Carb Float Bowl Breather/Overflow

George,

Did you perform the float needle valve pressure test after reassembling the carbs?  See MMH, section 73-00-00 paragraph 3.3.2.  It would seem that something went wrong during reassembly.  

Ok, since the fuel only come from the vent tube then I would absolutely check the brass float armature height. 10.5mm. I haven't found any real difference if it is 10mm or 11mm.

If these measurements are ok then I'd pull the viton tipped float needle valve out that controls the fuel delivery and polish the brass seat down inside. I have had 4 that I have done this with and problem solved.

Thank you, both.  I did not do the leakage test primarily because I don't have a pressure reducer to go that low.  But I did some troubleshooting today and started by opening the main fuel valves and letting gravity feed fuel to the carbs (high wing tanks).  Fuel again came out of both sides of the airbox, with the 1/3 side first and little if any, in either drip tray.  Next I took off the 1/3 exhaust manifold, drip tray and carb bowl because it (1/3) was the carb I suspected of leaking.  I checked for the 10.5 mm distance and adjusted the float bracket somewhat lower (as mounted), but not significantly.  Then I opened my fuel shutoff valves, and let gravity again feed the carbs.  Sure enough, fuel came out the 1/3 carb (not the airbox) until I rather quickly pushed up on the float needle bracket to seat the float needle gently.  Then it stopped.  No more fuel dripping.  So thinking that perhaps something in the float bowl had previously shifted mechanically during transportation and recent installation, I re-installed the floats and bowl.  Then I tested fuel again but this time with one electric fuel pump operating for 5+ secs, to make sure fuel pressure was enough to fill the bowl and check the float needle seating properly.   I did not see any leaks.  I repeated this and still did not see any leaks, but ran out of time to continue.  I'll try it again tomorrow, just in case I underestimated the time to fill up a float bowl with fuel coming from an electric pump.  But hopefully, it was some mechanical shifting during the first installation that caused this float bowl issue.  If not, then I may have to polish the float needle seat as Roger suggests.  Your advice is always welcome, thank you.

When I rebuilt my carb’s a year ago I had the same problem. I found one of the new float armature must have had the holes where the pin went through pinched a little to tight. It would occasionally cause the armature to slightly drag on the pin as it rotated when the float moved up and down. Opened the pin tab’s up a little and it let the armature move freely and the needle close every time.

Roger Lee wrote:

Ok, since the fuel only come from the vent tube then I would absolutely check the brass float armature height. 10.5mm. I haven't found any real difference if it is 10mm or 11mm.

If these measurements are ok then I'd pull the viton tipped float needle valve out that controls the fuel delivery and polish the brass seat down inside. I have had 4 that I have done this with and problem solved.

Hi Roger,

"..........brass float armature ........."

Is this what Rotax Heavy Maintenance,73-00-10, page 13, calls "float brackets"

".....height 10.5mm......"

Same page as above - "Spacing on the 912 Series: 0.4 to 0.5 mm"

Then next page 14;

Shows a diagram Figure 13.4: Spacing  (912 Series) with the measurement 0.5-0.1 mm

All a bit confusing.