About 30 minutes into a flight where the engine (Rotax 912 UL, 80 HP, ~1100 hrs on the engine) was running fine, it suddenly started running very rough and could not reach full power, maxing out at about 4600 rpm. After limping back to the airport, I’ve tried to find the source of the problem without any success so far. The engine will run, but only with violent shaking; it stalls out below about 3000 rpm, and now on the ground will not go much above 4000 rpm.
What I have done to troubleshoot the problem (with no success) is the following:
1. Fuel/carburetion system:
Initially I thought it was likely a carburetion problem (perhaps the slide in one of the carburetors sticking, or something clogging one of the jets or vent lines). I have taken apart and cleaned both carburetors, paying particular attention to cleaning the main jet, idle jet, throttle body, slide and slide body, and insuring all vent passages are open. In addition-
a. The float chambers are clean with no debris or corrosion, fuel is at the correct level
b. The floats ride at the correct level (floating with the side pins just above the gas surface)
c. The float valve tip looks good, and shuts off flow properly
d. The jets and needle appear to be in good shape with no obvious wear
e. The slides move freely up and down.
f. The diaphragms are flexible, no cracks
The above procedure did not improve the rough running.
2. Ignition System
a. I’ve pulled all the spark plugs and inspected them. The color is normal, gaps are within spec.
b. The spark plug electrodes and lower threads were cleaned to insure that there is no contamination from the heat conduction compound. This did affect the rough running.
c. The HT leads are flexible with no apparent deterioration
d. A proximity voltage probe indicates that all the spark plugs are receiving consistent high voltage pulses from the ignition system when running.
3. Carburetor Synchronization
About the only diagnostic indication I have is that when hooking up a set of vacuum gauges (as for doing a pneumatic synchronization of the carburetors), the one indicating vacuum for the left cylinder bank reads approximately zero, regardless of throttle setting (note that since the engine stalls out below ~3000 rpm, the vacuum might be expected to be somewhat low; the right cylinder bank vacuum changes between about 7” and close to zero depending on throttle setting). The carburetors are mechanically synchronized, but this behavior precludes pneumatic synchronization.
Thinking this vacuum indication might indicate an air leak, I’ve done the following:
a. I’ve checked the compensating tube for possible cracks/pinholes, etc. It is intact. I’ve also removed it and plugged the intake manifold connections at each end, with no effect on the rough running
b. I’ve carefully inspected the rubber carburetor sockets. These appear to be in good shape, no evidence of any cracking.
c. I’ve visually inspected the intake manifold for cracks, gaps where it attaches to the cylinders, etc. Nothing was found.
d. I’ve tried doing a propane test to try to identify any areas where air might be leaking into the system between the carburetors and cylinder heads, with no success in finding anything (although with the engine well above idle, this is likely not to be a very sensitive test)
So it appears not to be a problem with the carburetion system or with the ignition system, at least based on what I’ve done so far. The rough running is clearly a problem with engine operation, and not something like a gearbox or prop balance issue; the engine will only run (poorly with violent shaking) between approximately 3000 rpm and 4000 rpm. Since the problem originated suddenly, it seems unlikely that it would be caused by a slow deterioration such as worn or deformed valves (although I guess almost anything is possible).
I’d appreciate any additional suggestions or thoughts on things I might do that could help identify and fix this.
Thanks in advance for any help you can provide.
Paul
What I have done to troubleshoot the problem (with no success) is the following:
1. Fuel/carburetion system:
Initially I thought it was likely a carburetion problem (perhaps the slide in one of the carburetors sticking, or something clogging one of the jets or vent lines). I have taken apart and cleaned both carburetors, paying particular attention to cleaning the main jet, idle jet, throttle body, slide and slide body, and insuring all vent passages are open. In addition-
a. The float chambers are clean with no debris or corrosion, fuel is at the correct level
b. The floats ride at the correct level (floating with the side pins just above the gas surface)
c. The float valve tip looks good, and shuts off flow properly
d. The jets and needle appear to be in good shape with no obvious wear
e. The slides move freely up and down.
f. The diaphragms are flexible, no cracks
The above procedure did not improve the rough running.
2. Ignition System
a. I’ve pulled all the spark plugs and inspected them. The color is normal, gaps are within spec.
b. The spark plug electrodes and lower threads were cleaned to insure that there is no contamination from the heat conduction compound. This did affect the rough running.
c. The HT leads are flexible with no apparent deterioration
d. A proximity voltage probe indicates that all the spark plugs are receiving consistent high voltage pulses from the ignition system when running.
3. Carburetor Synchronization
About the only diagnostic indication I have is that when hooking up a set of vacuum gauges (as for doing a pneumatic synchronization of the carburetors), the one indicating vacuum for the left cylinder bank reads approximately zero, regardless of throttle setting (note that since the engine stalls out below ~3000 rpm, the vacuum might be expected to be somewhat low; the right cylinder bank vacuum changes between about 7” and close to zero depending on throttle setting). The carburetors are mechanically synchronized, but this behavior precludes pneumatic synchronization.
Thinking this vacuum indication might indicate an air leak, I’ve done the following:
a. I’ve checked the compensating tube for possible cracks/pinholes, etc. It is intact. I’ve also removed it and plugged the intake manifold connections at each end, with no effect on the rough running
b. I’ve carefully inspected the rubber carburetor sockets. These appear to be in good shape, no evidence of any cracking.
c. I’ve visually inspected the intake manifold for cracks, gaps where it attaches to the cylinders, etc. Nothing was found.
d. I’ve tried doing a propane test to try to identify any areas where air might be leaking into the system between the carburetors and cylinder heads, with no success in finding anything (although with the engine well above idle, this is likely not to be a very sensitive test)
So it appears not to be a problem with the carburetion system or with the ignition system, at least based on what I’ve done so far. The rough running is clearly a problem with engine operation, and not something like a gearbox or prop balance issue; the engine will only run (poorly with violent shaking) between approximately 3000 rpm and 4000 rpm. Since the problem originated suddenly, it seems unlikely that it would be caused by a slow deterioration such as worn or deformed valves (although I guess almost anything is possible).
I’d appreciate any additional suggestions or thoughts on things I might do that could help identify and fix this.
Thanks in advance for any help you can provide.
Paul
