Scale Matters

Lore, muffler pressure/clunk tanks problem, Mecoa problem, Cronenwett reels

By Orin Humphries
January 2026

Lore

O.W. Tosch

O.W. Tosch was a bush pilot in Alaska and there he achieved some fame before World War II. After the war at some point, he moved down to the lower 48 and opened a fixed-base operation in Minneapolis. But let me tell you of his initials as an aside. His parents named him after the Wrights, Orville and Wilbur, hence the initials. He never went by that; if he used a name at all, it was Orville. Mostly he went by O. W.

O.W. had a contract with the airline in Minneapolis to get weather data in the mornings for them before they began operations for the day. He had to do this rain or shine. He was to fly his airplane near the airport and climb up to a certain minimum altitude, taking atmospheric readings along the way. Then, he would land and turn in the data.

I asked Ben, (see previous articles), who was relating this story to me, “What about bad weather?”

“Rain or shine.”

“Did O.W.’s plane have instruments for it?”

“None”, Ben replied. “All he had was needle and ball, and airspeed! As he climbed, when he reached the overcast, he kept it level with the ball.”

Then came the bombshell! I asked Ben, “What did O.W. do if he got disoriented?”

“He threw it into a spin until he got below the overcast, and then he straightened it out. He then re-entered the overcast and tried it again, using only the ball and airspeed. He would spin down out of the clouds as many times as he had to until he finally made it high enough to break out into the clear and then continue his climb for data.”

“Well,” I asked, “how did he get back down to clear air?”

“The same way, using the ball to keep it level. If he lost it, he threw it into a spin until he broke out below, and then he straightened up and landed. He then handed in his data and went back to work at his FBO.

Ben said that when O.W. told him of this, Ben had said, “You were crazy!”

“We [bush pilots] were all crazy”, was Tosch’s reply.

Muffler Pressure Problem

Above and below: Test bench with oil trap tank. Photos by Orin Humphries unless otherwise noted.

Test conditions:

Fuel system uses muffler pressure
Fuel tank is a clunk style
Orientation of the cylinder – Upright, 90 degrees on profile model, clocked 120 degrees
Different brands of mufflers

Your model might have the following problems and it might not, depending upon the four conditions above. If you have it, the severity can vary from one plane to the next, making diagnosis difficult.

I will discuss the photo above shortly, but first, this:

For several years I have been having occasional, mysterious starting problems with a couple of my models. Starting will be per the usual, numerous times, and then, on some off-the-wall day, I can’t finger prime the engine, finger over the venturi and turning the prop. The engine will only run out the squirt prime, a few drops in the venturi from a small syringe. I will continue this for a while, and then it will start as if there had been no problem. I always ran the gamut of the standard procedures for clearing a blockage in the fuel feed line. This never solved nor explained the problem. Many times, during these situations, the engine did not respond to the main needle valve. The runs were quite lean.

Orin's Stearman at the 2025 Nats. Fred Cronenwett photo.

At the Nats this year I was trying for a spot on Team USA to the World Championships next year. I had flown this plane, my yellow Stearman, five times before, here at these contests, my last of three flights being in the Trials portion. The first three flights were in the Nats portion, where I was flying this bird in Sport Scale. After the AMA part I was using the Stearman again in the FAI portion, and this was to be the sixth flight altogether at Muncie. The engine would not catch. I could not draw fuel forward with my finger over the venturi. Everybody on The Team tried their best to assist my beating the clock to get airborne.

Finally, I recognized this mysterious problem, and I did what I had done in desperation before. I disconnected the fuel tube at the carburetor and hooked up a small syringe with fuel in it. With this, I back-flushed the fuel line and reconnected the line to the carb. (The only reason that I could do this was that my Stearman has an exposed dummy radial engine.) The engine caught normally, and I beat the clock, airborne at last.

Orin's Staggerwing at Roseburg. Flying Lines photo.

Fast forward: Back home much later, I was flying with Ben as my pitman at Sehmel Park’s ball diamond in November. My Staggerwing’s engine died twice. Everything was fine on my first two flights, but on both of flights three and four, when I throttled back for a touch and go and the plane was slow on approach, the engine simply died sharply.

I checked for a couple of possible causes of the flameouts and found no answer. Back home afterwards, a key symptom of muffler pressure operation coalesced in my head. We always see a few bubbles of oil going up the pressure tube from the muffler toward the tank. I have always wondered about what becomes of the accumulated oil once inside the fuel tank. I have assumed the oil just got mixed in with the fuel and “vanished.”

Actually, I had previously done an experiment to look at this oil in the pressure line question. See, again, the above photo of the tanks on my run stand. The photo shows the amount of oil collected in a preliminary trap tank, before the pressure is introduced to the fuel tank. The run time on the engine being tested was some 30 to 45 minutes.

Back to my Staggerwing’s flameouts, I checked and eliminated all the traditional causes of fuel starvation and was left with that oil collecting in the tank was blocking the clunk. At lower speed on approach, the clunk would have fallen down from a higher, outboard location within the tank. The clunk would at that time be located in the bottom outboard corner of the tank. That is where the oil would also be if that were the problem. The supposed oil blob would be drawn into the fuel feed tube and the oil would block the needle valve. (Or so I guessed at the time.)

For both models, I had in concert with times of a failure to start normally, the engines’ not responding to the main needle valve’s being changed through a range of a whole turn or more. What kind of blockage was causing these?

Back home, I had parked the Staggerwing in my garage, standing the plane up on its nose for lack of space and on a wingtip. A couple of days later, I went to get the plane and check the tank out. I saw a puddle of liquid on the floor under the nose of the model. I rubbed my finger in the puddle to get a sample and then spread the sample of some cardboard. The liquid felt thicker than our fuels. I then poured a sample of Brodak fuel onto the cardboard next to the stuff from under the Staggerwing’s nose. The fuel was thinner than the material in the puddle. Indeed, there was excess oil in the liquid in the puddle from the tank.

The conclusion then, is that oil from the muffler does gather in some of our tanks and it does not always get diluted by the fuel in the tank as we fly. How does oil get into the tank?

Picture a front view of a chamber muffler, as above. The duct from the cylinder brings exhaust gases into the muffler chamber, typically at the bottom of the chamber. This produces a swirling flow that proceeds outboard along the bottom of the muffler body. The body turns the flow upwards along the far side, outboard of the cylinder. The body then turns the flow inboard across the top of the body, towards the cylinder. Finally, the flow is turned downwards on the inboard side towards the duct, completing the circle.

Note that near the top of this image that the gases are going past the pressure tap. Pressure causes some of the gas and what it carries to exit through the tap and head toward the tank. This is how the oil can become a problem for many models.

The orientation of the muffler relative to the ground on a model can ordain an oil accumulation problem in our fuel systems. In some muffler brands this happens, and in some it might not. Perhaps the details of the muffler’s internal geometry are part of the root causes.

Examples:

All below have clunk tanks and muffler pressure:

My Stearman with an upright cylinder: SuperTigre muffler, problem
My Staggerwing with a Profile mounted cylinder: Mecoa universal muffler, problem
My Stinson with its cylinder clocked 120 degrees: Mecoa muffler, problem
My run stand with upright cylinder: OS muffler, much diminished problem

When I broke in my new OS .46 AX on my stand, I looked carefully at the pressure tube from the muffler to the trap tank. I at first thought that no oil was going through the tube. This impression was fostered by having seen so many significant, whole blobs of brown oil going up the tube from a clocked muffler in both my Stinson and my Staggerwing. Those blobs were the size of a healthy raindrop. Upon closer inspection, I first saw that the oil level in the trap was slowly increasing. Looking very closely at the pressure tube from the muffler to the trap, I noted very tiny clear oil droplets slowly migrating toward the trap. For a while, I had discounted this as a problem. (I also noted a small, persistent brown oil smudge inside the fuel tank itself, right at the point that the pressure inlet tube touched the inner tank surface. This smudge was from previous oil in the fuel tank’s pressure tube from before I had installed the trap tank and plumbing. Oil was not now bypassing my trap tank.)

Trap tank, later.

While breaking in my vertically mounted .46 AX on my stand, residual oil in the trap rose from a previous 3/16” depth in a leaned and tilted tank corner to ½” depth. This was over the course of some 13 or 14 8 fl.oz. tanks. Note the color change of the oils in the trap from the older oil to these break-in runs. The darker at the bottom is the older, of course, and the lighter on top is from breaking in my .46 AX.

Problem Solved

An old Veco 1/2-A tank replumbed for this work.

This is for reference, next. Orin's Staggerwing flies at Roseburg, Ore., Airport, during a Northwest Control-Line Regionals.

Oil trap tank added.

I installed an old NIB 1/2-A tank between the muffler and the fuel tank to trap the oil coming up the pressure tube. Look for the oil drain tube.

Orin's Stinson, also for reference, next.

Here you see the NIB 1/2-A tank that I have put in my Stinson between the muffler and the fuel tank. Note carefully the “wet pressure” inlet for the trap tanks, the “dry pressure” outlet tube, and the oil drain tube, in each system.

What about the plumbing details of these oil trap tanks? Well, you have to picture the trap tank under two conditions. One condition is In flight, and the other is ground attitude. In each of these orientations you must picture where the oil will be and where you want the pressure outlet from the trap tank to be. You want the pressure coming out of the trap tank to be well away from where the oil blob will be. You also need to provide for an oil drain tube in the trap tank so that you can empty the trap tank at the end of a flying session, in ground attitude, of course. The location of the inner end of the pressure inlet tube is critical, too, as will be seen shortly.

Pulling oil out of the fuel tank. Jeral Godfrey photo.

But first: An alternative oil draining method: Have your pitman hold the airplane in a nose-up, right -wing-down attitude. This will let the oil in the fuel tank move to the fuel pickup tube’s mouth inside the tank. This will take a few seconds. Disconnect the fuel line at the carb, (assuming that you have easy access to it) and hook up a small syringe to the fuel line. Now, you may withdraw the oil with the syringe. I cannot do this procedure with my Stinson as it has the engine all cowled in. However …

The oil is so thick that it likely will not want to come out at all. What will likely happen is that the suction will draw a small passageway through the oil and mostly just air will come out. You will need to introduce a little bit of fuel, a couple of fluid ounces or so, and vigorously shake the whole airplane around a while. This will slosh the fuel around and make it mix with the thick oil. Then, you can withdraw the oil/fuel mix.

Son of Old Faithful

Back on track:

After writing this, I went out into my garage to run my new OS.46 AX on my stand for the first time. Being a “numbers nerd,” it struck me to measure the amount of oil that you see in the picture of the stand’s trap tank. I couldn’t get the oil out to do that, because the pressure inlet tube did not extend down into the oil pool. I pulled the plug out of the oil tank and put a piece of silicone tubing on the inlet tube that extended its reach all the way to the bottom of the tank. I withdrew 13 ml.

(This would become a spectacular failure!) Verschlimbessern!

Upon reassembly of the tank plug and plumbing, I set out to commence the OS. The main needle was set at a guess, which proved to be unworkably rich. The engine died upon removal of the glow plug clip. When I tried to restart the engine, the starter motor spun the acorn nut off the prop. The engine had flooded at shutdown. I pulled the glow plug and spun the engine with the starter, and I was treated to a huge expulsion of fuel from the plug hole! What the…??

I started the engine for the second run, and all went pleasingly well, firing right off on 10% fuel at 40F degrees and with a hot glow plug. At the end of my usual routine’s 2 minutes and 30 seconds run, I shut the engine down to cool. I topped off the fuel tank for the third run.
As I turned the prop with my finger, fuel gushed up out of the venturi. What the…?? So, I again pulled the plug and spun the engine with the starter. I was treated to another geyser of fuel coming out to the plug hole that rivaled Old Faithful!

How can this be happening? This never happened before in decades of run stand operations. I ran the engine for the third time with normal result and then shut it down.

After shut down, within seconds, once again, the engine was FULL of fuel. Removing the glow plug again, one more Son of Old Faithful fountain of fuel cleared it. Okay, it was time to think this through.

I woke up my favorite neuron, Ned is its name, and put it to the task. (Now you know why I am an unemployed Stand-Up.) What can move that much fuel to the engine at shutdown? Answer: pressure in the tank. Question: Why is there pressure in the tank, now? At shutdown, all pressurized systems back-flush the tank pressure out through the line to the muffler. Why, then, isn’t this happening here?

Remember that I had extended the pressure inlet pipe all the way to the bottom of the oil tank. Oil had accumulated, as I am saying throughout this, and the oil blob covered the end of the inlet tube. The oil was so thick, like Sig castor oil in the bottle, that air pressure from the tank could not get through the oil to exit via the inlet tube to the muffler. There is no way, then, that oil can be drawn into the fuel feed tube to make a blockage at the needle valve.

The blockages that I have experienced for several years of the fuel feed plumbing is at the oil glob in the tank, an internal problem, not outside at the needle valve.

Lesson: Your oil tank’s pressure inlet tube must protrude to a point in the trap tank such that the inner end is well above the maximum level of oil that you will accumulate in a day’s flying session. Your tank’s residual pressure at stoppage must have free retrograde flow to the muffler.

Mecoa problems, and another

I had chosen a Mecoa .46 Engine for my Stinson project, having flown an earlier version in my venerable T-34C. Initially, I was pleased with the engine, but troubles were not far, way off. After four months or so of operation, I began having difficulty adjusting the idle mixture on the RC carb. The carb is an otherwise good two-needle design. I removed the carb and noticed that the big O-ring at its base was badly cracked from the outer edge inwards. A check with Ben revealed a common cause of that is found in full-size aviation. Fuel was deteriorating the O-ring material. I replaced the O-ring with one from Tacoma Screw, although new ring was thicker in cross-section. More on this choice presently. That wasn’t all the problems that were rearing their ugly heads, though.

My inability to get an idle needle adjustment worsened. The symptoms that the engine presented upon throttle-up from mid-range and from idle suggested that the idle needle was rich. I successively tried leaner adjustments to no avail, and there was an appearance of an unexplainable bogus idle stop setting. When I looked into the venturi with the throttle arm all the way OFF, I was surprised to find the STOP had moved to about 40% still open! How in the devil had it gotten to there? (The idle needle was in way too far.)

Over the course of a few months this mysterious bogus setting reoccurred several times. Finally, I noticed that each time I went to readjust the idle needle, it was mysteriously in a different position than what I had recorded in my log.

The idle needle is supposed to remain where you leave it, right? When I checked the idle needle, that’s when I discovered that it turned freely, unrestricted. The parts illustration had earlier shown that there is a tiny O-ring on the idle needle within the “rotor” body. I removed the idle needle and found the O-ring to be deteriorated. It, too, was made from the wrong material.

O-rings that are exposed to fuels and their ilk must be made from a material called Viton. Ken Burdick of Kamloops, BC, told me of this. Viton rings are not very common on the commercial market.

Parts Department

When the first adjustment problems started, I went online to the Mecoa Parts Department, and I ordered multiple O-rings of both sizes. I also ordered a whole new carburetor to be used as a reference for settings; I would never touch the factory adjustments that the carb arrived with. I would use my blow test method which I had previously published, here.

Nothing came. And nothing came. Finally, months later, a lady at Mecoa called me asking for an explanation of a statement that I had made in my order for the carb. (I had said that I wanted them to ensure that the carb was correctly in the factory settings for their design specs.) The lady explained what their situation is. “The main business of Mecoa is Defense Contracts and we are swamped. People have been pulled into the Defense work.” There was no one available to speak to my request. The Parts Department at Mecoa is non-functional.

Nothing has ever come.

Willy, our mechanic at the Heritage Flight Museum where I am now a docent, showed me how to get by with an oversized O-ring. He showed me how mount a tiny O-ring on, say, a scribe or other round peg, and with good light and a fresh X-Acto blade, carefully trim away the excess rubber material. I made a new O-ring for my idle needle this way, but it worked, “sorta.” I think it was still leaking a bit of air.

Lean sensitive

I experienced a number of inadvertent, short, lean-runnings multiple times as I struggled with these leaky O-rings. I will quote an excerpt from the factory instruction sheet. “A lean setting raises the engine temperature…This is very harmful to the engine…”.

I experienced some short, lean runs multiple times when, now I know that, the problem was oil accumulations in the fuel tank.
My Mecoa .46 has lost compression. It still starts with my electric starter. But I cannot risk possible further compression loss between now and the World Championships in England next July.

This all has brought me to make a difficult decision. I am making an engine change. A friend, Ken Burdick had offered to me an engine that was excess to his plans at this time. It is the OS.46AX. I bought that engine and have now broken it in on my run stand for installation on my Stinson.

In fairness, problems of lean runs with no response to the main needle, occurred on both my Staggerwing and my Stinson. My Staggerwing has an OS .45 SF engine, not a Mecoa. I assess that that half of all the lean runs were caused by oil accumulation in the fuel tank, and the other half of the time in the Stinson’s Mecoa engine, the cause was leaking O-rings in their carb.

All in all, my problems with the Mecoa carburetor and with their Parts Department cost me several months of flying and more than two quarts of fuel.

Side bar:

This OS .46 AX has the lowest, sweetest idle of any engine that I can remember! I can see individual little puffs coming out of the muffler. Be still my beating heart! And the break-in for the .46 and .55 AX series is greatly relaxed. First, in the 1990s, OS was instructing us to use 100 percent castor for our fuels’ oil. Not now: for the AX’s they say 50-50 oil is fine. Best of all, however, is the break-in methodology. They say to just run one tank thusly: without stopping, run rich for 10 seconds, run lean for 10 seconds, repeat for the entire, single tank. You adjust the main needle for each action, here.

You are done with the ground break-in!!!

For the airborne break-in, run slightly ric15h for 10 to tanks. Finito.

For myself, I was already at the field, it didn’t have to be flying to do the “airborne” part by flying. Get this: In 3 hours and 15 minutes I did this. 1) I drove 19 miles to the field; 2) set up; 3) ran the “ground tank”; 4) ran 10 tanks slightly rich still on the bench; 5) tore down and cleaned up; 6) drove 19 miles home.

For me, that was the sweetest big engine break-in session ever. Think about these OS AX engines, guys.

Time in

No. 2 sheet metal screws.

Early this year I came on a batch of these widely used screws where the Allen wrench spun in the pocket when screwing into 1/8” thick aircraft plywood. These screws were put out by Du-Bro which had bought them from a supplier. I noticed that the heads of these screws were abnormally flattened, as you see on the right one in the photo. The heads were not correctly domed, and the pockets were loose. No trial of all other Allen wrench sizes produced a match. I brought this to the attention of B&B Hobbies in Spokane, Wash., and was rebuffed. If you just hold the screws in your fingers, the normal Allen wrench will turn them. Again, the wrench will spin in the pocket against the greater torque of putting the screws into good plywood. Oddly, the longer, half-inch length of these button head screws have the same flattening, yet the wrench will not slip in plywood. It is only the 3/8” length of these #2s that have the big pockets.

The 3/8” length screw on the left in the photo shows the correct dome shape, which works. This screw was from a different supplier.
I called Du-Bro and reached a gentleman high up in the company. He was aware of the problem, but he explained why they were powerless to do anything about it. He said, “These screws are made by a jobber somewhere in the world, and now there is only the one jobber. The jobber will not make batches of under one million units. [We can’t sell one million units.] The set-up costs for such a run make it impossible for Du-Bro to afford a custom, smaller run by some secondary supplier.”

You may all still get properly made #2 x3/8” sheet metal screws form either of the following suppliers online: Micro-Fasteners.com, and McMaster-Carr.com.

Cronenwett Line Reels

Fred Cronenwett makes these line reels.

Fred Cronenwett writes the control-line Scale column for Model Aviation Magazine, as most of us know. He got a 3-D printer and decided to make new, different line reels. He gave me a sample to try, which I have. I find them to be well-made and thought out. They have three slots around the rims for hooking the ends of our lines, and these work out fine for me. I did advise that he make the next batch narrower for storage in our line boxes, which are sized for the Sig reels that were about 1/2 inch thick.
Fred can be reached through his email that accompanies his columns.

Fred actually made these for our trophies at the Northwest Regionals in Roseburg, Oregon last May. He had all the contest and trophy prize information printed on them.

Happy New Year to All!