Dude; Magnums are good engines but the require proper setup like any other.

First and foremost read the link to the manual posted earlier.

To break them in on 10-15% is okay, use an OS "F" plug.

I run two of these engines and a .61 4 stroke as well; I'm not as please with the 61 as I am with the 91s but to be brief plumbing is the key to a proper running inverted engine. The Tank Centerline should not be much more than 1/4" higher than the centerline of the engine. So - let us know if that is the case - if it is much higher than that you will experience a serious amount of load up at lower throttle settings regardless of the low speed needle.

An easy way to tell if the engine is running lean or rich AT IDLE with a 4 stroke is to pinch the fuel line at idle (this doesn't work for a 4 stroke at higher throttle settings). If the engine runs up quick then it's rich, if it dies quickly it's lean. Assuming a rich setting lean it until it dies during the test and then richen it up a bit to make it right. When breaking it in though you want to run it at least a half turn rich on the idle.

At full throttle you need a tach to set it right; Lean it out until it starts to die and then back it off about 1500rpm for breakin. Then for normal running back it off for about 300rpm.

Your mileage may vary.