912 rotax shakes

[Mcslamma]

Hey, Others have said increasing the size of the balance tube/fittings from the intake manifolds helps tremendously, also adding a balance master ring has been mentioned in the past, personally I have the 80hp with similar vibrations at the same time when pulling back the power, I did add an increased balance tube size, and it helped, also I added a balance master on prop hub, it helped some more. Of course the carbs need to be balanced and as has been mentioned the carb floats need to be the proper weight, and floating etc.. As well as the vent tubes on the carbs have an affect on things.. I am happy with the way mine runs now, but it has been an evolution to get there..lol

Oh ya, mine I an IV prop 72" cut back to 70" iFA

[Dave S]

Regarding the floats, a quick and dirty evaluation can be done on a 912 by carefully pulling off the float bowl so as not to spill any fuel. The floats have the small rods sticking out of their sides to actuate the needle valve and these rods should be riding right at the fuel level. If a float is heavy enough to submerge the small rods below the fuel level - the float is probably out of spec heavy.

[avidflyer]

Thanks for the replies! I've weighed the floats with my small electronic scale, and also my reloader scale that weighs in grains. Both say the floats are fine, and they give the same weights although when I do the conversion from grains to grams from the reloader scale, it goes out a few more decimal points... I suppose it is the IVO prop like some of you have mentioned. I had a powerfin prop, but sold it a month or so back. Wish I had it now to see if there was a difference. I'm thinking of trying a two blade warp drive, I have three matching blades and both 2 and 3 blade hubs, but I think a three blade 72 " square tip will be a bit to much prop for an 80 HP. JImChuk

[Rodney]

I’m running a 912S with Hal’s big bore kit. Sensenich 3 blade prop. Large crossover tube

Same vibration problem in that rpm range. I’ve tried the balance ring
on the prop n have a balance ring on the engine. Still vibrates.
I took the balance ring off the prop then carefully balanced the engine at
about 5300 rpm. I think it’s smoother without the balance ring
but still haven’t figured out the issue at 3500