I am just attempting to run in my Magnum .91 fs. I'm doing right in the plane, (shoestring). At idle it vibrates quite heavily, however as you increase the throttle the vibrations go away. I'm running a zinger 14*6 wood prop, (and yes it is balanced ). This is my first "big" four stroke, so I was wondering if this is normal, or do I need to balance the spinner and look further into the problem?

As always thanks a bunch!!!

At certain RPM's, all engines will reach a "resonant vibration node". At these points, engine vibration will be much greater. On the other hand, at certain RPMs, the engine will reach complete non-resonance, and the vibration will be almost zero. I set my idle to be on one of these non-resonant spots, even if I have to settle for a higher idle speed. If possible, I try to set the top end speed one way or the other to be at least not right on a resonant node.

Just another thought as to cause. When the engine fires, a torsional force is applied to the engine. This causes the engine to move clockwise slightly in it's mount. (as seen from the front.) This movement propagates as a wave down the length of the fuselage, (and the wings too) and is reflected back after reaching the end of the fuse. When the reflected wave reaches the engine, the wave will either add too, or subtract from, the motion of the engine at that instant in time. If the engine just happens to be bouncing back from a piston firing, then the reflected wave will add to this motion and tend to accentuate it. As stated before, this will happen at certain rpm's where the rpm is a harmonic of the wave transit frequency. This is essentially what resonance is.

As you run the engine more, it will smooth out somewhat.

My saito 91 started out the same way. you'll have to lean the low speed needle out at least 1 turn assuming the magnum is similar to the saito.
I was worried too when it first started up.

Thanks for the info guys,

I'll run it in a bit more as I only have one tank through it, and hopefully it will settle in a bit. I know its running very rich which does not help with the vibrations. We'll see what happens.

Thanks again.