Your engine does need a tune up. On stunt planes like Extras or Sukhois I run the YS 1.20 four stroke engines. I think the biggest tank I have is a 16 ounce. My rule of thumb is one ounce per minute of flight but it really isn't that much, more like an minute and a half and that's in a stunt plane. A 40 ounce tank in your plane is great if you plane on flying around for over an hour. A 12 ounce tank in the senior would be more in line for normal use. 16 ounces if you want real long flights. Your plane doesn't require a lot of power so about 2 or 2.5 ounces per minute would be more in line. A smaller tank would even fit where it's supposed to go.