First off - an apology - a friend reviewed this and found it an exhausting read (And I am exhausted by the problem) While it is a summary He is right.
I am a year into this problem. I am wondering if there is anyone out there that has had the same problem?
Owner C-IKMO, Rans S7S, 912ULS
Intermittent Vibration issue and troubleshooting summary.
This is a summary of the technical issues experienced with my 10 year old 912uls serial number 6781147 mounted on a Rans S7S on floats which was built for its original owner in 2013/14. It is a summary only, has been significantly condensed, and is not a complete troubleshooting chronological history which would fill many more pages.
All flying, and almost all the vibration observations detailed in this summary, were observed and completely by me, the current owner. All the troubleshooting and work done on the plane over the last 12 months was completed by the owner, xxxx ( Rotax certified mechanic/owner of xxxxxxxx ), and xxxxxxxx (who has completed some Rotax provided training and is a very knowledgeable fellow Rans S7s flyer.
Technical issue CIKMO as of Oct 9, 2024.
Current symptoms - late summer /fall 2024;
At air temperatures above 14 degrees Celsius the engine will occasionally begin vibrating when power is reduced to begin a descent to land from cruise. Once it starts vibrating it does not stop throughout the rpm range of the engine (ie power down and/or power back up) and application of carb heat has no effect. Normal smooth operation of the engine can return in as little as 15 minutes after shutdown. The vibration described throughout this summary is not severe but has caused electrical connectors to loosen, clamps to loosen, coolant to leak, and parts (ie air filter carb heat box) to fall off.
At air temperatures of 9 degrees Celsius to approximately 14 degrees while flying ;
Engine runs rough ( rumbling vibration) and gives the impression of “missing” when running on one ignition module. The problem is more noticeable when running on module # 2. Pulling on the choke, while running on only one ignition, reduces vibration by approx 50%. Pulling on carb heat reduces it by more than 90%. The application of carb heat or choke makes an instant improvement and, vice versa it is instantly worse when removed. Like turning on and off an electrical switch. Note - this symptom is noticeable above 4300RPM and peaks around 4800 RPM.
A hard climb to altitude, warming up the oil and coolant above normal cruise temps, reduces the vibration when running on one ignition substantially until the engine cools to normal temps again.
At air temperatures of 7 degrees and colder the vibration is worse and is noticeable with both ignitions on. The amount of vibration is close the same regardless if it is running on ignition number 1 or 2 and is very noticeable/rough . Application of carb heat is still effective but less so than when at warmer temperatures.
A hard climb to altitude warming up the oil and coolant above normal cruise temps has has no effect - vibration remains the same. Also taping off the radiators (reducing their area) to increase normal operating temperatures has no effect.
Fall 2023 symptoms
In the late fall of 2023, at lower temperatures, approaching 0 degrees Celsius the engine began continually vibrating. Application of carb heat reduces the vibration significantly but does not eliminate it. The vibration was much worse running on just one ignition module. The float flying season ended shortly after the engine began exhibiting problems. Fuel was drained from the plane for winter storage. Note - ethanol was introduced in all supreme gas in Nova Scotia in the latter part of 2023.
Spring/summer 2024 symptoms
During the summer of 2024 the engine would intermittently vibrate throughout a complete flight. This could be after, or before, another flight. In one case of 4 flights in one day it only vibrated on 1 (the 2nd of 1.5 hours duration) of the 4. Time between flights varied from 30 to 120 minutes. The longest time on the engine between vibrating flights was 10 hours and the shortest was back to back flights.
Troubleshooting
In the spring of 2024 the high speed needle was adjusted to position 4 in an attempt to change the mixture. This had no effect on vibration . Three weeks later the needles were returned to position 3 as the engine was running rough at lower rpm.
Black particles were observed in the carb bowls and gascolator bowl in the summer of 2024. Ethanol was introduced to supreme fuel in the late summer of 2023 and it was assumed that something somewhere in the fuel system (tubing?) was breaking down and causing the particles. The working theory at that point was that low speed jets were being occasionally blocked causing carb imbalance and creating intermittent vibration. On two occasions the low speed jets on one carb appeared to be blocked on inspection, however, no particles were found when they were cleaned out.
The black particle problem took some time to resolve. In the end all black tubing was replaced in the plane, the gascolator screen and gasket was replaced, and an inline filter was added at the firewall before the gascolator. No particles were ever observed in the inline filter. Note - There is a return line, with a reduction orifice, from the banjo fuel distribution stack that returns fuel to the gascolator to prevent vapour lock. All tubing was inspected as it was replaced and no problem was ever found with any of it. In fact it all was found to be flexible and in good condition.
The black particle issue was resolved without determining the source and without changing the symptoms over the long term although short term improvements were sometimes noted although it is not possible to conclude categorically that it was the result of clearing the low speed jets.
Additional trouble shooting steps.
Note - the initial problem first occurred November of 2023. At that point the engine had been working flawlessly for the 2 years I owned it. I acquired it with 175 hours total airframe and engine and first symptoms occurred at 405 hours. As of October 10 2024 the total time of the engine and airframe was 486.5 hours.
- The original carburetors were disassembled and cleaned/adjusted on numerous occasions. A small rub spot on the piston in the right carburetor was buffed out in the summer of 2024
- Both Carburetors were replaced with brand new units in the early fall of 2024.
- New throttle cables installed when carbs replaced
- Both Carb sockets replaced in the early fall of 2024
- Carbs have been rebalanced on multiple occasions.
- Oil has been changed and magnetic plug checked.
- The main fuel pump was replaced with a used unit in early fall 2024
- The electric fuel pump was bypassed for a while in the summer of 2024
- Ignition modules were exchanged with those from another aircraft.
- Power to the ignition modules was switched between modules.
- Spark plugs have been changed twice
- Spark plug caps have been tested - all 5 Kohms plus or minus 100 ohms
- Spark plug wires have been repositioned and all shortened 1/4 inch and caps reinserte
- Fuel lines have been repositioned
- All black rubber fuel lines have been replaced
- In-line fuel filter installed before gascolator at the firewall
- Informal Thermal testing on flow through the air filters was completed and the plane was tested without air filters in flight.
- Fuel system indicated pressure accuracy was tested and confirmed
- Prop pitch (tri-blade) has been checked on several occasion
- Drained and filled system with avgas for one fuel cycle x summer 2024
- Props nuts retorqued
None of these troubleshooting steps has eliminated the vibration symptoms which has changed with the season as described above.
CIKMO idiosyncrasies
KMO is often flown with two other Rans S7s both of which have 912uls engines installed. One of the other 2 has been modified with larger cylinders to increase power. The difference between the aircraft is minimal and all three use the ring style engine mounts. As noted above I flew KMO approx 125 hours before the vibration issue appeared and the idiosyncrasies noted below have not changed since I purchased the aircraft.
- KMO tends to exhibit higher fuel pressures than the other two S7s. Normal pressure during cruise is around 5 psi with a low of 3 in a climb. During start and initial taxi pressures can be 5.5 and occasionally higher. Changing the main fuel pump did not change this behaviour.
- KMO runs cooler than the other two S7s. Typical operating temperatures at cruise are in the 190 degree range and water and oil temps track closely. It takes KMO longer to warm to 120 degree oil temp after start up than the other S7s.
- KMO a spark plugs always tend to be greyish in color. No damage to plugs has ever been observed, gaps remain constant, and the deposits in the exhaust pipe appears normal.
