Partial Fuel Starvation

[BeeMan]

About two weeks ago, I was half-way home from a 1 hour flight when I noted a slight tach. fluctuation (2400 to 2450 rpm). I was in level cruise, 4,000 feet altitude, gas on both with tanks ¾ full, and leaned for cruise. I pulled carb heat and got an immediate increase in rpms with no roughness. I took this to mean that I had no carb ice but was already running lean. I pushed mixture to full rich with no noticeable change. Over the next 20 minutes the tach. fluctuation became more pronounced ‘till the engine was surging 1500 to 2000rpm. Switching tanks from both to left to both to right made no difference. Shutting off the carb heat caused the engine to quit so I left it on. The surge stopped when I pulled the power back below 1500 rpm to spiral down into the pattern at my home field (Lake Hood Strip, Anchorage, Alaska).
First step in diagnosis was to pull the cowl and remove the fuel line from the carb for a flow test. The fuel line flowed full on left, both, and right. Next we (I was working with an A&P) pulled the finger screen out of the carb float bowl. The screen was packed full of a tan fibrous material; enough to seriously impede fuel flow. Then we drained the carb float bowl and didn’t observe any notable debris or water contamination in the fuel. However, when the float bowl plug was reinstalled and the fuel line hooked back up, the float stuck and didn’t seat the float valve even after starting the engine (we thought the vibration might free the stuck float). The carb was subsequently overhauled, the gascolator cleaned, the tanks drained and then flushed, and the fuel lines were blown back from the gascolator to the wing root fittings. There was a small clump of tan fibrous material on the gascolator screen and small clumps were stuck in the top of the tank sump quick drain fittings.
While the fibers looked like insulation, they smelled like cotton when burned in a torch. We think the fibers came from lint off cotton winter gloves or from chamois used for filtering fuel by the previous owner/s. In hindsight I recall seeing one or two very fine fibers swirling in the fuel samples when I sumped my tanks. In those situations, I typically rocked the wings, sumped the tanks again and found the samples clean. Also, I made it a point to practice stalls and unusual attitude recovery as I became familiar with the aircraft so I may have stirred things up. I had a buyer’s inspection and the aircraft had an annual inspection (two different A/Ps) as part of my purchase. The finger screen was apparently not checked in these inspections. I am convinced this problem was developing for some time and I nearly had a forced landing because it wasn’t identified quickly enough. If you don’t know your screen is clean or when your screen was checked last, you should have a look at it.
I am a 500ish hour pilot with most of my time in a C180 in Alaska. The aircraft is a C170B that I bought back in March, 2010.

Cheers, Bill
N2575D

[Bruce Fenstermacher]

Bill I had nearly the EXACT same problem on the second flight of my first 170. My short story.

Bought the plan in Virginia. Wife and I flew it home on a 2.5 hour flight with no problems noted. Next day was a nice day and I had a brand new to me airplane so I went up to bore holes in the sky.

Decided slow Cub type flight was in order and so throttled back and slowed down to about 80mph for about 45 minutes of just lazy turns and low (for some) flight about 800ft agl.

Time to return to the airport and I advanced the throttle from somewhere low probably in the teens and the engine quit. Before the prop stopped I pulled the throttle back to where it was and the engine resumed ticking along. I almost landed in a short private strip but realized the engine was running fine at low throttle and I'd just flown 45 minutes that way and so at low throttle I flew back to the airport gaining any and all the altitude I could at the low throttle setting.

Back at the airport I starting looking at the fuel system. All was clean and running smoothly. Then I pulled the finger filter. That is where I found the same fibrous material you describe. I thought someone had jammed a cigarette filter in there. I removed it and showed a local A&P who had never seen anything like it before and couldn't think anywhere in the system it could have come from.

Well with the finger filter clean and the carb fuel bowl drained we started up the engine for a penalty run. After about 5 minute we checked the filter to find it clean. Flew the plane above the airport for about an hour and checked the filter and found it clean. Never in the next ten years did I ever find anything else in the finger filter and no explanation what it was I took out or how it got there.

[canav8]

Bill has the carb had all the AD's complied with when you had the buyer inspections? Especially since the new floats must be changed again. There should be a stamp on the data plate. How about the venturi? Is it a one or two piece? Your mechanic should have found the problem then. What is in your gascolator? is it clear? Just asking.

[jrenwick]

Dang! I've gotta stop dousing my cigarettes in the gas tank while I'm refueling!!!!

[GAHorn]

Some fueling facilities may have old and/or deteriorated filters which are releasing filter media into the fuel. This is usually due to the use of automotive fuel suppliers.
If you can identify when and where you have purchased fuel prior to the contamination you might be able to track it down. They are supposed to be on a regular inspection/replacement program but may have decided to "save money".

[BeeMan]

Bruce,

Happy to hear that your fuel starvation incident was resolved with no bent metal. As I began to recognize my problem was reduced fuel flow, I might have been better off to pull the power back and go home a bit slower as you did. However, I fly over some pretty unforgiving terrain and didn't want to be lower or slower than necessary. I posted the story so others might avoid those anxious moments we experienced.

canav8,

AD compliance was a stipulation of my purchase agreement, both A/Ps were satisfied on that note before I bought the plane. It already had the one-piece venturi but now has the updated nozzle and the metal float has been replaced with a composite float along with a new needle and seat. The carb overhaul wasn't necessary to resolve my fuel starvation issue, but I feel good about having checked the fuel system out thoroughly and the carb is where all the crap ends up IF it gets through the screens. The biggest surprise to me is that these fibers could fit through the gascolator screen. I wanted to be sure that they couldn't pass the finger screen too.

Cheers, Bill
N2575D

[Brad Brady]

Bill,
Don't know how you would get it in a 170 tank....But flannel shirts reinstalling a 182 bladder tank, will produce the same type of contamination you mentioned .

[Blue4]

Ok, you have my attention. Is there an easy way to check the finger screens? Pictures, please. Thankee.

[DaveF]

FWIW, I check my carburetor inlet screen every annual and have never found anything.

[GAHorn]

FWIW, I check my carburetor inlet screen every annual and have never found anything.

Not on my 170 but, on another airplane I discovered black "sand", presumeably from the 30-year-plus old gascolator-to-carb fuel delivery hose liner. Drained the carb and found a bit more in the bowl. This carb later stuck a float and required disassembly/cleaning/re-assembly before it performed correctly again. (And it required rebuilding the fuel shut-off valve which was frozen in the "on" position, presumeably due to corrosion from all the autofuel (and ethanol?) that airplane had used in it's past. Ethanol-laced fuel can be very corrosive/injurious to aircraft fuel system rubber components, and it also is conducive to water accumulation in the fuel which adds to the corrosion possibilities....so beware, if you use the stuff.)

[Brad Brady]

Ok, you have my attention. Is there an easy way to check the finger screens? Pictures, please. Thankee.

Not that I'm fimular with. ...you'r not flying a 180 are you? This will anserwr sevrral questions....

[N4005V]

For the the ragwing 170 owners they have a fuel pump with a screen in it that needs to be checked and cleaned at annual also.