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Carb fuel leak source


Registered User
Anchorage, Alaska
Been trying to track down a fuel leak for a few months on and off with my 160 hp cub. :crazyeyes:
It all started when I replaced my old carb with one from UNIVAIR. When running, fuel ends up on the gear and belly in little drops. I took the carb off and had a local shop look at it and they replaced the needle and seat and bench checked it and pronounced it OK. Now if I go out and fly for a couple of hours sightseeing I don't notice any fuel. If I go out and do touch and go's for even a little bit (3 or 4 trips round the patch at lake hood) I end up with dried gas on the belly. There isn't any fuel leaking static.
I can see where the gas is coming out of the carb air box and all the external connections to the carb are clean and dry so I "know" the bonus gas is coming from inside the carb. The only thing I can figure out is the fuel level in the float bowls is set too high and when the plane is pitched up on takeoff and landing the fuel ends up coming out of the carb. I haven't noticed any change in the engine temps or roughness during any of this....
Anybody ever run into this before? Any ideas about what is causing it? Can you recommend a great carb person in the Anchorage area?
Thanks in advance,
Assuming everything else is in order. Could you be leaking fuel from the carburetor parting seam??

A common problem exists here.

The paper gasket that seals the top half of the carb from the lower half compresses on installation. If the assembling technician simply torques the hardware, bends the locktabs, and tells you to install the carb, you would be installing a carburetor that WILL leak slightly. Usually this occurs on the external seam, and can be observed by fuel staining on the external surface of the carb body.

The way to eliminate this problem is to torque the hardware, let the thing sit overnight, then retorque the hardware, set the locktabs, and declare the carburetor servicable.

I've found plenty of things wrong with "overhauled", "yellow tagged" carbs. The above mentioned gasket problem, along with others like worn out floats, nozzles, etc. DO occur.

It'll cost you money to do it if you take 'em junk, but Alaskan Aircraft engines will put a Carburetor back together properly, with quality parts. Good Luck.

THe seam is clean and dry. I am not seeing any fuel external to the carb... All the fuel appears to be coming out of the carb airbox and I haven't seen anything leaking static. That is the reason I think it is a fuel level problem in float bowls. I am trying to remember if there is a vent for the float bowls from the last time I put a carb together (A&P skool a long time ago)
I use Alaska Aircraft Engines as well. They actually put this carb back together after the seam you are talking about leaked when it arrived "rebuit" from Univair....
I have had both of these problems on two different 18's and it all began with the conversion to a one piece venturi. The blue belly syndrome is really exaberated by the use of priming, but still occurred on cold starts without priming. Carburetor was eventually replaced after a number of trips to the shop. The gasket seepage is a another issue, this is leading me to believe that there may not be any good carburetor service people available in AK.

What about an accelerator pump out of adjustment?
You did mention that the problem is there on T&G's, that is when the
throttle gets full articulation and the accelerator pump in extra fuel.
Hate to say it but I know that the "blue belly" problem isn't yet fatal and don't want to waste anothe frost free day so I will be out test flying the problem tomorrow...
It seemslike way too much gas for the accelerator pump though..
Call Precision Airimotive. They have a tech dept that is real good at toubleshooting carb problems. I had a carb that was running real rich at idle blowing black smoke. They told me to check the enrichment valve. Changed it out and no more problem.

Tell us more about the fix on your overrich carb. I have to lean for takeoff all the time. If I forget, I can tell as soon as I add full throttle that I don't have full power. What is wrong with the enrichment valve is it worn or just bad.
While you guys are eyeballing carb problems. There have been some problems with rebuilt carbs lately. Throttle shafts have been installed that are oversize. When you add carb heat ----the shaft and body expand and carb locks up tight. You can imagine when this would happen---at one of the worst possible times. Check with your rebuilder. If they haven't heard of problem be careful as it is around. Three that I know of.

Gerald, It was the Carburetor Pump Discharge Check Valve Ass'y. Brass nut with a screw in it that sits under the accelerator pump on the front of the carb. If I recall correctly the spring gets weak and allows fuel to come out of the accelerator pump thus enrichining the mixture. I do remember replacing the check valve with one off of another carb and it solved the problem.

The problem I have seen with over rich mixtures have been with carbs OH'd when replacing the venturi not being flow checked. They tend to run richer for some reason. I also think that Precision tends to jet the carbs richer these days for liability reasons. I have known several people to solve the over rich problem by changing jets.

Precision Airmotive 360-651-8282 tell them you have a tech question.
I guess my touch and go theory is shot. I flew the other day and after about a half an hour of straight and level had blue on the belly. I cleaned it up and did 5 or 6 touch and goes at a pretty rough strip and then flew it home with no staining.... So I am back to square one. I am going to call precision when I get home again.
I am resurrecting this thread because my carb has the exact same symptoms. AKCub didn't post the eventual resolution to the problem. Are there any other suggestions? I agree that it sounds like the float level may be set too high but in my case, the problem developed three years after the carb was replaced. Can the float level self-adjust?
Guys there were problems with floats contacting or hanging up on the sides of the float bowl. Make sure your float can't touch the sides. You can also test the seal at the seat on the bench. Rig up a fuel container with a line fitting and duplicate the working head pressure.
Who do we like for carb work? I have heard there's a place in Tulsa that's good.

My IA took a look and the leak is of indeterminate nature, and also intermittent. He can break down the carb for a look but since he's not a "carb guy" doesn't have a lot of experience with that operation, and has suggested that I send it to someone more proficient with troubleshooting carbs.
What about a crack that is opening up when hot. This is something of a WAG but about all the conventional wisdom has been exhausted here. These carbs are so basic that you'd think there were missing parts the first time you open one up (been working on performance-type automotive carbs for 30+ years), shouldn't be too hard to find the source.
There appears to be a potential crack at a solder joint at the base of the "balance tube" as my IA called it, but we can't be sure. He didn't think dye penetrant would work in this case. It's clear that the source of the fuel is coming from inside the carb, possibly up in the venturi but about the only thing he could determine is that since the leak is intermittent it would be best to take it off and tear it down, or have it torn down. Possibly the float is hanging up at times, in which case the bowl would overfill.
OK not familiar with aviation carbs BUT in motorcycles when this crops up sometimes it's that the place where the bottom of the needle touches the float wears a little depression in the brass on the float (vibration over time wears that spot) and as the float raises and lowers it occasionally "hangs up" in the depression which holds the valve open a little. It can be very intermittent since different vibration levels can free it up from time to time. On a motorcycle a little sanding to make the depression in the brass a little less severe takes care of it.
A divit in the float tab is something I have never personally encountered or thought of. Aircraft and motorcycles are both high vibration environments so this would be a good candidate. It has to be either this or a crack. The next time I'm inside my carb. I'm going to see what kind of wear pattern is setting up on the tab.
Thx PerryB and Q440, those are great suggestions and I'll track on those. The carb comes off next week. Will post the results when I have them.