My other stutter incident, or rather series of incidents, first occurred about three years ago, when my engine just started running rough for no apparent reason in cruise. Naturally I pulled on carb heat and it smoothed right out. I ran heat for several minutes and continued the flight. It didn't seem particularly moist that day, but since carb heat took care of the problem, I just figured that's what it was.
It happened again several times intermitently, sometimes with 20 or 30 hours between incidents. Since some of the occurences coincided with a likely possibility of carb ice and the application of heat always fixed the problem, this went on for a year or so. Finally it started getting worse, and I had a frightening experience on a cold and wet day when I could barely keep the engine running.
I started investigating. I had never noticed much junk in my fuel system, and it had been completely drained and flushed in 1998 when I had to fix a leak in the integral tanks. The system ran very clean for several years after that, and I always used a 16 oz. GATS jar (check it out at Sporty's or Chief Aircraft) to sample all 8 (!) fuel sumps on my plane. Of course I poured the fuel back in each time, that's the whole point of the GATS jar and it has a filter fine enough to stop water, finer than the gascolator screen. I suspected some sort of fuel flow problem from the beginning, but a check of the gascolator showed only the most minute amount of some fibrous material, only noticeable if you rubbed your fingers across the mesh. The total amount was about the volume of a BB, and it was essentially INVISIBLE in a sample of gas!
I didn't think that was significant, so I started looking at the carb air box, which had been rebuilt and carefully sealed during the Oct. 2000 major overhaul (which also exchanged my old carb for an overhauled one....). Nothing out of the ordinary there, so I put everything back together and flew for several more weeks with no problem. Then it happened again, this time in mid-summer with a relative humidity of about 5%! I began to get really aggravated. I just KNEW this was not a carb ice problem, but every single time it ever occurred carb heat fixed it. I have a 6 probe EGT/CHT and a carb temp gauge and I spend a LOT of time looking at them so I consider it a very remote possibilty that carb ice could happen to me without some warning.
So I told my A&P that I was going to pull the carburetor and I wanted to go through it with a fine tooth comb....he thought that was probably a good idea, since I had been bugging him for a year as to any possible explanation for this weird phenomenon. Well, here is what we found. This recently overhauled exchange carburetor had a loose throttle butterfly, bad enough that he thought it would have come completely off or jammed in another hundred hours or so, it's float needle seat had deep grooves in it that looked like maybe 3000 hours worth, the needle had a groove worn at the seating area, also 3000 hours worth. In short, it looked like they just painted it and yellow tagged it. I was pissed! My previous carb had only 1800 hours on it and was perfectly fine. There was so much else going on at the overhaul that it just slipped though the cracks. Never Again!
However, there was no visible crud anywhere in the float bowl, and we ran several ounces of solvent through it and blew air through it every which way we could. When we pulled the finger screen, again a minute amount of fibrous material was visible, and of all things the decomposed remains of a FLY! Guess how long this carb sat on a shelf unprotected.
So we put new everything in the carb and it went back together and I flew for a year with no problems. This winter it happened AGAIN! Well maybe.... I don't know for sure because it could have been plain old carb ice that day. However, the sudden onset was suspicious, just the same way it had happened those other times. I talked with several other very experienced A & P's, who basically told me to do what we had already done. They did all agree on one thing, that given the extremely careful fuel sumping procedures that I have been using for years it was MOST unlikely that I had a fuel flow problem caused by filter impaction. I was not convinced.
Here is what I have started doing and what I have found so far: I got two large glass Mason jars and a couple of Planter's Peanuts jars (just because they are easy to hold). I went to the NAPA store and bought
two regular inline fuel filters and joined them in line. I bought a small funnel with a built in screen. Now, every time I sump the aircraft before flight, I take a full GATS jar (16 oz.) sample out of each drain, pour it into one glass jar, take it out into the sunlight and swirl it around, then hold it up to something dark (my sleeve) and watch. If the light is just right, I can see some minute fibers 1mm or less and other motes even smaller glinting in the sunlight! There is also the usual small amount of fine dirt. I take that jar and pour it through the two fuel filters into another Mason jar, and take THAT out into the sunlight and check it. Sometimes I will repeat the procedure, if there is ANY stuff still visible. I did pull my finger screen this winter just after that one incident and it appeared perfectly clean, but I am continuing to find superfine transparent stuff after a couple of runs of this filtering procedure, which I only started last month. Now it's hard for me to believe that the amount of stuff I'm seeing would actually plug a fuel screen to the point of cutting off flow, but I think that fibrous material, even as miniscule as what I am seeing, has the insidious property of slowly building up across the mesh of a filter screen under constant flow pressure until it reaches a critical point where it has mostly obstructed the filter and any additional stuff will rapidly close off the flow. After all, this is the principal behind many automotive products that are supposed to stop leaks in radiators, etc.
Once the flow stops, any sloshing movement such as pushing the plane back into the hangar causes some of the particles to drop off the filter and settle to the bottom of the gascolator or finger screen housing, waiting to get swirled back up onto the filter after some minutes or hours of the next flight. I had a similar experience many years ago with bad gas in my Toyota 4WD. It ran long enough to get me well out into the desert, then quit after a mile of sputtering. I waited 40 minutes or so, and it would then run another 5 miles before quitting. Eventually, after about 5 iterations, I got back to town and replaced the filter.
Although I can't yet prove that my incidents were due to fibrous contamination, it is obvious that Zero One Victor's was. He found the smoking gun on the finger screen in the carburetor. He, like I, had not pulled and checked that screen at every annual because there was no reason to suspect that it was dirty. There had never been more than the usual slight amount of dirt in all the fuel samples he and I took over a period of years. In my case, the gascolator screen showed only the most sublte indication that there was any fiber in the fuel, an easily overlooked item....since it is INVISIBLE! (until it build up to a threatening amount, that is.)
There have also been recorded instances of bad contamination of fiberglass tanks because of poor resin technique and/or the use of chopped glass where there should have been only cloth. Homebuilders beware!
So here comes some more of the old pelican's pontificating:
1. Always inspect closely in BRIGHT light and clean ALL accessible fuel screens at every annual, even though the finger screen requires that star safety washer on Marvel carbs to be replaced each time. Go buy a half dozen of them.
2. Since fibrous contamination is so difficult to see, use a large clear glass jar once in a while to really check what's coming out of the gascolator. Hold it up to bright sunshine or shine a flashlight up through the bottom of the jar after swirling it. If you see little tiny things that glint in the light but are otherwise invisible (not bubbles), BE SUSPICIOUS! I have noticed that the samples out of the gascolator have at least twice the amount of this material as any other sump, so it's the place to start looking.
3. Always fuel your aircraft yourself and never let any type of rag or paper towel near the opening; put the cap on before wiping up.
4. If you are going to pour back your fuel samples I suggest you make a setup like mine; the GATS jar provides inadequate filtration in my opinion. (at least for fibers) I'm thinking of getting a dedicated two gallon metal can so I can filter several days worth of samples before pouring it all back in. Or just use it in your lawnmower!
Do you think I'm being a little anal here? Well, you are absolutely correct, but I will tell you one thing: by the time I am through with this filtering procedure, some 200 gallons later, I will have the cleanest fuel system in the world or I will have proved that there really is something disintegrating in my tanks. Naturally, I would have already opened up the tank access to look for whatever if I had not personally cleaned them 7 years ago during the resealing. I even VACUUMED them at that time, then inspected every inch of them with a light and inspection mirror. The only way anything could have gotten in these tanks is though a fuel pump. Also, I am the ONLY person that has ever fueled this aircraft in the 13 years I've owned it.
I suspect that the amount of fiber remaining in the tanks is minimal, and therefore I'm willing to try this procedure for a couple of months before going though the hassle of opening up the tanks and searching for the source of the contamination. I will inspect both the gascolator and the finger screen once more before summer, and if there is ANY further sign of fiber on those screens I will open up the tanks.
Be careful out there!
Rocky
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