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I do not know the cost of dynamic balancing, but maybe you should do it yourself. Considering all other expenses, the cost a Dynavibe is only around $1,600.
I have dynamically balanced my prop on my 912iS. The amount of balancing needed is dependent on the propeller/spinner you are using. Some props are notoriously unbalanced. My prop/spinner combo was not too bad, but I could feel the difference (improvement). The reason that Rotax wants you to do it, is because Rotax has dual carbs and vibration will cause wear to the carbs which will cause the engine to run rough and vibrate even more. Balancing the prop will not compensate for carbs being out of sync. You must make sure your carbs are sync'ed first or you are just wasting your time (and money). Our club bought a Dynavibe and it has been a good investment. Most everyone has said that they could feel a marked improvement. Makes everything in your plane last longer.
We rent out our prop balancer here at Viking for those that don't want to take the plunge and buy one. -Alissa
Way late to this conversation. Not for you specifically Herb, but in general for any Rotax powered Zenairs, just make sure that who ever is doing the balancing understands geared engines. Or at least, that's my very limited knowledge of the situation.
Way back around 2000 my dad took his 601 with a Rotax 912 to an established maintenance shop that was thought to be competent but couldn't understand how to balance a geared engine properly. They tried a couple times and claimed the numbers were right. We ended up with a worse balance than we started with, which one could feel as a buzz in the rudder pedals, which might have led to the exhaust system cracking we experienced on a long trip just afterwards. Various frustration and expense ensued. (And left us suspicious of balancing shops unfortunately.)
My dad then just took the prop apart, an Ivoprop, and static balanced the blades, shaving bits off as needed. Just that static balance improved things to better than before taking it to the shop, and we left it at that.