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