Rotax 277 shakes badly below 4000 rpm

[in...@xp-electronics.com]

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?

[Corvid]

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.

[Remi Dargis]

On Tuesday, June 30, 2020 at 12:01:55 AM UTC-4, Corvid wrote:

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.

[Corvid]

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.

[101AirborneE8]

I had a Rotax 277, and it purred like a kitten, even when at low RPM's. Checking the prop for balance should be fairly easy, then I would make the gear box my next point of attention. The 277 can be a great little engine. Maybe also, check the engine mounts, so insure that they are in good shape, and snugged evenly. Only problem with that motor is, with only 1 piston, if anything goes wrong (like when the rings went out on mine), the sky can become a REALLY lonely place.

[Remi]

i checked prop balance and seems to be OK. gearbox - i am not sure what to look for. it has a little play (feels like between gears inside) when you move prop back and forth, but not much.
it feels like every power strike makes a bang that shakes it. if you run it 6000 it runs fine, when close throttle to idle, it will run smooth while winding down to 2000, but when it hits that rpm it starts banging again. sounds like it shakes every time it power strikes against prop weight. i ran it without prop and it was running fairly smooth....