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Carburetor flooding

Y

ydna

I'm New Here
Because of the Service Bulletin on carburetor floats, I weighed my floats at 5.6 grams prior to them being exposed to any fuel and before my first engine run-up.

During first run-up and after warm up, I did a carburetor balance and with full throttle, I had a 5040 RPM. On that day, I shut down and shortly after did another run-up with like results. Prior to start, I notice the electric fuel pressure was 3.2 psi. However, during run-up, I didn't record engine pump fuel pressure.

After the first run-ups, I readjusted the prop to attempt to achieve 4950 RPM.

Five days later, I ran the engine again. With an 1800 RPM, the engine fuel pressure was 5.3 psi. However, within three minutes of start, I had fuel being blown onto the canopy. I shut down. I discovered that both carburetor drip pans were full of fuel. My first thought was that I had defective floats. I dropped the float bowls and both were full of fuel. I allowed the floats to dry and re-weighed them. The weight remained at 5.6 grams per pair of floats which are within tolerance.

I called Lockwood and spoke to an individual who asked me to do a float check by putting fuel in the bowls to determine that the side pins were above the fuel surface which would indicate floats are good and are less than 7 grams weight. My pins appear to be resting on the fuel surface. Thus, I was confused because the weight is within tolerance but the pins are touching the fuel rather than above.

The individual at Lockwood also said that perhaps my prop was causing a vibration. Accordingly, I reset the prop so both blades are the same.

I also emptied the bowls and reinstalled them and turned the electric fuel pump on. When the fuel pressure stabilized to indicate the needle seat had shut off the fuel flow, I turned off the pump. I removed the bowls and measured the height of the fuel which was just over one-half inch from the top. At the 3.2 psi, the needle seat functions.

I re-ran the engine and with-in three minutes in idle, fuel was again in each tray. I shut down.

During some research, I located an article by SKYDRIVE from UK which has a picture of a good and a bad float. This indicates the good float will have the pin at the fuel level. Thus, I am confident my floats are good and my fuel pressure is good.

HELP. Since my floats are good, what could be causing the flooding of the carburetors?
 
ydna said:
Because of the Service Bulletin on carburetor floats, I weighed my floats at 5.6 grams prior to them being exposed to any fuel and before my first engine run-up.

During first run-up and after warm up, I did a carburetor balance and with full throttle, I had a 5040 RPM. On that day, I shut down and shortly after did another run-up with like results. Prior to start, I notice the electric fuel pressure was 3.2 psi. However, during run-up, I didn't record engine pump fuel pressure.

After the first run-ups, I readjusted the prop to attempt to achieve 4950 RPM.

Five days later, I ran the engine again. With an 1800 RPM, the engine fuel pressure was 5.3 psi. However, within three minutes of start, I had fuel being blown onto the canopy. I shut down. I discovered that both carburetor drip pans were full of fuel. My first thought was that I had defective floats. I dropped the float bowls and both were full of fuel. I allowed the floats to dry and re-weighed them. The weight remained at 5.6 grams per pair of floats which are within tolerance.

I called Lockwood and spoke to an individual who asked me to do a float check by putting fuel in the bowls to determine that the side pins were above the fuel surface which would indicate floats are good and are less than 7 grams weight. My pins appear to be resting on the fuel surface. Thus, I was confused because the weight is within tolerance but the pins are touching the fuel rather than above.

The individual at Lockwood also said that perhaps my prop was causing a vibration. Accordingly, I reset the prop so both blades are the same.

I also emptied the bowls and reinstalled them and turned the electric fuel pump on. When the fuel pressure stabilized to indicate the needle seat had shut off the fuel flow, I turned off the pump. I removed the bowls and measured the height of the fuel which was just over one-half inch from the top. At the 3.2 psi, the needle seat functions.

I re-ran the engine and with-in three minutes in idle, fuel was again in each tray. I shut down.

During some research, I located an article by SKYDRIVE from UK which has a picture of a good and a bad float. This indicates the good float will have the pin at the fuel level. Thus, I am confident my floats are good and my fuel pressure is good.

HELP. Since my floats are good, what could be causing the flooding of the carburetors?
Click to expand...

I'm no expert, but my carb bowls are always full of fuel when I shut down and when I have popped the bowls to examine the floats.

I think the only way they would be empty is if you shut down by pulling the fuel valve and starving the engine - which is not the proper way to shut down.

Perhaps the bowl gasket is not properly seated? Are you reusing the cork gasket? Technically you are supposed to replace that every time you pop the bowl...Lockwood makes one that is made of styrene or some other synthetic material that will allow you to pop the bowl a few times.

Do you see fuel dripping from the overflow lines?
 
I would definitely not re-use the original cork gaskets -- too thin, wimpy, and difficult to re-set - particularly in tight quarters. Do your carbs leak with just the boost pump "On"? Really only two potentials carb leak points: At the bowl/carb gasket interface or from the carb vent tube. There are more potential leak points (fuel line attach fitting, choke gasket, ??), but they seem unlikely.
 
fuel leaks

Hi can you connect the carbies to the fuel lines and with someone or something holding the floats up turn on the electric fuel pump to see if the needle and seats are shutting off the fuel flow when the bowls are full I think it would be a rare occurance that both needles and seats fail together
also when you say you dried the floats before weighing them do you mean just a quick wipe of the fuel on the surface because if you allow them to dry for any period of time any fuel that they have absorbed will have evaporated giving you a false normal weight
 
ydna said:
Because of the Service Bulletin on carburetor floats, I weighed my floats at 5.6 grams prior to them being exposed to any fuel and before my first engine run-up.

During first run-up and after warm up, I did a carburetor balance and with full throttle, I had a 5040 RPM. On that day, I shut down and shortly after did another run-up with like results. Prior to start, I notice the electric fuel pressure was 3.2 psi. However, during run-up, I didn't record engine pump fuel pressure.

After the first run-ups, I readjusted the prop to attempt to achieve 4950 RPM.

Five days later, I ran the engine again. With an 1800 RPM, the engine fuel pressure was 5.3 psi. However, within three minutes of start, I had fuel being blown onto the canopy. I shut down. I discovered that both carburetor drip pans were full of fuel. My first thought was that I had defective floats. I dropped the float bowls and both were full of fuel. I allowed the floats to dry and re-weighed them. The weight remained at 5.6 grams per pair of floats which are within tolerance.

I called Lockwood and spoke to an individual who asked me to do a float check by putting fuel in the bowls to determine that the side pins were above the fuel surface which would indicate floats are good and are less than 7 grams weight. My pins appear to be resting on the fuel surface. Thus, I was confused because the weight is within tolerance but the pins are touching the fuel rather than above.

The individual at Lockwood also said that perhaps my prop was causing a vibration. Accordingly, I reset the prop so both blades are the same.

I also emptied the bowls and reinstalled them and turned the electric fuel pump on. When the fuel pressure stabilized to indicate the needle seat had shut off the fuel flow, I turned off the pump. I removed the bowls and measured the height of the fuel which was just over one-half inch from the top. At the 3.2 psi, the needle seat functions.

I re-ran the engine and with-in three minutes in idle, fuel was again in each tray. I shut down.

During some research, I located an article by SKYDRIVE from UK which has a picture of a good and a bad float. This indicates the good float will have the pin at the fuel level. Thus, I am confident my floats are good and my fuel pressure is good.

HELP. Since my floats are good, what could be causing the flooding of the carburetors?
Click to expand...

I feel your frustration. I feel like I was dealing with exactly the same thing about 1 year ago. In the end, I completely disassembled the carburetors, found nothing obvious, reassembled and everything worked. All I could think was the bowls or the floats were not properly set/aligned. With the carburetor in your hand, it was much easier to ensure everything was assembled perfectly.
I changed out the needle, mechanical fuel pump and ripped everything in the system apart thinking I had a clog somewhere in between the carburetors.
This is still something I wish I had root cause on.

I hope you find something you can share.
 
I did not replace the gaskets because they were undisturbed and still attached to the carburetors. I feel I checked the operation of needle seat by emptying the bowls and reinstalling them and operating the electric fuel pump. When the fuel pressure stabilized at 3.1 psi, I felt the needle seat had shut off the fuel flow. I again dropped the bowls and measured the fuel height at approximately one-half inch from the top of the bowl. Without tech data, I would expect that to be a normal measurement.

Question: Would the engine driven fuel pump with a 5.3 psi overdrive the needle seat?

I did confirm the choke is resting on the off stop position.

Question: Could running the engine without the cowling installed affect the altitude compensating feature?

All my run-ups have been with the balance tube disconnected and the Carb Mate attached for carburetor balancing. Note the first two successful runs five days earlier had the same configuration.

The weather is currently not inductive to performing any engine runs with or without the cowling, etc.
 
ydna said:
I did not replace the gaskets because they were undisturbed and still attached to the carburetors. I feel I checked the operation of needle seat by emptying the bowls and reinstalling them and operating the electric fuel pump. When the fuel pressure stabilized at 3.1 psi, I felt the needle seat had shut off the fuel flow. I again dropped the bowls and measured the fuel height at approximately one-half inch from the top of the bowl. Without tech data, I would expect that to be a normal measurement.

Question: Would the engine driven fuel pump with a 5.3 psi overdrive the needle seat?

I did confirm the choke is resting on the off stop position.

Question: Could running the engine without the cowling installed affect the altitude compensating feature?

All my run-ups have been with the balance tube disconnected and the Carb Mate attached for carburetor balancing. Note the first two successful runs five days earlier had the same configuration.

The weather is currently not inductive to performing any engine runs with or without the cowling, etc.
Click to expand...

The newer pumps operate at a higher pressure. It runs around 4-6 psi.

I think you might solve your problems by ordering 2 new gaskets from Lockwood. Those cork ones leak and are flimsy and should be replaced every time you pop the bowl. There are some rubber ones around too but they did not seat as well for me.
 
ydna said:
Question: Would the engine driven fuel pump with a 5.3 psi overdrive the needle seat?



Question: Could running the engine without the cowling installed affect the altitude compensating feature?
Click to expand...

As long as the needle/seat is not defective (having two is highly unlikely) 5.3 PSI should not be even close to overcoming the seal.


Running with the cowl off has no effect on the operation of the carb's.

There are other things that can cause the symptom you are havinging.

If the engine is not running smoothly (a properly set-up 912 engine / prop combination is very smooth at 1800 RPM and above), the shake can cause carb problems like you are seeing.
This can be the result of a badly out of balance prop., mismatch in blade pitch, carbs not properly synched, ignition miss, etc.

Is it possible the carbs are not actually synched as well as you think? Have you applied a vacuum to your test gauge to check that both sides read the same? (you can use a Tee to connect a single vacuum source to both sides and verify they read the same).

Also, the engine will always run just a bit smoother with the compensation tube reconnected, but it is probably not enough to make a difference in this case.
 
The max fuel pressure allowance is 7.2 and I have seen 8 psi without leaks.
Common causes of fuel in the drip trays are:
You may have pinched the bowl gaskets,
The bowl may not be seated in the grove,
Engine running rough enough to cause the floats to shake bad enough they can not control the fuel level,
Bad needle valve (I doubt this is it in this case)
Float control arm height wrong (may have been bent during your carb maint.)


If the first three items above are absolutely okay then I would pull the carbs and check the float arm height. The carbs need to be off the engine and turned upside down to do this. The float arms should be 10.5mm from the carb edge to the top of the float arm, vertically.
This is easy to measure and accomplish as a task.
 
ydna said:
I did not replace the gaskets because they were undisturbed and still attached to the carburetors................

All my run-ups have been with the balance tube disconnected and the Carb Mate attached for carburetor balancing. Note the first two successful runs five days earlier had the same configuration.

......................
Click to expand...

I'd definitely finish getting the carbs balanced and the balance pipe connected and then see what happens.

Also, be very careful about disturbing float bowls and not replacing the float bowl gaskets - just because they look okay is no excuse not to replace them, in my humble opinion. If they leak, it causes problems.
 
Thank you all for the suggestions. I have re-synchronized mechanically the carburetors. I'll order gaskets tomorrow.In the meanwhile, I purchased a new digital level to recheck the prop. That will be a few days before the weather will permit another run-up, etc.
 
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