Carb Float Bowl Breather/Overflow

hi Sean

So good news, but conflicted as to what it means?

Any changes on the airbox i would assume will change the resonance on the carbs.  Could be as simple as the slight change in harmonics that the float bowl is seeing changed the resonance for the better. (reduced foaming) In my experience this would seem logical.  

In think the high pressure air would be more likely to cause a rich mixture rather than lean.  The slide is activated by the delta from the abmentle air on the underside of the diaphragm relative to the venture low pressure.  The slide may have a tendency to ride up rather than down relative to the incoming air in ram air 

You should be able to check the actual air temp and compare it to your outside air, this will tell you if you have such a wide spread as 15C.  In flight this will depend on your undercowl pressure of airflow.  For a small aircraft the relationship of outgoing air to incoming is usually about 3 to 1.  (3 out to 1 in for best cooling a flow past coolers) You can check your undercowl pressure simply by placing a spare mechanical altitude gauge in the cockpit and run a tube to the area where your carbs are breathing.  If the reading is the same then you are golden, nothing wrong.  It is the temperature while in normal flight that is important in my opinion. 

Cheers

I have an under cowl temp (UCT) gauge - probe close to carb air inlet. I am able to compare OAT with UCT. That's how I get my figures.

As for pressure - dont have a spare altitude gauge - will see if I can borrow one from somewhere - results will be instructive.

"In think the high pressure air would be more likely to cause a rich mixture rather than lean.  The slide is activated by the delta from the abmentle air on the underside of the diaphragm relative to the venture low pressure.  The slide may have a tendency to ride up rather than down relative to the incoming air in ram air "

Very persuasive argument - thanks for that.

One small question, in relation to above - how does the 914 handle high carburettor air pressures?

Sean

I regards to the 914 the carb intake is inside the airbox as well as the vent tubes are also connected to the airbox (from the top on each side)  The turbo pressure then is seen at the carb inlet as well as the float chamber.  As long as it is balanced the carb works the same as with an unboosted engine.  This is why I believe what you are seeing is more related to a resonance that is exciting the fuel in the bowl.  I am not aware of any major differences with your airbox design to the somewhat similar one Rotax devised with the exception on the inlet tube to get the fresh air.  Perhaps something in how it is suspended on the carb sockets?  

As a note, the 914 sees at best a 40 inch HG at full take-off boost.  Normally it runs about 33 HG for max continuous (5800 vs 5500 RPM on the map in the TCU) The Rotax airbox has an Adele clamp on the top of the ring mount, the airbox has a lug that bolts to the clamp and this supports the airbox weight so it is not suspended entirely on the carb sockets.  The rubber carb sockets are for vibration damping and not really heat soak as some speculate.  

Cheers

RW,

"The Rotax airbox has an Adele clamp on the top of the ring mount, the airbox has a lug that bolts to the clamp and this supports the airbox weight so it is not suspended entirely on the carb sockets."

My airbox weight was largely supported by the carburettors, with only one separate rubber mounting, near the front of the engine.

I would like to reinstall it BUT only if I can find someway to support its weight, without acting on the carburettors.

I see that the Rotax Airbox is supported, via the Adele, to the Ring Mount.- unfortunately my Sonex does not have a ring mount. Perhaps a bracket off the firewall - will look into it.

PS - having quite a lot of ERROR messaging & losing content.

“I have an under cowl temp (UCT) gauge” - brilliant!
A couple of years back I set up a test with wires taped along the cowling and to keep the temp sensors in place. If I remember correctly the different of OAT and airflow into carb (AIC, 😉) taken right at the filter was 10 to 12 C - far less than I had expected to be honest.

However, I looked at the temp relevance starting from ISA conditions and potential power loss through temp. My conclusion at the time was that the temp advantage, despite the less than expected delta, makes a difference. It does not change the world but is more than academic. I’ll see if I can find my documentation on this.

By the way, both numbers and butt confirm better engine performance (and climb) during winter times. Thus, I would suggest to find a way to mount the airbox in a way that it supports the carbs (not vice versa) and enjoy the advantage.