Carb Float Bowl Breather/Overflow

Sean, indeed it could be.  Given you have a ULS I would assume you have an overload clutch.  Follow the MML and MMH to lock the crank, then check the friction torque and be sure your preload on the shims inside are in the mid to high range of adjustment.  (heavier props can cause more rapid wear.  

Question as to when this occurs.  If this is happening on decent, lowering your RPM from high crusie nd you are dropping down from peak torque (5200) at about 5000/4800 this may be a sensitive area.  If this is where you have issue then for sure reshim to the max point and work with your mounts.  I dont know what you currently have but if it is a Lord or Berry mount some work in tension and compression and others work in shear.  if it is the type that work in tension and compression usually these have an internal sleeve inside the rubbers.  in years gone by we used to shorten (increase pressure) to make the mount more rigid in KitFoxes (when they moved from 80 HP to 100 HP engines) 

Not to scare anyone but Rotax has a publication for checking, directed mostly towards the high compression engines such as the ULS, engine vibration.  Have a look at the last bit, it is the engineering way to check but for most is too complex for anyone other than engineers. 

Cheers

Thanks RW,

It's a Sonex supplied (made?) mount that, adapts the Aerovee designed mount, to Rotax.

In an effort to reduce side to side engine movement at start/stop, I have made the following changes:

 # Higher durometer rubbers.

 # Cup shaped bottom retaining "washers".

 # To adjust thrust line, I have a pair of 1.2mm thick fender washers "lifting" front.

My prop has been dynamically balanced.

From my perception engine/prop runs very smoothly - I do not notice any vibration on throttling down below 5200 rpm.

Attention RW  & Roger L,

I asked this question earlier - as yet no response. 😈

What I am intersted in exploring is, what might be the symptoms for;

 # Relative high airbox pressure -  due to positioning airbox air intake at front of cowling opening (RAM)

 # Relative low pressure in the air box - caused by an overly restrictive air filter.

hi Sean

It's a big guess so to do a test you need to know the pressures inside the box.  In the old experimental days we would use a altimeter.  Take the probe and place it into the airbox somewhere, it has to be sealed in so you will need a fitting or existing fitting you may have made on that airbox.  You now have a way to check if the air within the airbox is the same as ambient.  If the engine is showing the same as outside air you are golden, no high or low pressure.  That is the ideal pressure.  

If you have high pressure you will  have your prop in ambient and the engine air out of balance, this may cause issues with your calibration.  It may perhaps cause your vibrations that we presume give you the fuel overflow.  Low pressure will rob you of some power, how much is not easy to determine.  Again it may then put the engine out of balance to your propeller ideal loading.  

You may think that a manifold pressure gauge would be a better one to connect, however MAP is not the same in the airbox side of the butterfly,, they are always different by  0.8 inch HG or more.  That value is very dependent on throttle position and the air in the inlet, unless turbocharged, should be the same as ambient. 

Let me think on it a bit more.

Cheers

RW; Thank you for your thoughts/time.

I am well aware that the air pressure at the carburettor inlet should be the same as in the float bowl (hence the importance of float bowl breather location).

I think I may have seen a comment somewhere, that Rotax carburettors do not like high (RAM) air pressure.  I assume this comment included float bowl pressure. I do not recall why this would be the case or what the effect /symptoms would be. My speculating would be, a lean fuel burn with high EGTs, don't know if fuel could be ejected from the float bowl into air box. Power at TO/Climb would not be affected due to relativly low forward speed.

I think low air pressure, at the carburettor intake, may be easier  - I would anticipate/guess a rich cool running engine/ low power/black coaked plugs. What I don't know is if there may be a tendency for fuel to be ejected into the air box.

Engine performance observations:

Starts & runs smoothly. 

Good TO/Climb/Cruise performance.

Left EGT slightly higher than Right. Same for Cylinder Head (coolant) temperatures.

High rpm/fine pitch, low forward speed (80 knots) can result in a high EGT warning.

!00 hr service/inspection - plugs black & sooted up. 

Carburettor pneumatic balance - during 4000-5000 RPM pressure check (dials equal),  vacuum greater, compared with previous 912 ULS. Dial/Bourdon gauges - indicator does not go as high, towards atmospheric - subject to imperfect memory.