Rotax 277 shakes badly below 4000 rpm

On 06/30/2020 03:39 AM, Remi Dargis wrote:
On Tuesday, June 30, 2020 at 12:01:55 AM UTC-4, Corvid wrote:
On 06/29/2020 03:09 PM, wrote:
Hi everyone, i am new to ultralights (i was only flying skyhawks
and warriors until now). i now own Fisher ultralight with Rotax
277. i never flew it, i never saw it flying and i do not know
previous owner who actually flew it. when i got the airplane it
had carb removed but everything else looks in good shape. i put
carb back and was able to successfully start the engine. engine
runs nice and smooth at 4000 rpm or above, but when it drops
below 4000 it starts shaking badly. it runs stable but shakes to
the point that some screws from the panel shook out. i have read
that single cylinder 2 strokes shake a lot, but to me it looks
too excessive. i took a prop off and it runs much smoother
without it. i am thinking prop may be out of balance? the problem
is that i do not have any experience or knowledge to tell how
much vibration is too much on those engines. does anyone know
how to determine if this vibration is normal?

There's a link for tech support at https://fisherflying.com/

The prop could be balanced perfectly, and have the flex or mass to
interact unfavorably with the engine, engine mount,
speed-reduction drive. Or anything else.

my problem is that i don't know how much vibration is consider
"normal". i am reading online that others are complaining about 277
shaking, some say this is normal and just a nature of this motor. i
don't want to start replacing things if this is normal. is there any
criteria, anything to determine if this is normal? that shaking
sounds like "compression vibration" - makes one bang on every stroke.
or if you turn ignition off at higher rpm, when it starts to wind
down below 4000 starts banging and does that till the last stroke
when it stops.

4000 sure sounds high for banging and shaking. I know what I'd replace
if that's normal.

crossposted to rec.aviation.homebuilt , a once-bustling group which
still has lurkers waiting for questions like this.