I'm guessing there is an issue in the tank.

Maybe a crack in the uniflow tube inside the tank which uncovers roughly half way through the flight. Once the crack is in air, the it can easily bleed air into the rest of the tank. At that point the tank begins to act like a normally plumbed tank (with the vent located midway in the tank) for the rest of the flight.

If you can, try to change tanks and see if the problem clears up.

Or something like that!

I agree to a point, check the fuel system, but Matt knowing how thourough your approach has been on other issues, I am betting its not a tank issue. I refer you to the first rule taught me by Pat,, if it leans towards half flight, add oil. this is classic symptoms for low castor in the fuel, the engine builds heat as it goes through the flight and there is not enough castor to cool it. My guess would be that it leans in a manuever and doesnt come back down again..
add 3 or 4 oz of castor to your fuel ( per gallon that is_) and blend it,, I am willing to bet you will have a sweeter run. Some needle tweaking will need to be done and if you are marginall on fuel load to complete the pattern you COULD run into some issues there. but, it sounds like a classic case of needing a dose of castor.

Hi Matt - How big is the exit area of the muffler?  Too much backpressure can cause heat retention/ runaway and its indistinguishable from too little oil.  Tom H

Well I popped the back of the tank off, and none of the tubes were leaking.  My only other guess is to take muffler pressure off and see how it runs like that.

Tom Hampshire's muffler tip is also solid information! So, what sort of muffler ya got on that OS .35S, Matt? If it's a tongue muffler, try flying without it, or swap to a tube muffler. I like Randy Aero's tube mufflers. The one I put on my .46LA got the outlet drilled out and a 'rubber ducky' installed. That same muffler will fit your .35 and therefore, the outlet should be big enough, but the straight outlet will oil down your whole plane. Thus, the rubber ducky, and therefore the outlet drilling.  Steve

Steve, I'm running a Big Art tongue muffler that we put a pressure fitting on.  I don't want to put on a tube muffler since I don't want it become more nose-heavy than what it is, which is why i have a tongue muffler on.

I guess I'm getting confused on which actual OS engine you are using---is it an old OS-35S, or the newer OS-35 FP? I ask because the 35S isn't that heavy and I wasn't aware that you could get a Big Art muffler for the 35S since it doesn't have "normal" muffler lugs unless you tap the upper and lower "plugs" on the exhaust port (don't know what else to call them)! The 35FP has a standard bolt-on muffler while the 35S came with a strap-on muffler.

The reason I ask is that in your first post you say  "I have run OS .35s in 2 other planes ....." which I can interpret in at least 2 ways!

Hi Alan, I can understand the confusion, I am running an OS .35S, not the FP.  This engine my grandfather bought from Mike Tufano, who has passed on, but he drilled through the exhaust ports to put a bolt on muffler on.  We also have a 35-40 year old OS .35S that I guess had a bolt on muffler.  I originally had with that engine a wrap around muffler on my first plane, but it kept on coming loose so he had an aftermarket muffler for it.  That engine could have had bolt on mufflers and that is what there was on that one particular OS .35S.

During a flying session today, I took muffler pressure off and the motor ran beautifully.  Now I got to get the tank shim right.