Float Bowl Fuel Leak

[Skippydiesel]

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.

[Murray Parr]

Did the increased leakage coincide with when you changed the durometer hardness of the Barry mounts? Also, how heavy is the prop? Maybe some resonance happening at certain RPM combinations? Just thinking out loud here...

[Skippydiesel]

Hi Murray,

The prop is an Airmaster CS/ Whirlwind 3 blade, Model: AP332 SCTFH, Blade type: WWR64W Weight: 11.5 kg

Its heavy, by most Sonex standards but within Rotax limits for prop weight. It has been Dynamically Balanced.

I don't think the prop has much, if anything, to do with this problem.

No, the change to a harder durometer (to reduce engine movement at start/stop) had no effect on the fuel spillage - did reduce movement..

The sudden increase in fuel spillage, coincided with a 2 hr flight, The Oaks/Cowra. Outbound at 4500 t /130 knots indicated in "lumpy" air - Return, 10 min climb @ WOT 900ft -9500 ft/124 knots indicated with a long/fast (>165 Kn) descent down to 3500 ft at end.

The one "FIX" I have yet to try, is replacing the home built carburettor air box (Rotax look alike) with K&N conical filters, to see if this helps.

The engine bed matter has only just come to my attention - Dont know how many Rotax / Sonex bed rail adapters are flying out there. The rail type bed mount, with its poor control of movement/vibration, is a rather cheap/nasty alternative, to what should be used with a Rotax 9, the ring style mount.

I would like to hear from any owners, who know/think they might have a fuel spillage problem.

[Skippydiesel]

From Rotax Owners Forum:

Conversation on Ring Mounts & Bed Mounts (Sonex/Rotax)

[i]
"In acceleration the load is being increased all the time. On deceleration the unloading is what causes abrupt swings in the harmonics and this is primarily seen just from 65 to say 70 percent power (4800 to 5100/5200)"

"The ring mount solves most of this in that it holds the engine from the ignition side and this fore and aft movement, or pulse, is much easier to damp. The long bed mount arms under the engine from the firewall forward vibrate lot and a lot of these designs crack mount tubes. (not just on Zenair) After years of working with OEMs to switch to the ring mounts we have reduced most of the issues with cracking and failing mounts. The desire to build the kit for less was the prime motivator for OEMs to use bed mounts but in my opinion it is a mistake. "
[/i]
"...............fuel foaming, or overflow, indeed they may be related to a harmonic. "

"Today bed mounts are rare (thank heavens) and almost all OEM have the Rotax ring mount or a clone ring mount to fit special airframes."

Dont think Sonex got the memo.

[Skippydiesel]

It may be instructive for owners of Rotax powered Sonex to read:

https://legacy.rotaxowner.com/si_tb_inf ... 12-010.pdf