912UL Carb issue

[tim_fc3s]

I have a fairly early 80 horse 912 in my KF4. The plane is at a mechanic shop and they told me the carbs are leaking fuel when idling. They showed me a video and then I went out to the shop to see it for myself. I had just rebuilt the carbs with all new seals and needles, it seems like the fuel pressure might be over powering the float needle but I'm not really sure. I put a fuel pressure gauge inline and the whole rpm range it sat at 5-5.5 psi. I do not have a return line in the fuel system and everything I'm seeing online says that rotax mandates them now. The question i have is, had anyone else had this issue, what pressure is your fuel system, if you did install a return line, how big is the "orifice" to restrict the flow in the return line? It is not a consistent thing either, sometimes one carb does it, sometimes both and a few times neither did..

[109JB]

Crank the idle mixture screws in. I had an 80 hp that did the same and the book number for initial setting of the idle mixture was way way too rich. Ran like a sewing machine once the mixture was leaned a lot.

[avidflyer]

I think you should install the return line. I have one on both of my Kitfoxes. I believe the orifice is .040" if memory is correct. Better check on that though. Rotax says is should be there for reason. Also, a major cause of fuel leaking from the float bowls is over weight floats. The two together should way 7 grams maximum. If they are heavier, they of course sink lower in the fuel which raises the level of fuel in the float bowl. JImChuk

[tim_fc3s]

Crank the idle mixture screws in. I had an 80 hp that did the same and the book number for initial setting of the idle mixture was way way too rich. Ran like a sewing machine once the mixture was leaned a lot.

I'll give that a shot tomorrow when I head to the airport.

[tim_fc3s]

I think you should install the return line. I have one on both of my Kitfoxes. I believe the orifice is .040" if memory is correct. Better check on that though. Rotax says is should be there for reason. Also, a major cause of fuel leaking from the float bowls is over weight floats. The two together should way 7 grams maximum. If they are heavier, they of course sink lower in the fuel which raises the level of fuel in the float bowl. JImChuk

I am heading to the airport tomorrow to install a return line, I did make a .040 orifice, hoping thats the right size. I did see a few other people say that seems right. The floats are all in spec also, but will double check.

[atosrider]

my early 912UL was in storage for many years with the previous owner, then brought back ot life for a few hours, only to be on hold waiting for me, the new owner to rebuild the plane .. so 2 summers now with 50 and 60 hours .. first was ok but then factory rebuilding made sense at the start of the second summer ... floats were weighed .. bypass pressure relief added, as in the first summer no issue so this was a preventative addition ...
ok, enough history, now to floats getting saturated and heavy .. they had different suppliers with different material compounds over the years .. so if the weight is correct then look at the bend in the clip that triggers the inlet valve .. very fussy setting, and I would be skeptical of most aircraft mechanics being "tuned" into this technique of turning the carb upside down and eyeballing that the bendable lever is parallell / horizontal ... it just doesn't seem very aircrafty having specs like that ..
fuel pressure bypass .. for about 30 years the 912UL didn't need this bypass .. there were some pump supplier changes over the years .. and the story I picked up from visiting the original and local rotax distributor .. have cooler fuel for less chance of vapour lock, maybe it's hot and you wanted a restart fairly soon after shut down ... my unit has an analogue fuel pressure gauge, the readings before and after were never out of range ... I can see the new return line going up to the right wing tank, and puzzle over how the air bubbles get in there, but it's nice to see them merrily moving along .. maybe it's from cavitation through the choke point?

one of our clan put up an excellent thread on fine tuning the 912's, it would be nice to dive into the process like he did, but I'm no into installing O2 probes in the exhaust system, seeing as the results I'm getting seem ok .. the interesting point that gentleman made was, factory specs are one thing, but every motor has its own ideal settings .. this makes sense particularily in our cases, where the airflow under and through the shroud must be different in every unit, there are so many factors that can influence ..

eventually my idle screws were turned in a bit from factory settings ..
bending the float clip is the quickest and cheapest potential fix we could ever ask for ..

looking forward to your reported results

[jiott]

That line you see going into the right wing tank with the bubbles in it is not the return line; it is a vent line for the header tank. The return line typically goes into the header tank, or some folks run it into a gascolator (not recommended).

[tim_fc3s]

I installed a t-fitting in the feed line from the fuel pump to the carbs, tapped the barb that will be my return line, drilled a brass machine screw with a .040 drill and then ran the line back to the header tank this morning. We just fired it up and not only did it stop over flowing but the engine sounds WAY better at idle. After lunch we are going to check the vacuum sync of the carbs. Thank you for the suggestions, feels good to actually have a small win right now haha.

[tim_fc3s]

20250404_111637.jpg20250404_111628.jpg20250404_111609.jpg20250404_111602.jpg
The upper line in the fire sleeve is the return line, which goes to hard line back to the header tank.

[tim_fc3s]

Annnd I spoke too soon. As soon as I started it up to do the sync it started leaking out of the left carb again.