same with this setup?

Anyone have any experience like this? How did you resolve it?

Best,         DennisT

On a proper "Uniflo" set-up it is usually best to have the uniflo vent inboard and ahead of the fuel tank in clean air. If you experience too much variation upwind/downwind, insert a restrictor in the uniflo line: i.e. if you're using a 1/8" OD tube as most do, slide a section of 3/32" inside of it. A well set up uniflo on a stunt engine should hold the in-air setting throughout the flight until the uniflo line uncovers, richen slightly, then lean out and quit. The duration of those last two is determined by the internal uniflo line position.

I have a stash of the old KK one way valves which I use on open vented uniflow.  I have had excellent results running a Fox 35 with ST NVA on muffler pressure (no oneway) with uniflow.  Your engine should go slightly rich once the uniflow uncovers.

While the problem may have already been solved, I would throw out a couple more thoughts. Some engines (maybe if not adequately run-in), benefit from a bigger venturi, because the engine passes more fuel and thus more oil and more cooling. Seems odd, but it makes sense, too. Less prop load might do the same, and give better performance. All 10-6's aren't the same.

I have never understood the logic of a uniflow clunking about. We know that with metal tanks, the uniflow vent end inside the tank is what really matters when getting the rpm the same upright/inverted. What would happen with the end of the uniflow vent moving around inside the plastic tank? Seems to me like the rpm would vary a lot, depending on where the uniflow clunk happens to land as it's thrashed about.

Finally...think about how a clunk can pickup fuel, considering the radius of the clunk. The bigger diameter the clunk, the more fuel should be left in the tank when it shuts down. Therefore, the bigger the uniflow clunk, the sooner the uniflow vent thinks you're about out of fuel, and the engine breaks lean. You might experiment with different clunks, particularly with tapered ones. I don't understand how the sintered bronze "filter clunks" would work for our purpose, because they'd "uncover" over a much broader fuel level. Ok for R/C, where it would not intentionally be run out of fuel, but uncool for our use. You're not using one of those?  Steve

I also don't understand why the uniflow clunk works, but a friend uses them and gets excellent engine runs. 

Guys,
Well, found the problem, head bolts not fully tight. Found this by dumb luck as I turned over the engine, seemed soft on the compression yet would flip fine. I followed my hunch and put the head into a container of water and turned it over, saw the head bubbling. Re-tightened all the head bolts, checked it again, all tight. I also added the 1/32" plywood mount pads to have some "crush" room under the mounts just in case. Flew it today, solid run on muffler pressure.

Best,         DennisT

From reading most of but not all of the posts on this subject, I get the feeling that fellas using plastic tanks vented Uniflow are not mating the clunk of the uniflow vent line with the fuel pickup line.  My practice has been to use copper wire and bind the fuel pickup line with the uniflow line with the uniflow clunk about 1/4" inch behind the pickup line...This has worked well for me using 10% nitro w/24% castor on Fox 35's & OS 35-40 FP equipt stunters....The 1/4" distance of the uniflow clunk from the pickup clunk seems to be a workable distance in a plastic tank works fine for me...