I dunno. My Mark IV, VI and VII Combat Specials ran pretty well, I thought. Well, the VII did until it ate a circlip, and the VI did until I had a midair and the shaft bent a little. (We had to sand it down, chucked in a drill, to get it to work again.) Then again, I wasn't one of those running 40% nitro and complaining about breaking shafts and whatnot. (And then Henry came along with his .36 and blew everything else away.. well, the Mark VII was about as fast, but there weren't many made..) You used to be able to get a new Mark IV mail order for $40, but to get it into form you had to break it in, run it a season, get it chromed, etc. And if you wanted a real fast one, you got the crank EDM'ed out (or bought a high-zoot crank from Brasher), or got a ABC piston/liner from Doc Passen, got a titanium prop screw and used 40-50% nitro. I didn't, way too expensive for me. One plug a run, more or less, is what I remember, that on Missile Mist.

For a while, the .15 Schnuerle was the engine of choice in the US for FAI, but you needed a special head, special props and 50% nitro to get them to run fast. Duke even made a special version with a Mark IV crank in it (with a rear bearing but no front bearing) just for the US team. Their weak point was the venturi flange - it would often break off the case in a crash. A couple of engines I bought off another flier had bolts mounted in the exhaust flange, adjusted to butt up against the venturi to prevent just this from happening. They became obsolete with the fuel rule, the muffler rule and the venturi rule. So did the Nelson .15, which was a lot faster than anything else out there, including the current F2D engines.

Idle? We don't need no steenking idle...

Iskandar