Leaky Carb

[Harbi]

Hi Folks,

I am getting the bird annualed. My IA is concerned about my carb dripping fuel while the plane is setting. When I did the recover-refurb three years ago I put a steel needle in the stromberg carb ( With help from an A&P). I was talking with a friend and he says it is hard to keep the steel needle from leaking a bit of fuel (metal against metal).
The carb had the derlin needle in it when we replaced it(not neopreme), but I went ahead and put the steel one in anyway. I have the auto gas STC.

I have been flying like this and the engine runs fine at all power settings, I just make sure I shut the fuel off at the end of every flight. My IA says I might want to think about getting the carb overhauled, but will this solve the problem? I have owned the plane for ten years and as I recall it dripped fuel even with the derlin needle, but maybe not as noticible as with the steel needle.

Any insight would be greatly appreciated. I have no qualms about getting the carb overhauled if it would solve the leak or if this is a saftey issue that must be addressed.

This is a c-85 12F


Thanks a bunch
Wade

[alwaysoar]

Re: Leaky Carb

There's a thread on this with a lot of explanation and cure-alls. However, this quote from the Stromberg overhaul manual should put your IA's concerns to rest: " In certain installations a condition exists where the fuel level in the carburetor is above the present idle air bleed location. While the airplane is parked, it is possible for the fuel to drain...on to the ground." In other words, do what you were doing and don't sweat it. Check out the thread. In addition, you can move the idle air bleed hole higher up but that's a hassle and not worth the trouble.

[cpirrmann]

Re: Leaky Carb

When I swapped engines on mine, my carburetor sat dry for about a year. When all was reassembled and fuel was turned back on, after the gascolator filled up and a short delay (assumed the bowl filling up), fuel would run in a stream out of the carburetor. We figured the float was stuck and tapped the side of the carb to no avail. We finally had to take it apart and found the float stuck and in the bowl was a gravelly substance plus a lot of corrosion on the brass screen clogging the about 50% of it. It seemed to me it was a power loss waiting to happen. I don't know the last time it was opened and until now there was never any problem and never any reason to open it. There isn't even anything on any annual suggestions about the screen (there is now). If the fuel leak hadn't occured, it might never have been checked.

If it's been a long time since the carb was opened up and not knowing the history of many our airplanes, it might be advisable to have it looked at.

Does anyone know of a recommended period to have the carb overhauled or serviced?

[Ragwing nut]

Re: Leaky Carb

I wouldn't worry to much unless it is leaking a bunch. Shutting off the fuel is the best way to deal with it and it also keeps the shut valve lubricated by operating it. Overhauling it won't garuntee the needle won't leak anymore. You might try taking it apart and lapping the needle into the seat.

Mike

[Garry Crookham]

Re: Leaky Carb

Wade,

When the Delrin needle is installed, a weight is supposed to be installed on the float arm. Also, the Delrin needle needs a round edged seat. The stainless steel needle is used with a sharp edged seat and no weight on the float. You might check to see if the proper seat is installed in your carb and if there is a weight soldered to the float arm (shoud not have the weight with the steel needle).
If the float level is misadjusted (too high), the carb can weep an excess amount of fuel with the engine stopped and the fuel valve on.
Hope this helps.

Garry Crookham
Tulsa

[Buell Powell]

Re: Leaky Carb

If you dont find that the problem is with the carborator float or needle and seat you might check to see if the primer seals are leaking. The original primers had leather seals and would deteriorate if they sat very long with no fuel. I think the easyist way is to take the line off and plug it and see if it quits dripping. Hope this helps.
Buell Powell

[hangarb7]

Re: Leaky Carb

Sir,

Trying to stay up with all of the reasoning here.

Are you suggesting that there may be an air leak thru the primer seals, thus providing a break that would allow the carb to leak, or at least overfill?

Interesting thread.

Thanks,

Jack D
N44057
BC12D

[Buell Powell]

Re: Leaky Carb

Hi,
What may be the problem is the seals on the primer shaft may allow the primer not to seal and allow it to continue to draw fuel from the gascolator. Most primers have rubber O'rings that are very easy to replace. My original had leather cup seals which I didn't like so I replaced the primer with a modern one. If it turns out that the steel needle and seat aren't sealing you can use valve lapping compound and lap the seat and needle like a valve and seat. You might look to see if something has gotten between the needle and seat also. If that doesn't look like the problem see if the float is sticking or isn't set right. I think also there is an AD on adding a weight so the needle and seat will have more pressure against each other to seal better-you might check with a carb shop about this to see if it applys to your carb. The first thing I would check is the primer because it's isn't very much trouble to eliminate.
Good Luck!
Buell

[Harbi]

Re: Leaky Carb

Thanks for the great info. I think the first thing to check is the needle, seat and float. Since the derlin needle was replaced, I will have to see if there is a wieght on the float.
Thanks again, Wade

[NY86]

Re: Leaky Carb

As Gary pointed out, if you go from a Delrin needle to a steel needle, you have to install the correct seat for the steel needle and *remove* the weight from the float. There is no AD, and the weight is only applicable to the Delrin needle. The Delrin needle supposedly uses the same seat as the neoprine needle, but it is the responsibility of the person working in it to determine that the correct seat is installed. I've seen all combinations and then some...

There is another service bulletin (aluded to by alwaysoar) about relocating the air bleed which originally, in the 3 point attitude, was very close to the fuel level in the bowl. Any drift toward the high end of the fuel level range or any leakage would cause the bowl to overflow through the air bleed. Carbs that are so modified are supposed to be marked with a 3/4" yellow dot on the outside. Many are not...

My experience is that the most sure fire way to eliminate float valve leaks is with the latest needle and seat. Earlier seats, while still "right" seem to cause more leaks than they are worth. A couple of hours fussing with them soon eclipses any savings over a new one.

Another oft overlooked cause is a worn float fulcrum bushing or pin, allowing the float to cock slightly and not fully seat the needle.

[Richard Boyer]

Re: Leaky Carb

Out of curiosity, why would you need to remove the weight. I realize that it isn't needed, but it shouldn't hurt either.....right?

[NY86]

Re: Leaky Carb

Out of curiosity, why would you need to remove the weight. I realize that it isn't needed, but it shouldn't hurt either.....right?

The weight is to compensate for the difference in weigh between the 2 needles. Leaving it on with the steel needle will change the way the float moves in response to changing fuel demand. The level in the fuel bowl will have to drop farther to effect the same opening of the valve, so it'll tend to lean out under increased load.

[Richard Boyer]

Re: Leaky Carb

Interesting. Would it make sense to just swap out the float with a new one instead of removing the weight? (not that I'm going to mess around with this anytime soon.....).

[NY86]

Re: Leaky Carb

Interesting. Would it make sense to just swap out the float with a new one instead of removing the weight?

A new one? I guess, if you have a new one. Removing the weight is not difficult, though.

[DanBrown]

Re: Leaky Carb

Got to fly yesterday after a month's layoff due to combination of wx, family health and work. Wonderful day -- cavu (20+ out here qualifies!!) smooth air above the pattern, and just enough wind to make landing practice fun.

After shutdown, I got a fuel drip of about a drop every 5 seconds out the breather (the line guys filled it a little more than usual). Stopped with the cutoff valve pulled out. Is this too much? I couldn't find the thread that alwaysoar referred to -- any help??

Thanks,

There's a thread on this with a lot of explanation and cure-alls. However, this quote from the Stromberg overhaul manual should put your IA's concerns to rest: " In certain installations a condition exists where the fuel level in the carburetor is above the present idle air bleed location. While the airplane is parked, it is possible for the fuel to drain...on to the ground." In other words, do what you were doing and don't sweat it. Check out the thread. In addition, you can move the idle air bleed hole higher up but that's a hassle and not worth the trouble.