914UL carburettor gremlins
in 912 / 914 Technical Questions
6 years ago
SHORT STORY
My 914UL initially had a problem with sudden onset power loss during takeoff run. It turned out to lose power whenever the throttle was advanced enough to run the engine above approx. 4500 RPM (no MAP-guage installed in aircraft).
After changing floats and spark plugs, it now unexpectedly has run rough, mostly after activation of secondary electric pump.
On two occasions the following has been observed: With engine stopped (before running it for days), when both fuel pumps are switched on, after a short while, fuel will start dripping from a drain hose that leads to a drain on the airbox where it connects to the left carburettor. On the second time this occurred, there was also a small drip from the drain hose connected to the same carburettor’s spill bowl and I could see as well as feel fuel running down the outer wall of the float chamber.
When repeating the same thing only (10-15) minutes later, there was no visible drip, on both occasions.
Carburettors have been torn down, floats changed and float bracket spacing checked. On second teardown, the new floats were also weighted and checked for correct parts type (they were supplied by previous owner).
When mechanic tore down the left carburettor he noted that the fuel seemed yellow-ish. The fuel drained from primary and reserve tanks is not. We currently have no explanation for this.
Any ideas?
LONG STORY
I have a 914UL mounted in an experimental Europa Classic (mostly) Trigear which I bought this February.
I soon noticed that the engine ran a bit rough on run-up mag check with left magneto circuit (with RPM drop below POH allowed values). I felt that it got gradually worse but my local mechanic listened to my observations and cleared me for flying. However on the next attempt, after that consultation, sometime in late May, I had to abort a take-off as the engine suddenly lost a lot of power (like, negative surging) when I moved the throttle all the way to full (at about 100 percent out of 115 percent, yeah I know that doesn’t sound right, but it’s designed that way due to the turbo). Further ground testing concluded that the engine started behaving strange at about 4500 RPM and up (so the problem could go unnoticed during a standard 4000 RPM run-up, except for the bad mag drop which might have something to do with it all). Also the Flydat showed that EGT on the left side was about 50-60 degrees lower than the right side. This being at ground idle, taxi and run-up.
On my home field, I have an experienced and well respected mechanic who is also an authorised Rotax repair centre (though not with much 914 experience). However, he had a too busy summer with all the glider tow, parachuting and fire watch aircraft running in and out of shop, so we only got around to him assisting me in removing the carburettor “lids” and checking the diaphragms which were OK, as well as swapping spark plugs and, in one case, the spark plug cap which was worn.
In August, the previous owner – and builder – got tired of his old aircraft being grounded and drove over to help. We tore down both carburettors, changed floats and checked float bracket spacing. Then we synched the carburettors and the whole thing seemed fixed. The engine ran fine (though probably we didn’t thoroughly check the EGT values…) and I took the aircraft up for two touch and goes and one full stop. Oh the joy!
But… A few days later I took it up for a test run outside of the CTR. I did notice there was still a difference in EGT between the left and right side. However, the engine seemed to run just fine. Or well, both before the first and second test flights I noticed some slightly rougher running after switching on the secondary fuel pump during run-up. This was of mitigated by simply turning off the pump again. I suppose I should have concluded something was wrong and stayed on the ground, but off I went testing normal cruise, slow flight and some manoeuvres before returning to the field with a big smile on my face. EGTs were almost perfectly synched, especially at cruise power. However, on a long base, the engine started coughing and running rough.
The one thing that had changed from the slow flight at altitude was that landing procedures of course dictate that the secondary fuel pump be turned on. I cannot say for sure that it got better when I turned the second pump off, which I tried while searching for suitable fields and talking to ATC about the problem. The engine didn’t seem to want to fully give up so I only got around to declaring a state of urgency and promptly got the aircraft down on the runway. I’m not really sure today whether I landed with or without the secondary pump running, my mental priority being getting home in one piece.
Back on the apron, after dismounting (and swearing) I noticed a wet spot on the tarmac below the left (as in port) side of the nose, basically below the left carburettor spill cup and airbox drain hoses.
Now, my suspicions were, and still are, that the left carburettor is getting too much fuel and somehow overflows. Next time at the field, after talking to the mechanic about the issue, I simply went to the aircraft and turned both fuel pumps on. Within the minute, fuel was dripping out of the drain hose from the left side drain (I suppose it is some kind of drain) on the airbox where it connects to the left carburettor. When trying to show this to the mechanic maybe 10-15 mins later, the dripping did not reoccur.
The mechanic went on to disassemble the left carburettor’s float chamber, checked the float markings and weighed them. He also checked the float bracket spacing and, like me and the builder a few weeks earlier, found it correct. We later reassembled the carb together and test ran the engine, which seemed to run ok.
However, the next time I went to the aircraft and turned both pumps on, a much greater stream of fuel ran out of the drain hose from the left side of the airbox. Also, this time, there was fuel dripping out of the drain from the spill bowl/cup under the carburettor and I could also see and feel fuel running down the outside wall of the float chamber.
Once more, when testing again 10-15 mins later, there was no sign of any fuel dripping/leaking.
Any ideas as to what is causing this would be most welcome!
My 914UL initially had a problem with sudden onset power loss during takeoff run. It turned out to lose power whenever the throttle was advanced enough to run the engine above approx. 4500 RPM (no MAP-guage installed in aircraft).
After changing floats and spark plugs, it now unexpectedly has run rough, mostly after activation of secondary electric pump.
On two occasions the following has been observed: With engine stopped (before running it for days), when both fuel pumps are switched on, after a short while, fuel will start dripping from a drain hose that leads to a drain on the airbox where it connects to the left carburettor. On the second time this occurred, there was also a small drip from the drain hose connected to the same carburettor’s spill bowl and I could see as well as feel fuel running down the outer wall of the float chamber.
When repeating the same thing only (10-15) minutes later, there was no visible drip, on both occasions.
Carburettors have been torn down, floats changed and float bracket spacing checked. On second teardown, the new floats were also weighted and checked for correct parts type (they were supplied by previous owner).
When mechanic tore down the left carburettor he noted that the fuel seemed yellow-ish. The fuel drained from primary and reserve tanks is not. We currently have no explanation for this.
Any ideas?
LONG STORY
I have a 914UL mounted in an experimental Europa Classic (mostly) Trigear which I bought this February.
I soon noticed that the engine ran a bit rough on run-up mag check with left magneto circuit (with RPM drop below POH allowed values). I felt that it got gradually worse but my local mechanic listened to my observations and cleared me for flying. However on the next attempt, after that consultation, sometime in late May, I had to abort a take-off as the engine suddenly lost a lot of power (like, negative surging) when I moved the throttle all the way to full (at about 100 percent out of 115 percent, yeah I know that doesn’t sound right, but it’s designed that way due to the turbo). Further ground testing concluded that the engine started behaving strange at about 4500 RPM and up (so the problem could go unnoticed during a standard 4000 RPM run-up, except for the bad mag drop which might have something to do with it all). Also the Flydat showed that EGT on the left side was about 50-60 degrees lower than the right side. This being at ground idle, taxi and run-up.
On my home field, I have an experienced and well respected mechanic who is also an authorised Rotax repair centre (though not with much 914 experience). However, he had a too busy summer with all the glider tow, parachuting and fire watch aircraft running in and out of shop, so we only got around to him assisting me in removing the carburettor “lids” and checking the diaphragms which were OK, as well as swapping spark plugs and, in one case, the spark plug cap which was worn.
In August, the previous owner – and builder – got tired of his old aircraft being grounded and drove over to help. We tore down both carburettors, changed floats and checked float bracket spacing. Then we synched the carburettors and the whole thing seemed fixed. The engine ran fine (though probably we didn’t thoroughly check the EGT values…) and I took the aircraft up for two touch and goes and one full stop. Oh the joy!
But… A few days later I took it up for a test run outside of the CTR. I did notice there was still a difference in EGT between the left and right side. However, the engine seemed to run just fine. Or well, both before the first and second test flights I noticed some slightly rougher running after switching on the secondary fuel pump during run-up. This was of mitigated by simply turning off the pump again. I suppose I should have concluded something was wrong and stayed on the ground, but off I went testing normal cruise, slow flight and some manoeuvres before returning to the field with a big smile on my face. EGTs were almost perfectly synched, especially at cruise power. However, on a long base, the engine started coughing and running rough.
The one thing that had changed from the slow flight at altitude was that landing procedures of course dictate that the secondary fuel pump be turned on. I cannot say for sure that it got better when I turned the second pump off, which I tried while searching for suitable fields and talking to ATC about the problem. The engine didn’t seem to want to fully give up so I only got around to declaring a state of urgency and promptly got the aircraft down on the runway. I’m not really sure today whether I landed with or without the secondary pump running, my mental priority being getting home in one piece.
Back on the apron, after dismounting (and swearing) I noticed a wet spot on the tarmac below the left (as in port) side of the nose, basically below the left carburettor spill cup and airbox drain hoses.
Now, my suspicions were, and still are, that the left carburettor is getting too much fuel and somehow overflows. Next time at the field, after talking to the mechanic about the issue, I simply went to the aircraft and turned both fuel pumps on. Within the minute, fuel was dripping out of the drain hose from the left side drain (I suppose it is some kind of drain) on the airbox where it connects to the left carburettor. When trying to show this to the mechanic maybe 10-15 mins later, the dripping did not reoccur.
The mechanic went on to disassemble the left carburettor’s float chamber, checked the float markings and weighed them. He also checked the float bracket spacing and, like me and the builder a few weeks earlier, found it correct. We later reassembled the carb together and test ran the engine, which seemed to run ok.
However, the next time I went to the aircraft and turned both pumps on, a much greater stream of fuel ran out of the drain hose from the left side of the airbox. Also, this time, there was fuel dripping out of the drain from the spill bowl/cup under the carburettor and I could also see and feel fuel running down the outside wall of the float chamber.
Once more, when testing again 10-15 mins later, there was no sign of any fuel dripping/leaking.
Any ideas as to what is causing this would be most welcome!
