Fuel Flow questions/PROBLEM

[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

[HighWing]

I had exactly what you are describing once after a carburetor rebuild. Talk on the email list - back then - suggested they may have sent the wrong float needles with the rebuild kit. I replaced the new ones with the original ones and back to the way it was for another 500 hours. Apparently the rubber tips on the needles come in various hardness and the new ones were not correctly mated to my engine. Sorry, I can't help with which is which - too tlong ago.

[jiott]

Little Rocket you don't say if you have installed the proper orifice in the return line yet? If you have not, then the fuel is freely returning to the header tank and preventing the fuel pump from building up the proper pressure. Do you have a fuel pressure gauge? What does it read? The system will not operate properly without the proper fuel pressure at the carbs. I don't know if this will take care of your problems or not, but it needs to be done regardless. An interesting simple test would be to temporarily totally disconnect and plug the return line and run the engine. The return line is only there to prevent vapor lock anyway. Many folks have run for years without a return line. Rotax believes it is important to have, but you must follow their instructions when installing it.

[jrevens]

Little rocket... I think that HighWing or Jim are probably on the right track. Maybe you have bad or incorrect floats or needle valves. I didn’t mean to imply that the lack of a check valve was causing your specific problem, but just that it was not right. With basics such as that not done properly, who knows what else is incorrect. Since the engine runs so well on just that return line, it does seem likely that the normal line and pump(s) are delivering too much fuel and/or at too high a pressure for your carburetors, causing them to overfill. Somehow or other you reached a “sweet spot” with just the small return line and gravity flow and the engine seems to run properly. You said you installed new carbs. Are they “new” or rebuilt? It seems like you may have carburetors that are not correct for some reason, or you possibly have a mechanical pump on the engine that is producing too much pressure. As N213RV said, check the fuel pressure downstream of the mechanical pump.

The only thing I can think of, if the pressure reaching the carbs is actually too low because of the wide open return line, is that maybe the fuel level is low enough in the carbs to cause extremely rough operation at that time, but the roughness and shaking is severe enough to cause gas to come out of the overflow tubes even though the level is low in the float bowls. Others who are more familiar with the Rotax might be able to answer that. As Jim said, try it without the return line.

As far as your question about that return line - the newer engines have a port with the orifice built-in that is on the fuel line that comes from the pump, at the point where it tees off to both carbs. Also, it has nothing to do with controlling fuel pressure to the carbs or “getting rid of excess fuel”. It’s the pump that should be producing correct pressure and flow to feed the carbs without overpowering the float valves, plus enough extra to provide the orifice-controlled amount of fuel that recirculates back through the return line.

[Dave S]

Just a thought,


If the installation of the return line was done without a check valve; and, the engine was run for any time (or a considerable time) in this configuration, the carburetors were getting unfiltered fuel through the orifice. Any crud coming back through the return line would be limited to the size of the orifice (0.040"/0.041").



The reason for the check valve is to prevent unfiltered fuel from returning to the carbs through the orifice and plugging the orifice or dirting the carbs since the tap for the return line is between the fuel pump and the carbs and the other end at the header tank is bypassing the gascolator and filters.


There may be at least some potential for crud having entered the carburetors.

[little rocket]

Thanks everyone.
Lets start with the orifice where exactly should it be located what does it look like?
A bit more about what I know about the problem from previous owner.
He was the second owner, and the plane sat for almost 2 years without being flown .
He replaced the header tank with the Kitfox new aluminium one ,then he one by one chasing the problem replaced the mechanical pump with new one ,then added new electric fuel pump,then added bypass line then replaced,carbs with new carbs. the problem had gotten better but not totally cured.He had replaced the metal leading edges on the prop and figured that the prop was out of balance causing a vibration .
So when I bought it I balanced the prop and yes it was way out of balance and the balancing made the engine smoother.
BUT here is the kicker, the plane being new to me I had the fuel valve shut off the whole time that I balancing the prop.I then put the carb mate on it and balanced the carbs and got it running like a top in all ranges [again fuel valve OFF] Finally I was ready to fly her.So I fueled up and sumped the tanks then went to sump the gaslater and no fuel came out of the gaslater.Well how could that be ,so I checked the fuel valve and it was turned off,I'm old so figured I must have turned it off the night befor after I did the carb balance.
So turned it on and fired it up and noticed it was running rough then saw fuel coming out the bottom cowl .Shut it down removed the cowl figuring that the gaslater valve must be leaking because I had just drained it.It didn't appear to be leaking ,so with cowl off I started it to find leak and right away ran rough and pushing fuel out the overflow.So shut down and thought about things and decided to shut fuel valve OFF and start it .Fired up purred like a kitten.Shut down and started fallowing all the fuel lines to determine where the fuel could be coming from determined it was coming from the return line and here we are .
Another question is I see guys mentioning fuel filters ,where are you installing the filter.
I'm from the world of lycomings and continental so this stuff is all new to me.
The fox has NO fuel pressure gauge so I don't know what pressure I'm getting at the carbs.
All the high wings I have owned with lycomings and continentals have just used gravity to feed the engine and it seemed like the fox will run just using the same ,so why all the pumps ect?
AGAIN THANKS GUYS FOR ALL THE HELP
I'm old and not to bright so any PICTURES of the orifice and location will help.