Fuel Flow questions/PROBLEM

[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.

[Dave S]

Little rocket,


Don't be too hard on yourself....I think you have an engine setup that had "a lot of cooks in the kitchen" and possibly some non-standard installation procedures. Just a guess but don't assume things were done correctly before you got the plane.



On your question about what the orifice looks like, it is part of the the newer Rotax Factory fuel manifold which a is small cluster of banjo fittings affixed to the balancing pipe which connects the right and left intake manifolds. The best I can suggest is to go to the engine manuals available on line at the rotax owner web site



https://www.rotax-owner.com/en/suppo...engine-manuals


Find the correct installation manual, then go to the pages for the fuel system and there should be a diagram of the parts you are looking for the identity of - in their various diagrams, I believe rotax refers to this as a "restrictor" aka "orifice"



Probably a good idea to get back to some basics on this. Once upon a time there was no fuel return line with a restrictor on 912 engines. At least be aware of that. Our 912ULS does not have one. Runs just fine normally - on occasion I have had it happen that the engine has flooded after a shutdown on a really hot day when I tried to start it about 8-10 minutes later - what can happen without the restrictor in unusual situations like this is the fuel locked in the hose between the the carburetors and mechanical fuel pump will take up enough engine heat to expand/boil/vaporize the fuel in that line and overpower the carb needles flooding the carbs. It's strictly a hot soak situation.



It is puzzling about the engine running with the fuel valve shut off. About all I can think of is that the return line system must be missing both the required check valve and the restrictor/orifice. It isn't too likely that the engine could run on gravity flow through the little restrictor.


Maybe I carried on too long; but, I have found with mechanical stuff that when things are puzzling - start with the factory setup information - Reading through the on line rotax installation manual (if you don't have the original copy) and checking if the fuel system is installed correctly might be a real good place to start.


Also - don't let my long winded discussion distract you - there might be other things not quite right with the engine. Multiple problems can often mask something that might not seem apparent.


Oh...also on the fuel filter - the original Kitfox installation instructions for this was to have an automotive filter in line just before the mechanical fuel pump. Many people use a gascolator on the lower firewall instead - that should work just fine and has some advantages.

[HighWing]

The thought that first came to mind with your latest post is that the engine runs fine on gravity feed - return line. Maybe it is a fuel pressure problem. During the rough drippy runs is this with the aux fuel pump on. If so, this might be the problem as the typical aux pumps come in a variety of output pressures. The one I have is Facet Part Number 40104 - 4 psi max 105 psi min - as shown on the Aircraft Spruce page.

[little rocket]

Thanks guys.
Found a guy not to far away with an avid with same engine and he had just installed the orifice so I went over and had a look at his install and what the orifice is all about .Saw how/where he had a fuel filter installed,also liked a couple other things that he had done .Nice to be able to see how someone else has done things.
No doubt I DON"T have an orifice installed ,no way would it have been able to run gravity feeding from the header tank threw that little pinhole. So I will order an orifice,meanwhile I will try to just plug the line off and see what that does if I can find a plug that size in my parts junk.
In answer to the electric pump putting out to much pressure ,I shut it off and just ran on the mechanical pump and made NO difference ,ran crappy with and without it.
Thanks for the links to the pump in Spruce and the link to the rotax stuff.
THANKS AGAIN GUYS

[mr bill]

Can't help thinking it may be fuel soaked floats.