Fuel Flow questions/PROBLEM

[little rocket]

I have just bought a Kitfox 4 with a 912 Rotax.Have not flown it yet.Previous owner had problems with fuel coming out the overflow vents and engine running rough.Things that he tried were ,installed fuel return line to bottom of header tank.Installed new mechanical fuel pump,installed an electric fuel pump,installed new carbs.
Anyway the engine still overflows fuel out the carbs at idle .But the interestingly I found out by mistake[forgot to turn fuel on ] is that with the fuel shut OFF she purrs like a kitten from idle to full power and everywhere in between and with the carbmate on it both carbs are in sync at idle ,mid range [3000 rpm] and full power. Also no overflow from carb vents.Now it should not even be running let alone purring like a kitten with the fuel shut off. I pulled the fuel line after the shut off valve and verified that it works and it does turn the fuel flow completely OFF.
I checked the plumbing to and from the header tank to see if it was right and it is .From the header it comes into the cabin to fuel shut off valve then to new electric fuel pump,then to gaslater then to new mechanical fuel pump,then to a T that goes to the fuel return and to another T to the carbs.
The only way I can see that it could be getting fuel [WHEN THE VALVE IS OFF] is from the return line.
It is plumbed at the bottom of the header tank and is then goes to a T between the 2 carbs in the engine compartment.So it must be gravity feeding from the bottom of the header tank.
So i'm wondering if this is causing the problem of the fuel coming out of the overflow on the carbs.Instead of returning excess fuel to the header tank it is gravity feeding more fuel to the carbs when the main fuel it turned on .So I wonder if I could connect the return line somewhere else so that it retuns excess fuel rather then feeding fuel to the carbs?
It has the newer aluminium header tank on it.It just doesn't seem right to me to have the return line going to the BOTTOM of the header tank?
Any help would be greatly appreciated.
THANKS

[jrevens]

Just a couple of points... first, the return line must have a check valve on it that prevents back flow. Secondly, the flow needs to be restricted through a specific size orifice. Don’t know if it has anything to do with your problem, but those things are definitely not correct. I don’t think that the return line going into the bottom of the header makes much, if any, difference.

If you have a model 4, with reversed intake headers, the carbs are hanging out a little further from the engine, giving a little more potential for overfilling carbs because of possible increased movement/shaking, perhaps during “rough” idling. Also make sure that the fuel pumps are correct and not higher pressure than they should be. You stated that the carbs were new, so hopefully you don’t have a float or needle problem.

[N213RV]

You definitely need a one way valve on the return line. If the line is coming out of the header tank, it is being pressurized from the column of fuel in the wing tanks and it is not working as a fuel return, but rather another source of fuel to pressurize the carbs.

To me, it sounds like you have too much fuel pressure. I am currently working thru a similar situation on my Rotec engine with a new TBI that required a fuel return. I have 2 electrical pumps, but when either is on, fuel leaks out the TBI at idle. When I shut both off and just use tank head pressure, it works great. I don’t like the idea of no fuel pump, but it flys in all aspects of flight without one (ie low fuel, full throttle, max climb angle, etc). I’m considering going with a lower pressure fuel pump for take off and landing to see if that prevents the problems I am seeing..... I think I too am over pressurizing the system. (Also my fuel flow goes way up when the fuel pump is on as I am returning all the excess unused fuel back to the tanks). When I shut the fuel pumps off, I see fuel flow and fuel burns about where they used to be.

[jiott]

As was said, the model IV with reversed intake headers has always made it more sensitive to vibration at the carbs, and it is well known that too much vibration causes the float needles to dance on the seats and not shut off properly. So I would do several things: make absolutely sure you have excellent sync of the carbs; even though they are new, take off the float bowls and weigh the floats (Rotax has gone thru 3 or 4 iterations of float designs in the last couple of years); make sure the engine mount isolators are not old and stiff and contributing to vibration; the carb mounting rubber sockets need to be soft and able to damp vibration; of course also do what has already been said-definitely need a check valve in the return line AND a properly sized orifice (I believe it is 0.040") the small return hose is NOT a substitute for the orifice.

[DesertFox4]

Another thought. Check your carb balance tube for any holes or loose connections. I once had a zip tie wear a pin hole through a metal cross tube on my model 3 with a 912 ul. Ran like crap.

Also a properly balanced prop can aid in reduction of fuel overboarding on the carbs. The DynaVibe prop balancer has helped some frustrated Rotax owners elinimate shaking carb issues. Of course a good carb sync is foundational but if all other avenues have been eliminated, prop balancing couldn’t hurt.

Also was wondering how many hours are on the engine.

Hope you find the cause.

[little rocket]

Ok
I installed a check valve in the return line and started engine then shut fuel valve off to verify that the check valve was working and after a few minutes the engine quit from fuel starvation ,so the check valve is working .
BUT the problem of running rough and pushing fuel out the carb overflow is still happening .
I put the carb mate on and the carbs are in balance at an idle and at 3000 RPM but still runs rough and pushes fuel out the vent tubes.I just did the prop balance 2 days ago. Also checked balance tube ,even made a different one to see if that made a difference,it didn't. Engine has 600 hours.
I hear everything you guys are saying ,BUT the fact that with NO check valve and the fuel valve shut off it purrs like a kitten in all RPM ranges.So the only fuel it is getting is from gravity feed from the UNCHECKED return line.
With the fuel valve off there is NO fuel flowing to the electric pump or to the gaslater or the mechanical pump.
With that system/fuel valve turned on it runs like crap both without the check valve and with the check valve.
I tried shutting the electric pump off and just using the mechanical pump but same crappy running engine.
It doesn't overflow the carbs at higher RPM but when you throttle back it starts overflowing the carbs again .
But with NO check valve and the the fuel shut off so it can only get the fuel from the unchecked return line threw gravity none off the above problems are present.
Seems to indicate that it is getting to much fuel and can't get rid of the excess threw the return line.
Where are guys tying into the fuel system for your fuel return line?
THANKS

[little rocket]

Thanks
The return line is a very small line ,way smaller then the fuel line so I think that part is OK but there must be NO check valve in it or it couldn't be returning fuel that lets the engine run when main fuel shut OFF.
On the rest of your observations those are all good points .I shut the electric fuel pump off so it is only getting fuel from the mechanical pump and it still forces fuel out the overflow. So that eliminates the to high pressure electric fuel pump.On the float/needle the fact that is runs perfectly when getting its fuel from the overflow would seem to show that those are not the problem.Also the carbs have cable supports on them to help with vibrations.
I will locate a check valve to install.
THANKS AGAIN