The International Cessna^® 170 Association

I purchased the STC to use Swift UL94 this last summer under the assumption that it would be better long term for engine maintenance. UL94 recently became available and my home field in Livermore, Ca. (LVK). It costs $1.70/gal more than LL100 here but if it runs cleaner and avoids valve work that is probably worth the cost. Right?

At the end of September I flew for 1.8 hours and on returning, I topped off the tanks with my first purchase of UL94. 13.9 gallons. I didn't fly again for 3 months. ugh On 12/31/24 I flew for 2.2 hours and again toped off with 19 gallons of UL94. That seems like a bit more than usual consumption but I was circling for photos and shooting a bunch of landings.

A couple of days later I went to the hangar and noticed a blue spot on the drip pan under the cowling. I opened the cowl door and looked inside. There was a about a 6"x6" area of blue die on the inside of the cowling right below the bottom outlet of the carb heat airbox. That is way more than I had ever seen before. In fact I rarely see any at all. Coincidence?

The only thing I did at the end of the flight that was out of the ordinary was that as I got close to the hangar on taxi back from fuel, I turned the mags off briefly to make sure the switch turned them off. As the engine was quitting I turned the key back on and then on arriving at the hangar perhaps 100' away I starved it to shut down which is what I usually do.

I removed the cowling and looked for any sign of leaking around the carburetor of fuel lines. There was no sign of any leaks except inside the carb airbox. It had dripped directly out of the air box onto the cowling. I took lots of pictures and consulted my mechanic. He suggested cleaning everything up and seeing if it leaked anymore.

It did not leak overnight fully lean with the selector valve off. I turned the selector valve on both tanks and waited another day. Still nothing. I pushed the plane out and ran the engine while my mechanic watched. Idle, then 1000 rpm for 10 minutes to warm it up, then up to 1700 for a mag check and ran it another 5 minutes at that level. Then back to idle and starved it to shut down. No sign of further leaks.

My mechanic speculated that perhaps the float stuck for just a minute but had cleared itself. I did not want to fly outside the pattern to test that.

A couple of days later I put the cowling back on and flew 0.3 hours staying in the pattern so I could test at full power. The next day there were a couple of small spots of blue die at the very back of the lower cowling, behind, but not directly under the airbox. Maybe there was some residue hiding from my previous cleanup? There was nothing inside the airbox.

Reading the other bulletin about UL94 in the Hangar, I'm wondering if the tiny cadmium particles mentioned could cause a leak or a stuck float.

Has anyone else had a similar experience with or without UL94?

In looking closely at my photographs again, I notice the after cleanup (first attachment) stains appear to originate from between the layers of aluminum where the horizontal and vertical pieces are riveted together at the back of the bottom cowling. Perhaps some fuel avoided the cleanup and reappeared when flying speed air pulled it out of the seam. I hope that's it. I'm planning to contact Swift to see what they say.

LIkely either a stuck carb float valve…or someone stroked the throttle after shutdown, etc. (activating the accel-pump)

I Always close my selector valve when storing the airplane and that’s particularly important inside a hangar. On rare occasion, especially after a long-period-storage,…..turning the selector back to “Both” during pre-flight has demonstrated a fuel lean dripping from the lower cowl due to a stuck float-valve. A sharp “rap” with a wooden chock or fuel-sampler handle stops it….as does simply starting the engine (vibration).

Hope this helps.

I thought I should post an update about the fuel leak I experienced at the first of the year after having talked to multiple A&Ps as well as having an email exchange with Swift.

Bottom line is everyone seems to agree I has a stuck valve causing a temporary leak and I should fly more. What a good idea.

As George suggests, I always turn my selector valve to off after each flight. the plane is stored in a hangar. Swift suggested the fuel leak may have washed built up blue die out of the air box. Doubtful.

I will cut and paste Swifts response below which mentions multiple different carburetors. Mine is a Marvel Schieble and has all the associated ADs complied with...

"I suspect as does Bill that his float needle valve stuck open for a little bit.

A couple of other issues could be which carburetor he has installed on this Cessna 170 as several models applicable have a service mod required to drill a second passage in the carburetor to prevent an unseated needle valve when the aircraft is idle. This bulletin applies to tail wheel aircraft where the nose of the airplane is high when parked. Prior to the bulletin most people just turned the fuel off when the airplane was not in use. This mostly applies to Stromberg carburetors which he might have. The other issue if he has a Marvel Schiebler carburetor is that there was a later update that replaced the rubber tipped needle valve with one made from Deldrin. The rubber tipped one seems to swell a little especially with 100LL and caused the needle valve to stick. Generally speaking, when you go back to a lower aromatic fuel (perhaps UL94?) it returns to a normal size but often times will then leak. This same applies to wing sump drains. In Bill’s case I suspect that the leaking needle valve just washed the former fuel stains of 100LL (Blue) from his air box and that is mostly what he was seeing. Brian"

Given all the issues circulating around, I'm going back to 100LL in the short term.

What color is 94UL? Purple is the answer I just googled. I'd be curious what a purple 94UL stain looks like. Those stains look blue even though the fuel was mixed 100LL and 94UL. It's a puzzle.

Not a 170 but my Vagabond with an A-65 with Stromberg carb, has on occasion of inactivity of more than a month, had the needle stick. Theory is as shut of the fuel selector, that some fuel evaporates leaving the fuel bowl empty and the needle sticks open. We find this as we turn the fuel on at preflight. What happens we believe is fuel bowl fills but is not cut off by the needle and fuel spills over and out the carb venturi. We cut off the fuel valve till we are ready to start then put it back on and prop the engine to start it. We let it run a minute then shut it down to confirm the carb is no longer leaking fuel. We restart and go flying with now more trouble until the plane sits.

I have experienced a stuck float-valve after the airplane has sat a long term (month or more) …when the fuel selector valve is turned back “on” for preflight.
A sharp “rap” with a chock…OR…starting the engine…. in any case, a vibration of any sort will “free” it back into normal behavior.

(Another good reason to have a quality fire extinguisher nearby anytime an airplane is being started.)

Bruce Fenstermacher wrote: ↑Thu Mar 06, 2025 8:15 pm What color is 94UL? Purple is the answer I just googled. I'd be curious what a purple 94UL stain looks like. Those stains look blue even though the fuel was mixed 100LL and 94UL. It's a puzzle.

The picture of the inside of the lower cowl that interstellardust posted is definitely a 100LL stain.

That's a lovely color purple Miles.

Looks like Miles is using too much MMO.

GAHorn wrote: ↑Thu Mar 13, 2025 11:48 pm Looks like Miles is using too much MMO.

Absolutely NONE in the past 30 years.

I also googled what color is UL94 and got purple. Interesting since the placard that came with the Swift fuel STC says clear. Also attached are photos of the fuel taken during my last pre-flight inspection. It looks absolutely clear to me.

Given some of the other complaints I have read about flight schools ditching unleaded due to increased valve recession combined with the extra $1.70/g it costs, I have returned to using 100LL for now.

It also concerns me that while both Swift and GAMI fuels are tested for compatibility with 100LL, they are not tested for compatibility with each other.

The color change is fairly recent, sometime in the past year. I’ve been running 94UL ever since our airport fuel club started selling it (5-6 years??) and have had absolutely no ill effects. The lowest COLD compression on my 600+ hour O-300A on any cylinder over that time has been 77/80, borescope checks all good, and oil consumption 10-12 hours per quart.