Float Bowl Fuel Leak
Posted: Tue Sep 03, 2024 2:39 am
This question / information is for Rotax 912's mounted to the Sonex engine mount bed adapter.
My new 912ULS has had a mystery carburettor float bowl leak, from the breather tube, since about 3 hrs to now, 104 hrs.
A whole range of "fixes" (including carburettors back to Rotax) have been tried - all have had some success but non have cured the problem.
Today I read a 13 year old discussion, on Rotax Owners Forum - This is an excerpt, that relates to the use, by some aircraft manufacturers, of engine bed mounts (like Sonex) rather than ring mounts -
"The aircraft were fine when the gearbox was shimmed to max but shortly after some wear internally the engine would vibrate so bad that fuel would spew from the carb vent tubes and shake badly. This was noted as the pilot would be coming down to just below peak torque on throttle down. Descents were scarry for some. The solution, since it was too late for the kits to be changed and the OEM did not want to redesign, was to install the overload clutch. (circa 2003 large starter and overload clutches were added) This absorbed the inputs internally to allow them to use the ULS with the existing bed mounts."
HAVE ANY OF YOU EXPERIENCED ANYTHING SIMILAR???
The only reason I know I have had fuel leaking is that I installed a carburetor float bowl breather catch can. I have been able to get the fuel discharge down to about 10 ml/hr (manageable) that is until the last 2 hr flight, when it suddenly jumped to about 50 ml/hr.
If you don't have an air box (as I have) you likly have the breather going to atmosphere, as per Rotax instructions, the small amount of fuel ejected would be atomised/vapourised. Other than the possibility of a mystery fuel smell from time to time you may be unaware of fuel being discharged from the float bowl.
Other than startup/shutdown shake, my engine seems to run very smoothly, however the ability of humans to detect vibration at certain frequencies is very poor.
My new 912ULS has had a mystery carburettor float bowl leak, from the breather tube, since about 3 hrs to now, 104 hrs.
A whole range of "fixes" (including carburettors back to Rotax) have been tried - all have had some success but non have cured the problem.
Today I read a 13 year old discussion, on Rotax Owners Forum - This is an excerpt, that relates to the use, by some aircraft manufacturers, of engine bed mounts (like Sonex) rather than ring mounts -
"The aircraft were fine when the gearbox was shimmed to max but shortly after some wear internally the engine would vibrate so bad that fuel would spew from the carb vent tubes and shake badly. This was noted as the pilot would be coming down to just below peak torque on throttle down. Descents were scarry for some. The solution, since it was too late for the kits to be changed and the OEM did not want to redesign, was to install the overload clutch. (circa 2003 large starter and overload clutches were added) This absorbed the inputs internally to allow them to use the ULS with the existing bed mounts."
HAVE ANY OF YOU EXPERIENCED ANYTHING SIMILAR???
The only reason I know I have had fuel leaking is that I installed a carburetor float bowl breather catch can. I have been able to get the fuel discharge down to about 10 ml/hr (manageable) that is until the last 2 hr flight, when it suddenly jumped to about 50 ml/hr.
If you don't have an air box (as I have) you likly have the breather going to atmosphere, as per Rotax instructions, the small amount of fuel ejected would be atomised/vapourised. Other than the possibility of a mystery fuel smell from time to time you may be unaware of fuel being discharged from the float bowl.
Other than startup/shutdown shake, my engine seems to run very smoothly, however the ability of humans to detect vibration at certain frequencies is very poor.