Rotax 912 TBO (15 years) and On Condition

[IBob]

13 hours ago, IBob said:

I may have this wrong, but:

 

I'm pretty sure his problems started with his assumption those are drain tubes (and he's still thinking that and calling them drain tubes).
My understanding is they are nothing of the sort:

They should be connected to wherever the carb is drawing it's air from, their function being to ensure that the float chamber is at the same pressure as the incoming air. (So, for instance, if you have an airbox with a filter, they are connected to the airbox but after the filter.)

Put them anywhere else and you'll get exactly the sort of behaviour he's talking about, as the float chamber is then running at a pressure higher or lower than the carb air, resulting in rich or lean mixture.

 

Feel free to straighten me out if I haven't got this right.

 

Correction to my statement above:
On further reading it seems the carburettor vents do also provide a drain function in the event that the floats fail, reducing carburettor flooding and allowing the engine to continue running (albeit very roughly) rather than stopping the engine.
The Sav has the vents plumbed to the airbox, after the air filter, with a separate drain line from the bottom of the airbox venting to atmosphere below the cowling.

Where there is no airbox, common practise seems to be to attach short hoses to the carb vents, and tuck these under the float bowl clips.

[Thruster88]

In certified aircraft any fuel drain hoses from fuel pumps (seal failure) or fuel injection systems are ALWAYS routed to the bottom of the cowling for obvious reasons.  

[skippydiesel]

10 minutes ago, Thruster88 said:

In certified aircraft any fuel drain hoses from fuel pumps (seal failure) or fuel injection systems are ALWAYS routed to the bottom of the cowling for obvious reasons.  

The reasoning is obvious and reasonable however Rotax go to some length to advise appropriate location of these vent tubes and its very short tubes terminating close to the carburettors. The most important instruction is to have the termination point(s) in the same air flow/pressure preferably similar to the carb intake airflow/pressure.

Airframe manufacturers routinely stray from this instruction - my aircraft has a Y piece to join the two vent tubes, with the single "tail" tube terminating in a custom made, perforated, length of brass tubing, that is fixed low down close to the bottom of the firewall - seems to work okay but definitely at odds with the Rotax bible.

[IBob]

14 hours ago, IBob said:

FWIW, I believe it is the pressure on those vent lines that is lowered when a fuel leaning system is added to this engine...and that is done by introducing a small amount of vacuum taken from the balance pipe after the carbs (but before the engine).

I should add that this makes for a very unsophisticated leaning system, in that any change in throttle setting alters the manifold suction, which then alters and upsets the fuel leaning, resulting in rough running and possibly even stoppages.

Manufacturers of one such system I saw state that the mixture must be set (back) to full rich before any changes in throttle setting.