The Cessna 120-140 Association Forum

I’ve had the subject carb rebuilt twice in the last six months for leaking. If I leave the fuel selector on in between flight I find fuel stains on the bottom of the cowling. Minor at first but gets worse over time. If I turn the fuel selector off, it does not leak, however, as soon as I turn it on the fuel pours out of the carb for about a minute or so. The engine runs fine with normal fuel consumption. Also since the rebuild, the mixture control has no effect. Prior to the rebuild, I could use the mixture control to lean out during taxi. This helped prevent carbon build up. Looking for some input on what my next steps could be and identifying a rebuild shop that someone has had success with in the past. Thanks!

Bob Kachergius is often called "The Stromberg Doctor" (below). Many (...most, all?) who have flown behind his OH'd Strombergs report that they no longer leak. That said, many that haven't been "blessed" by Bob do leak...and, "we" just put up with it. There are different needles that can be changed & that has helped...some say that auto fuel is an issue with this, too. As you have experienced, turning the fuel valve off stops the leak...'til it's turned back on. It's important to remember to turn it off, of course. ...so, probably the ultimate "fix" is having Bob's OH. Great guy, BTW. There may be others equally able, too. My own was OH'd by a very good and experienced shop -- sometimes it leaks, sometimes it doesn't. So far I've remembered to turn the selector valve off (which we should probably always do anyhow?). There, FWIW. Mac

Robert Kachergius, Uni-Tech Air Management Systems, Inc. of Orland Park, Illinois. Bob has both engineering as well as an aviation maintenance background including an A&P and IA, and for the past number of years specialized in the rebuilding and overhauling of Stromberg carburetors.

If you go to the tech articles you can download the Stromberg PDF's. They talk about the needle valve leaking problem and how to fix it. Shutting off the fuel valve between flights is the right way to handle this leak, and is also a good safety habit. Concerning the mixture, there must have been something wrong with the carb if you were able to lean below 1500 ~ 1800 RPM. They don't work like a Marvel, the mixture will do nothing at low RPM's and below 5K feet. Again, you can read the Stromberg PDF's about the mixture circuit.

I shut my fuel off as I'm taxing the last few hundred feet to shut down, this runs the fuel in the bowl down a bit and closes the needle valve.

The only thing I've heard about Bob that's not praise is that he's not cheap. Pretty much the only game in town, though. As you already know, most field overhauls aren't worth the price.

When you're dealing with a gravity feed system ALWAYS turn the fuel off when you're not using it. Every float valve will eventually fail unless it's first replaced as a preventive measure. 100% guarantied. Gravity, on the other hand, has proven to be 100% reliable since the beginning of time.

this runs the fuel in the bowl down a bit and closes the needle valve.

Actually, it opens the valve. (The valve opens when the bowl is less than full, to refill the bowl.) This may contribute to the valve sticking and the bowl overflowing next time you turn the fuel back on (if you have that problem). Still, it's not a bad practice. If the bowl is empty the engine can't start accidentally.

Franklin wrote: "I shut my fuel off as I'm taxing the last few hundred feet to shut down..."

John wrote: "...not a bad practice. If the bowl is empty the engine can't start accidentally."

Excellent advice! I shut my fuel off the last bit of taxi to the hangar...and, until the engine stops. I know several others who do, too. We all have and/or heard stories of P-leads failing & I've experienced the surprise of turning the mag switch off and having the engine keep on running . With fuel in the carb, an unexpected "live" mag & the wrong touch on a prop disaster potential is alarming. (...important to be sure that the shut-off is actually shutting off, too ). Mac

Thanks to all who replied. I will make sure I start turning the valve off as I approach the hangar and wait for the engine to stop. If I understand one of the responses correctly, this may also solve the fuel running out of the carb when I turn it back on. Is this correct? If not, any corrective action I can take for this issue? I’ve also tried to reach out to Bob K. Thoughts on replacing the Stromberg with a Marvel? Thanks again!

7739 wrote: ↑Wed Aug 28, 2019 2:29 pmIf I understand one of the responses correctly, this may also solve the fuel running out of the carb when I turn it back on.

It should not have any effect on this problem one way or the other. How much fuel pours out of the carb now? What do you do about it? This is an indication the float is sticking. The usual cure is to rap the carb bowl with a big stick...

If not, any corrective action I can take for this issue?

Figure out why it sticks and correct it... You're going to have to open the float bowl. The float may be rubbing on the side of the bowl or the float valve may have a ridge worn in it that catches on something.

Thoughts on replacing the Stromberg with a Marvel?

Sacrilege!

The Marvel is a whole different animal. It works differently. It requires constant fiddling with the mixture control. It costs twice as much to overhaul and you'll pay at least one Stromberg overhaul just to acquire it.

The fuel pours out for about 15-20 seconds or so then shuts off. Pretty good puddle on the hangar floor. Since it was rebuilt in April of this year and returned a week later to the repair shop you would think that that it would have been checked for sticking float. I’ll check with the local A&P to see if he feels comfortable taking apart the bowl. My gut says they won’t want to tackle this “old” stuff. I’m reluctant to send it back the repair shop a 3rd time. I’ll send it to Bob K before I do that. Thanks for all the advice!

Bob: Sent you a private message.

I've heard that turning off the mags to stop the engine will allow some fuel to enter the cylinders and this raw fuel will "wash" the oil film and might cause rust. FWIW, I always turn off the fuel and allow the engine to run until it doesn't. Then mags and master off.