Suppose you use a 2" round clunk tank, and it sits on top of the usual maple rails, the center of the tank is well above the venturi.  This would normally indicate a lean run while inverted- as the clunk will be about 1" BELOW the level of the spraybar.

Why does it work?

I have done this only once, with a Brodak Legacy/ST51. The tank actually sits a little between the maple bearers, but not by much.  It works, but I'm cautious about doing it again.

Floyd in OR

Guys,
lets remember the way a uniflow tank works is where the position of the Uniflow tube is,, not where the fuel pickup is. so with a clunk tank the clunk can bounce around all over the place and its where the uniflow vent is that matters.

Steve, fair enough, I made an assumption there, in the case of a clunk feeding with conventional venting not sure how that affects the run, I have NEVER run one that way, though some do I imagine,, hmmm

It makes perfect sense that the center of the tank should be aligned to the height of the NV Assy., but that's almost never the correct tank height. Much more so with Uniflow tanks. With "standard" venting, it's fairly close. It's generally different for every engine, and I have had a muffler change require a tank adjustment. Also changed from a ST .60 to a ST .60 Clone and had to adjust the tank height.

Several stunt noteables have experimented with adjusting the spraybar height instead of the tank (via longer venturis) and found that didn't do the job. Those noteables were Fancher and Rabe, in independent tests. I believe both mentioned their findings in their columns in AAM...or was it MA? Safe bet is to use Uniflow, and if using a plastic tank, make the uniflow vent fixed, but adjustable for height. Solder a brass nut onto the tube outside the stopper, so you can just apply an ignition wrench and give it a tweek without disassembly...maybe right in the plane. The uniflow vent outlet inside the tank is what the engine 'sees' as the center of the tank, but it will probably have to be higher than expected. If you use a round tank, figure a way to index the tank when you remove/reinstall. They do respond to a red hot branding iron. 

FWIW, one out of the two Sullivan clunk tanks I've used leaked like a pig. I changed to the dreaded Hayes tank that Da Dirt recommends, and found joy. I had to adjust the tank height the old fashioned way, and I wish Joy had been blonde and about 5'8"/130 lbs. But it works amazingly well, for no reason I can determine. It's not uniflow. Muffler pressure goes in at the top center of the tank, so it's covered for half the flight, and uncovered for the rest of the flight. I guess I can learn to accept the facts and not worry about the why's......eventually.  Steve

I've never had good luck using muffler pressure. The uniflo tube is open to slipstream.  This one has the tank inserted through a 2" hole in firewall.  Have to remove motor, but I can service the tank OK.

Floyd in OR

Oregon state motto: "Sunshine?  We don' need no stinkin' sunshine!"