Carb Float Bowl Breather/Overflow

Hey Guys,

maybe it is just a question of Carb Synch.

I had noticed that in the past, while doing the synch. While the engine was shaking due to bad synch (at the very beginning), it was spitting fuel. As soon as the synch was done, no fuel anymore....

Just as an "idea"....

Always blue skies!

Gerhard

Hi Gerhard - Thank you for your interest.

The carburettor balance has been done very carefully, twice.

The first time before the aircrafts first flight a little over 12 months ago and again as part of this investigation, I sent my carby's to the Rotax dealership (see earlier responses) on return/reinstallation, carbys balanced.

The engine runs /performs very well, with very little vibration - can't ask for better.

This annoying, small fuel "spillage" does not stop the aircraft flying.

hi All

So if you have exhausted all possible factors perhaps it is time to go back to some old info to understand what may be happening.  I will take you back to the early days when the 912 ULS was introduced in 1999. (yes it has been that long)  The base engine was sold with the low power black starter and no clutch.  Only the S, certified, was supplied standard with a clutch, it was optional on all the rest.  A lot of OEM had huge vibration problems when they swapped out the 80HP and installed the 100HP.  Looks the same, has the same bolt pattern, what could go wrong.  Well a lot actually.   Go and download the SL, yes service letter, on the subject SL-912-010.  It is a great read.  Attached for you is one excerpt of this SL, I highlighted a part most tend to ignore or perhaps did not know.

Cheers

Hi Roger

I refer to an earlier conversation in this thread:

"My Son, an enthusiastic dirt bike rider/home mechanic, advises me that his carburettor used to eject fuel quite often.

He insist this is normal and the result of the float bowl being tilted (TO/CO), such that the float comes away from the valve, allowing extra fuel into the chamber, which then exits the breather tube, when the bowl returns to level.

His fix - extend the breather line into a loop, such that the top of the loop is slightly above the float bowl - he is urging me to try this - your comments?"

Your response (in part)

"There is no reason to put some different type of vent tube. Putting a different type of tubing setup is a bandaid. Just fix the cause and all's well. Most of the Rotax flying around the world don't have issues so you would need to find the cause. None of my clients have any fuel venting issues." 

https://www.rotax-owner.com/videos-topmenu/rotax-minute/454-carb-vent-lines

I have just viewed the above video on proper Carburettor Vent/Breather tube installation. 

The only "light bulb" information for me, was the view (behind Mr McLaughlin) of the 912 breather tube to air box - the breather tube curves upwards, rising to the top of the carburettor, before descending down to the air box - this would seem to be very much in line with my Sons advice  - your comments?

You should not have any fuel coming out of the vent tube. None of my clients do. I have only seen this when things aren't set correctly. It shouldn't be an everyday thing.