Carb flooding and venting, Kitfox4 912 UL

[Bob]

After a 5 year rubber replacement, we started the engine to check for leaks and balance the carbs. In the course of the replacement, we had removed a fuel regulator which the previous owner had in the line to the carbs from the fuel pump. Checking with Lockwood Aviation, we had been told that was non standard and not needed to regulate fuel pressure. After starting, the engine ran well for several minutes. Then both carbs began venting and flooding. I consulted the previous owner as to why the fuel regulator was installed. His response was the the carbs had begun to vent and flood. So, he had installed the regulator to reduce the fuel pressure and stop the flooding and venting. The solution I have in mind is to replace the both carbs.

I am wondering if anyone has encountered a similar situation, or would like to comment. Thanks.
Bob

[jiott]

Before replacing the carbs, I would suggest you replace the little rubber-tipped needles AND the seats in both carbs. Very likely they are worn and don't seal well at the normal fuel pressure.

[Bob]

I bought the 5 year Hose and Rubber Component list from Lockwood Aviation which included the fuel pump, float needles, diaphragm, Carburetor Overhaul Kit, etc. I think the seat might be at fault. Lockwood thought so too, with no guarantee that they could fix it. Rather than loose more time, I'm going to bite the bullet with new carbs. Toys that don't work are kind of useless. Thanks for your comment.
Bob

[rv9ralph]

Did you check the floats to ensure they do not sink. Rotax-Owner.com has the Service Bulletin detailing the process.

Sinking floats will not close the needle valve that keeps the fuel bowl from overfilling and spewing fuel out the vent lines.

Ralph

[desertdave]

That's what happened to mine. I pulled the carb bowls off and one of the four floats was sitting at the bottom of the bowl. Sounds like Bob wants to get back flying ASAP with ordering new carbs.

[Av8r3400]

Did they supply the red or black needles? I believe the red ones are softer and work better with the reversed intake manifolds on a Kitfox 4.

[avidflyer]

If you want a deal on new carbs, Jason Busat from BadAss powersports sells new carbs with intake manifolds and fuel pump for $1250. I got a set from him and they are new. He strips these parts off new engines and puts fuel injection on the engines. Here is the stuff I got from him a couple of months ago. JImChuk

PS a pair of floats should not weigh more then 7 grams together. I've seen them weigh as low as 2.8 grams each. I would verify this is not the problem. Also, do you have a return line? New style fuel pumps make more pressure than old style ones.

[Bob]

Did you check the floats to ensure they do not sink. Rotax-Owner.com has the Service Bulletin detailing the process.

Sinking floats will not close the needle valve that keeps the fuel bowl from overfilling and spewing fuel out the vent lines.

Ralph

The floats floated. Thanks for the comment.
Bob

[Bob]

If you want a deal on new carbs, Jason Busat from BadAss powersports sells new carbs with intake manifolds and fuel pump for $1250. I got a set from him and they are new. He strips these parts off new engines and puts fuel injection on the engines. Here is the stuff I got from him a couple of months ago. JImChuk

PS a pair of floats should not weigh more then 7 grams together. I've seen them weigh as low as 2.8 grams each. I would verify this is not the problem. Also, do you have a return line? New style fuel pumps make more pressure than old style ones.

The weight of the floats checked in at 2.8g each. The new pump has three accesses IN, OUT, and VENT. The stuff looks very nice. The new carbs were $ 1100 something. Thanks for the reply.

[Bob]

Did they supply the red or black needles? I believe the red ones are softer and work better with the reversed intake manifolds on a Kitfox 4.

The new needles were black.

Thanks.

[Bob]

Before replacing the carbs, I would suggest you replace the little rubber-tipped needles AND the seats in both carbs. Very likely they are worn and don't seal well at the normal fuel pressure.

The rubber tipped needles were replaced. Lockwood said it might be the seats. Whether or not they could repair them, they said, was not guaranteed.

Thanks,
Bob

[avidflyer]

I've replaced the seat on a Bing type 94 carb without problems. Made a puller, ran threads into the seat/jet, and pulled old one out. Pressed new one in. I did this so I could run my Jabiru with gravity feed. JImChuk

[DesertFox4]

My question is why not reinstall the fuel pressure regulator as it solved this issue for the previous owner? It’s initial installation tells me the fuel pump is over pressurizing the carbs.
If your fuel pump is putting out too much pressure to the carbs, they will overboard fuel regardless of the needle color or condition or how many internal parts you changed in the rebuilds.
What’s going to really make you angry is spending a boat load of money on new carbs and solving nothing. While a pressure regulator may be non-standard on the majority of the Rotax installation, some of Rotax’ mechanical fuel pumps produce more pressure than the carbs can handle. If it ran for many trouble free hours before, it should again if you restore it to it’s previous configuration. If you still have that regulator, put it back in where it was before with the original fuel pump and give it a try. Too many changes at once and you will never know what solves the issue or worse, nothing was solved.
If that regulator bugs you being in the fuel system, try a new fuel pump first. Lots less money compared to new carbs or even used carbs that you may have to rebuild anyway. Do the simple things first. Frustration costs money and time.

[Slyfox]

I agree put the pressure reg back on. If you really want to know what's going on, check the fuel pressure before the carbs. that will tell all

[Bob]

My question is why not reinstall the fuel pressure regulator as it solved this issue for the previous owner? It’s initial installation tells me the fuel pump is over pressurizing the carbs.
If your fuel pump is putting out too much pressure to the carbs, they will overboard fuel regardless of the needle color or condition or how many internal parts you changed in the rebuilds.
What’s going to really make you angry is spending a boat load of money on new carbs and solving nothing. While a pressure regulator may be non-standard on the majority of the Rotax installation, some of Rotax’ mechanical fuel pumps produce more pressure than the carbs can handle. If it ran for many trouble free hours before, it should again if you restore it to it’s previous configuration. If you still have that regulator, put it back in where it was before with the original fuel pump and give it a try. Too many changes at once and you will never know what solves the issue or worse, nothing was solved.
If that regulator bugs you being in the fuel system, try a new fuel pump first. Lots less money compared to new carbs or even used carbs that you may have to rebuild anyway. Do the simple things first. Frustration costs money and time.

As we began the whole process of replacement, we saw the regulator and a fuel filter on the line from the pump to the carb. At that point we checked with Lockwood and asked if that was a standard setup, or necessary. According to them, it was neither. The carb was supposed to be, designed to be, happy with the pump, unfettered. When the new carbs are installed, we will have the fuel system pretty much as it came from the factory. I hope it works.

Thank you for your reply.
Bob

[Slyfox]

standard procedure for diagnosing this problem is to do a fuel pressure test anything short of that and you are throwing your money out the window. sorry I don't believe in your way of fixing something.

[Bob]

That's what makes the world go round.

Thanks for the reply,
Bob

[cwsnyder2001]

Hi Bob,

I have been struggling with carb flooding and venting also.

I was curious if the new carbs solved the problem?

Charles

[Dusty]

When I replaced my old engine with a new one i had flooding and venting issues.
Everything was as per rotax spec but the fuel pressure was higher (7 psi vs my old pump 5 psi)
I think these issues were a combination of high fuel pressure and ground angle with Bush wheels and extended gear .

My solution was to replicate the setting off my old motor. The pump was modified to provide 5 psi(with no return line). I also lowered my float level by1/8 of an inch with no difference in running or egt in flight. Before I lowered the levels I couldn't taxi upslope without rough running and black smoke. 270 trouble free hours later:D

[Bob]

Hi Bob,

I have been struggling with carb flooding and venting also.

I was curious if the new carbs solved the problem?

Charles
No, the new carbs did not solve the problem. Today, my mech put in a gauge and checked the pressure of the new fuel pump which I installed. It is 6.5 psi using the starter. He thinks the problem may have to do with the angle of the a/c as a taildragger; ie, the floats not seating properly because of the angle, about 13 degrees. Tomorrow, he and I will put the old Mr Gasket fuel pressure regulator the previous owner had on the line from the pump to the carbs. We will read the pressure it shows. Then we plan to remove it and put a new Mr Gasket regulator in its place and match the pressure. We think the cabs would work normally, and do in an inflight, level attitude. But, you can't taxi and take off with the gas dripping out all over. I'll let you know how it goes.

As you can imagine, we've talked to a bunch of folks from Kitfox and Lockwood. Lots of scratching of heads at both places.

[cwsnyder2001]

Dusty,

Thanks for your response.

Bob,

Maybe this is a conspiracy to sale Mr. Gasket products :) My long story...

I checked/weighed the floats. They were good. I replaced the float needles even though the original needles and seats looked great and adjusted the float height correctly. I still had the same problem of fuel going overboard.

I put a Mr. Gasket 0-15 psi analog gauge in the carb supply line to verify the digital gauge, followed by a Mr. Gasket regulator to allow me to control the pressure.

Without the engine running. the analog gauge and digital gauge registered the same using a primer to supply pressure and I could pressurize to 7 psi without fuel leaving the carb.

When I ran the engine, the analog gauge was pulsating from 0 to at least 10 psi rapidly (needle was a blur). Starting at 6 psi on the regulator, I went down in half psi increments to 3.5 psi and the carbs quit flooding.

I think the high pressure pulses are unseating the float valves and maybe another fuel pump is due or maybe a fuel pressure dampener... - but I don't think I should have to run a regulator.

I would appreciate anyone's thoughts about replacing the fuel pump, pressure dampener, etc.


Charles

[Av8r3400]

This has been an interesting discussion.

I have the "new" style mechanical fuel pump, I purchased it back in 2014-2015 time frame. My Dynon D180 has a fuel pressure sensor and readout on the display. I constantly show 3-5 psi of fuel pressure.

I would agree that 7+ psi is excessive and could very well overpower the floats.

[Dave S]

Bob & Charles,

I think, with the information known now, there is very little doubt about the excessive pressure being the cause of the fuel overboarding. FWIW, Rotax has snatched defeat from the jaws of victory a couple times in the past by shipping fuel pumps which produced excessive pressure. If I recall, there was a rotax document back a long time ago explaining the need to replace certain pumps because of excessive pressure. They have screwed up factory supplied fuel hoses between the pump and carbs too - but that is another issue.

I don't buy the angle that the aircraft sits at on the ground argument. There is a rotax spec (don't recall where at the moment but it may be in the engine installation or operator's manual) which indicates an angle deviation from vertical far in excess of what was measured - don't recall the number but I believe it was somewhere in the neighborhood of 40 degrees - the carbs are pretty darn good. Considering that Kitfoxes can climb at deck angles in excess of taxi deck angles without problems like this - oh well. After all, its an airplane engine that normally spends time in less than vertical positions.

Another little known tidbit is that unless a person knows for sure how a gauge or electronic readout of fuel pressure is arrived at, there is some possibility that the data is displayed in dampened format (either electronic averaging or having a restrictor orifice to reduce the vibrating needle syndrome) Your gauge which reads the high and low peaks apparently does not have any damping function and may be reading true to what is happening.

A correct, direct reading diagnostic pressure gauge does take the guess work out of things like this.
Not a bad habit of using such a gauge for annual condition inspections as well as whenever a pump is changed.

[Bob]

Hi,
We think we solved the problem. We checked the pressure the old Mr Gasket was providing, 4psi. Then we connected the new Mr Gasket, set at # 3
position which gave us 4psi. The carbs didn't flood, but there was some dripping from the vents at idle. The engine ran smoothly, except at idle. which was slight. Regardless of what others have said, we think that the position of the taildragger is the cause of our difficulty. And the regulator seems to be the answer. We have to put it back together tomorrow. Plan to fly it on Saturday. I'll keep you up to speed.
Bob

[Bob]

I flew this morning. There's some tweeking to do, with regard to idle speed. The engine ran well.

[cwsnyder2001]

Bob,

Just following up with my end results.

I installed the new pump (893115, same as the old pump), with an analog gauge, fuel regulator and another analog gauge in the pump output line. Carbs are balanced.

The new pump still pulsated as expected but the pulsations were not as extreme. With the regulator at maximum pressure (7 psi), approx. 5.5 psi was seen at the carbs.

Like you experienced, I was still getting overflowing at idle.

I lowered the regulator in half psi increments until the overflowing stopped (3 psi) and I ran the engine through the entire rpm range with the pressure varying from 3 to 3.5 psi.

The regulator stays.

Charles

[Birdseyeview]

I have the same issue going on so I'm also looking for answers too and it looks like you got your answer with a new pressure regulator. However, I'm curious if you came to the conclusion that the average pressure was just too high and the regulator brought that down or do you suppose it was the pulsations were too high and the regulator just calmed that down too? When you measured the pressure and saw the pulsations how did you get a stable overall pressure reading? If the issue is ultimately caused by high pulsations might a fuel pressure damper be a simpler, more compact and more reliable solution than a regulator? I'm not questioning your solution, just curious about details you discovered along the path to your solution.

[avidflyer]

Bob,

Just following up with my end results.

I installed the new pump (893115, same as the old pump), with an analog gauge, fuel regulator and another analog gauge in the pump output line. Carbs are balanced.

The new pump still pulsated as expected but the pulsations were not as extreme. With the regulator at maximum pressure (7 psi), approx. 5.5 psi was seen at the carbs.

Like you experienced, I was still getting overflowing at idle.

I lowered the regulator in half psi increments until the overflowing stopped (3 psi) and I ran the engine through the entire rpm range with the pressure varying from 3 to 3.5 psi.

The regulator stays.

Charles



I can't remember, did you have a fuel return line or not? And if you do, where does it go to? Thanks, JImChuk

[cwsnyder2001]

Jim,

I have a return line routed back to the return inlet on the header tank.

Larry,

In the test with the old pump, I only had the pressure gauge before the pressure regulator and just interpolated the average pressure while watching the wildly swinging needle and digital gauge.

In the test with the new pump, I added a fuel gauge after the regulator. The pressure after the regulator was "calmed down" (steady, no fluctuations). I attempted higher pressures on the regulator (and would really like to not have it in the system at all) but the carbs (left carb first) would start dribbling at idle - particularly cold idle as the engine is a little twitchier.

Roger Lee on rotax-owner.com mentioned a dampener is some of his responses to others on this issue but its location was said to be before the fuel pump which did not make sense in my situation. It seems I need to lower the pressure anyway.

I also checked the location of the fuel pump vent by blocking and unblocking it with no effect.

Charles

[Slyfox]

Bob,

Just following up with my end results.

I installed the new pump (893115, same as the old pump), with an analog gauge, fuel regulator and another analog gauge in the pump output line. Carbs are balanced.

The new pump still pulsated as expected but the pulsations were not as extreme. With the regulator at maximum pressure (7 psi), approx. 5.5 psi was seen at the carbs.

Like you experienced, I was still getting overflowing at idle.

I lowered the regulator in half psi increments until the overflowing stopped (3 psi) and I ran the engine through the entire rpm range with the pressure varying from 3 to 3.5 psi.

The regulator stays.

Charles
just to let you know. the facet pumps for these rotax 912uls is rated at 3-4.5. This a facet pump that Vans aircraft uses in the rv12. so with that I'm sticking with the fact that you want 3-4.5 pressure at the carbs, above that and you will overcome the needles and have flooding. have a nice day

[Slyfox]

I'm going to hit one more thing on all this mess. I read all the time about how bad the carb floats are on the 912. There is a huge discussion on the Vans list about this. Rotax says to weigh the floats. Well I for one want to see a sinking float. I always check my carbs at oil change, meaning I remove the bowl and be very careful to keep the level of fuel high. I than set the bowl on a flat surface and look at the floats, a sinking float will show you a sinking float. I've read oh man I weighed my floats and they are over what rotax calls for, well let me tell you, I've tried to weigh my floats and got varied results. Tough to do. I go by sight. I think most people with flooding carbs are in fact having over pressure problems. Now I'm not right there looking at things and testing so I can't say for sure. My thought is if your running over 4.5 psi than change it. Also for someone to say that adding a regulator is out of sorts , well so be it. I think a person should be making things right whatever it takes. I've personally seen these new fangled pumps that rotax put out just plain put out too much psi to the carbs. my take, take it or leave it. I know what I'm doing with mine and that's what matters. my rotax runs like a million bucks.

[Dave S]

Thanks for your comment on the floats, You are SPOT on. :)

The practical way to determine if they are OK or not is seeing how high they ride in the gasoline after carefully pulling the bowl without spilling - if the little brass protrusions on the sides of the floats are level with the fuel - they are correct.

The problem with weighing which can scatter the results all over the place is because of varying amounts of evaporation that occurs between taking them out of the bowl and getting them on the scale. Air movement, temperature and time can all mess up an accurate weighing as varying amounts of fuel go away.

The floats unmolested height in the fuel is an accurate real world function. What you said.

[Bob]

Bob,

Just following up with my end results.

I installed the new pump (893115, same as the old pump), with an analog gauge, fuel regulator and another analog gauge in the pump output line. Carbs are balanced.

The new pump still pulsated as expected but the pulsations were not as extreme. With the regulator at maximum pressure (7 psi), approx. 5.5 psi was seen at the carbs.

Like you experienced, I was still getting overflowing at idle.

I lowered the regulator in half psi increments until the overflowing stopped (3 psi) and I ran the engine through the entire rpm range with the pressure varying from 3 to 3.5 psi.

The regulator stays.

Charles We placed the new regulator in line to the carbs. It reduced the pressure to 4psi. No leaking carbs, only occasional slight drips from the vent line at idle. I've flown about 3 hrs this setup. Engine runs well.

[Slyfox]

Thanks for your comment on the floats, You are SPOT on. :)

The practical way to determine if they are OK or not is seeing how high they ride in the gasoline after carefully pulling the bowl without spilling - if the little brass protrusions on the sides of the floats are level with the fuel - they are correct.

The problem with weighing which can scatter the results all over the place is because of varying amounts of evaporation that occurs between taking them out of the bowl and getting them on the scale. Air movement, temperature and time can all mess up an accurate weighing as varying amounts of fuel go away.

The floats unmolested height in the fuel is an accurate real world function. What you said.

Yes I have had 2 floats now since 2004 that sunk. I will always believe that the only way to know, and that's how I found those two floats, is to remove the bowl and look at the float. Back in the old days we use to always find a carb float on a vehicle that was bad by doing that. take care.