912ULS Engine Vibration
in 912 / 914 Technical Questions
6 years ago
So I've searched and read everything and have been working on my engine now for a few months.
It started late summer of last year. Up until then the engine ran smooth and perfect. I run a 5 year old 912ULS with 305 hours on the clock since new with a 2 blade ground adjustable sensenich prop mounted to a Revo trike. Prop is pitched to 1/32" in 3 feet (don't remember the calculation in degrees but it's WAY better than what is required - something like 5 hundredths of a degree). Prop has been dynamically balanced by an A&P.
Near the end of the summer engine started to rough a little rough and was progressively getting more noticeable throughout the flying season. CHTs were a bit uneven up to that point - within 5 degrees of each other so I knew the carbs were out. Synced the carbs and it improved things a bit but not totally. Continued to fly about 10 more hours before I stripped her down for the annual. Everything checked out perfect in the hangar - Magnetic plug was clean - no buildup. Filter was clean too. Started here up and fast taxied her up and down the runway - Very rough running. Figured time to hit the carburetors.
Took the carbs home and stripped them, cleaned them and replaced all rubber, o-rings, Float valves, etc. Even replaced the carb mounting flanges and cross over tube rubber. Cleaned, inspected, probed every orifice and everything looks as good as new! Reinstalled and rebalanced carbs and the engine still runs very rough from idle 1700-4000RPM....it's worse the lower you go. Carbs don't seem to be the issue. I even removed and sealed the intake access bolts (for balancing). I also flushed the fuel lines by running the electric fuel pump before installing to ensure nothing got introduced into my newly cleaned carbs.
Mag check seemed normal - about a 300 rpm drop and maybe 20-40 rpm difference between Mags A&B. I then checked every Spark plug (less than 50 hours on them - all clean and in new condition). I took off the plug boots, cut back the wires 1/2" and reinstalled the boots to ensure I had a good connection. No joy.
I've got 5 fresh Gals of Pump 93 Octane in the tank. Fuel delivery is perfect....no difference with the engine fuel pump or electric backup running.
The aircraft has suffered no trauma and has been well cared for and meticulously maintained. I'm thinking it's down to two items:
1. Ignition coils - perhaps failing/misfiring? Unlikely but want your opinion.
2. Gearbox - read a lot of posts on here of dogs getting beat up. I don't have a spring Gage to measure friction torque - getting one in a couple of days and will check and measure but by hand I think I might be below 300 inch pounds - I can turn the prop the 30* with one finger and light pull. Will report back when I have the measurement. I trained in this plane. The previous owner had 88 hours in it (no idea how he ran it) but did run AVGAS. I've been pretty religious about idling her @ 2000 rpm+ since I've owned her. I am also hearing a lot of noise from the gearbox from 1700 - 2500 rpm.
I guess my question is what else could I/should I check before what I fear will be a costly repair. Also - considering switching props to a much lighter option and moving away from the Sensenich.
Looking for any and all expertise. None of us is as smart as all of us!
Thanks.
00Drew
It started late summer of last year. Up until then the engine ran smooth and perfect. I run a 5 year old 912ULS with 305 hours on the clock since new with a 2 blade ground adjustable sensenich prop mounted to a Revo trike. Prop is pitched to 1/32" in 3 feet (don't remember the calculation in degrees but it's WAY better than what is required - something like 5 hundredths of a degree). Prop has been dynamically balanced by an A&P.
Near the end of the summer engine started to rough a little rough and was progressively getting more noticeable throughout the flying season. CHTs were a bit uneven up to that point - within 5 degrees of each other so I knew the carbs were out. Synced the carbs and it improved things a bit but not totally. Continued to fly about 10 more hours before I stripped her down for the annual. Everything checked out perfect in the hangar - Magnetic plug was clean - no buildup. Filter was clean too. Started here up and fast taxied her up and down the runway - Very rough running. Figured time to hit the carburetors.
Took the carbs home and stripped them, cleaned them and replaced all rubber, o-rings, Float valves, etc. Even replaced the carb mounting flanges and cross over tube rubber. Cleaned, inspected, probed every orifice and everything looks as good as new! Reinstalled and rebalanced carbs and the engine still runs very rough from idle 1700-4000RPM....it's worse the lower you go. Carbs don't seem to be the issue. I even removed and sealed the intake access bolts (for balancing). I also flushed the fuel lines by running the electric fuel pump before installing to ensure nothing got introduced into my newly cleaned carbs.
Mag check seemed normal - about a 300 rpm drop and maybe 20-40 rpm difference between Mags A&B. I then checked every Spark plug (less than 50 hours on them - all clean and in new condition). I took off the plug boots, cut back the wires 1/2" and reinstalled the boots to ensure I had a good connection. No joy.
I've got 5 fresh Gals of Pump 93 Octane in the tank. Fuel delivery is perfect....no difference with the engine fuel pump or electric backup running.
The aircraft has suffered no trauma and has been well cared for and meticulously maintained. I'm thinking it's down to two items:
1. Ignition coils - perhaps failing/misfiring? Unlikely but want your opinion.
2. Gearbox - read a lot of posts on here of dogs getting beat up. I don't have a spring Gage to measure friction torque - getting one in a couple of days and will check and measure but by hand I think I might be below 300 inch pounds - I can turn the prop the 30* with one finger and light pull. Will report back when I have the measurement. I trained in this plane. The previous owner had 88 hours in it (no idea how he ran it) but did run AVGAS. I've been pretty religious about idling her @ 2000 rpm+ since I've owned her. I am also hearing a lot of noise from the gearbox from 1700 - 2500 rpm.
I guess my question is what else could I/should I check before what I fear will be a costly repair. Also - considering switching props to a much lighter option and moving away from the Sensenich.
Looking for any and all expertise. None of us is as smart as all of us!
Thanks.
00Drew
