Zenith 701 with a 190 hour 912UL. In service now 4 1/2 years. Opened the hangar door last week and noticed fuel spots around the front tire. Airplane had been sitting about a week since last flight. Source was the carb on the 1/3 cylinder side. Fuel was slowly dripping from the K&N air filter and dripping down to the muffler then down the nose strut. Removed the filter and had fuel seeping from the idle air jet and needle jet air tube. The carb on the other side was dry. Wing tanks had about five gals each, i.e. a half load. Dropped the float bowl, checked the shutoff valve operation (it worked), weighed the floats (they were good), no debris in the float bowl. Put it all back together, no leak. Ground run, all good. Buttoned it up, flew it, landed, checked- all good. It’s been dry since (several days now). Anyone have any ideas what could cause this? The gurus at Rotax-Owner.com tell me it’s the result of not shutting off the fuel supply to the carbs when parked and the weight of the fuel up in the wings is allowing the fuel to overfill the bowl shutoff valve. They tell me they’ve seen this before. But they offer no explanation as to how it occurs or why it is random. When I pointed out that the carb float shutoff valve is rated to 5.8 psi continuous pressure, and since I have a fuel pressure gauge I can read static pressure on the carbs, and it’s 0.7 psi with my half tank fuel load, and only one carb was leaking, and I did not not fix anything, I just took things apart and reassembled them, and the problem went away- I received a “believe us, we have seen this before- you need to shutoff the fuel from the wing tanks to the engine when parked.” I’m not knocking them, they have far more knowledge than me, but it doesn’t make sense. Although their proposed solution would work, I would have to modify my plane to accomplish it- and if their theory was correct, why weren’t both carbs leaking? Or why wouldn’t a full load of fuel cause leaking almost instantly? But I’ve topped off my tanks and let it sit overnight more than once and no leaks ever appeared. I postulated that a tiny piece of debris maybe got lodged between the float bowl cutoff valve tip and valve seat, and when I dropped the carb bowl to investigate, that instantly fully opened the valve and so any trapped debris would have rinsed away in the falling fuel. But they did not comment on that.  Anyone else experience this same issue?

Views: 103

Reply to This

Replies to This Discussion

I go with the lodged debris theory.  I replaced my rubber hoses a few years back and ran into a similar carb leakage situation.  When I pulled the bowl, I found a tiny piece of black rubber inside.  It was more than likely debris left over from my hose replacement action.  No carb leaks prior or since.  I have a low wing 601XL, so even lower static fuel pressure than yours.

Hope it all remains dry!

Dave G.

Thanks Dave. Yeah your static pressure would be about zero! It’s still dry and I’ve flown it twice since, and keep the fuel load at about 4-5 gals per tank- although I would think the total fuel weight would be the 0.7 pressure on my gage as it measures from the fuel distribution manifold at the center of the engine. 
My hesitation to add shutoff valves for each tank are 1. More weight 2. More complexity 3. Increased risk of making an inflight fuel management mistake.  My current setup is simple and works. I’m going to leave it as-is for now and see what transpires. 

I had the same issue before..i have always had a a shutoff valve(L-R-both) however, never turned it off at the end of the day-until the leak started. Puzzled as you,went through all the same diagnostics. i then turn it off at the end of days flight.no more problem.that was 300 hrs. Ago.makes sense to me how the gravity weight of fuel pushing on that little needle could give way over time. Don't overthink it- install a shutoff valve and rest easy. My 2 cents

Thanks Shawn. So after dropping the float bowls and cleaning/checking everything, one or both carbs still leaked continuously until you shut off the fuel supply to the carbs? 

What bothers me about this issue is that as fuel passes through the float bowl shutoff valve it should enter the float bowl only because the floats have lowered due to low fuel in the float bowl. Additional fuel allowed in by the shutoff valve then raises the floats, which in turn applies closing pressure on the shutoff valve tip against its valve seat- Assuming fuel pressure remains below 5.8 psi. This is how a properly functioning system works. So if my carb suddenly started leaking one day, years after operating properly, to me, it can only be one of three things- the fuel system pressure is above designed system limits; a system component is worn or damaged; or a foreign object has obstructed the shutoff valve and prevented it from functioning as designed.

If I believe my fuel pressure gauge, then ten total gallons of fuel results in 0.7 psi static pressure on the float bowl valve. So I’m assuming full tanks (20 gals) would produce about 1.4 psi static pressure. Well below the system design limit of 5.8 psi. Actually, if I turn on my electric boost pump I will increase pressure on those float bowl shutoff valve to almost max- 5.8 psi.  Yet they won’t leak.  I’ve done this every day now since this happened. She’s still dry.

So yes, I believe closing my “fire cock” as Rotax calls it will stop the leak. But so would draining the entire fuel system after each flight. And neither makes sense to me. I’m convinced that shutting off the fuel to the engine to prevent a continuous static leak is just masking the actual cause of the leak. For now I’m leaving the fire cock open when parked and if my leak ever returns I’ll definitely come back here with an update.

Two weeks and several flights later. No leak or seep has reappeared. Maintaining half-full (5 gals/side) fuel load.

I have Jabiru engine with Bing carb and it was dry for the first 5 or 6 years then started staining the intake tube and dripping slowly. Jabiru instructions are to turn fuel off after flight - which I didnt normally do.

The float needle valve is Viton tipped coating and commonly these are replaced if seepage occurs. Jabiru procedure is to replace both needle valve and seat and I just ordered and replaced the needle valve. It didnt stop the seepage so I now shut fuel off after last flight.

You probably had a temporary debri issue, however eventually one carb or the other then both will likely develop this seepage, with avgas it leaves a green or blue dye behind, mogas probably nothing obvious. In engine running conditions, this seepage is not likely to alter the float level, its very small.

Two months later, still no seeps, no leaks.


New from Zenith:

Zenith Planes For Sale 

Classified listing for buying or selling your Zenith building or flying related stuff...

Custom Instrument Panels
for your Zenith

Custom instrument panels are now available directly from Zenith Aircraft Company exclusively for Zenith builders and owners. Pre-cut panel, Dynon and Garmin avionics, and more.

Zenith Homecoming Tee:

Zenair Floats

Flying On Your Own Wings:
A Complete Guide to Understanding Light Airplane Design, by Chris Heintz

Builder & Pilot Supplies:

Aircraft Insurance:


West Coast USA:

Pro Builder Assistance:


Transition training:

Lavion Aero

K&S Aviation Services

Aircraft Spruce & Specialty for all your building and pilot supplies!

How to videos from HomebuiltHELP.com

Developed specifically for Zenith builders (by a builder) these videos on DVD are a great help in building your own kit plane by providing practical hands-on construction information. Visit HomebuiltHelp.com for the latest DVD titles.

© 2024   Created by Zenith.Aero.   Powered by

Badges  |  Report an Issue  |  Terms of Service