I have a new Surpass 11 non-pumped in a P-47. I'm using a 16 oz sullivan V-tank with a fuel dot. 1.5 gallons of Ritchs brew thru it. It ran great for the first 5 flights or so. At that time I had a dubro fuel valve and noticed it was spewing out fuel so i changed it to the fuel dot. Apc 13x7 and graupner 13.5 x8 used. Runs good for the first half of the tank.

There are too many things that can cause the sputtering, my P-51 did that with an inverted OS 91 in a power dive. I added a $28 Perry Oscillating pump and it seemed to take care of it, however, it could be a lot of things in your case. Try replacing all of the lines and make sure your clunk isn't stuck somewhere in the tank. Another thing is you might consider is a bigger prop. When the engine overrevs, you could be floating a valve causing the engine to sag and surge in the dive. I recommend you try a 14x6 or 13x8 with the OS 91. The 14x6 Master Airscrew looks nice and scale like on the warbirds as well as howls at high rpm which adds to the excitement. It won't go very fast but it will sound fast. A 13x8 Master Airscrew Scimitar will make you go fast.

Good luck.

I had this engine a while back and if it has an idle screw, richen up the idle mixture. If I am not mistaken, that is what I did, to fix the problem I had with this engine in my 60 size extra.

I just started running my new OS 91 Surpass a few weeks ago. The engine got over 10,000 rpm with a 13x8 APC prop and that is to high. I use 10% Cool Power fuel. I used a 13x10 and got about 7000 so I had the LHS order me a 13x9 APC and engine runs @ 9400. A few on RCU said this engine should run below 10,000 rpm. Also, vibration very high @ 10,000. Mine is in a Ultra Sport 60. My original OS plug (came with engine) blew aftyer about 10 runs, went to a OS#8 and that didn't work well. Went to LHS and shelled out $7.99 for a OS F plug and all is well again. Engine now has 15 to 20 runs on it. Good Luck.