Vibration and strange EGT readings at small RPM range. (rotax)
An interesting issue for those who know Rotax's for the last few hours of flying, the 912ULS has developed a niggling vibration, this vibration shows up when moving from full power to max power in the take off roll, usually at about 3/4 throttle, once at full power, its ok. in cruise at 5100 rpm it is very slight, 5200 RPM is smooth as should be, just noticeable vibration. at 4700 rpm, its as smooth as a rotax should be, between 4700 and 5100 rpm, the vibration is very noticeable..
to try to fix this, i have done the following.. Balanced carbies, balanced the prop, checked prop track and pitch, all good. Leads have been tested anre are within limits, but not new. carbies have been recently overhauled by rotax. engine still starts fine, very quickly, even when cold.
so, 2nd last flight, i performed a high power mag check at 5100 rpm results were.
Mag 1 off, EGT's all changed the same amount.. back on.
Mag 2 off, EGT on cylinder #2 steadily fell from 700 deg C to about 300 deg c, back on, all back to normal.
(#3 EGT failed completely and reads 0, and still read 0 for the next testing)
so, 1 spark plug change later. (all plugs looked fine), back at cruise power, again, same test, same result. hmmm
, back on both mags, i started experimenting with RPM settings to see at what RPM the vibration was the most noticeable, and the results were interesting.
4700 rpm, no vibration,
4800 rpm, small vibration..
4900 rpm, largest vibration, but also, the same EGT drop on #2 Cylinder!! from about 700 to 500 deg C.
5000 rpm, same, still lower EGT on #2.
5100 rpm, #2 EGT comes back up to normal temps around 700 Deg. in line with other readings.
Any rotax engine guru, have an idea on what might be happening?
Thanks.