Had to try AV100LL and see for myself…..

[octofoil]

I’ve previously posted about the issues I was having with poor idle performance for trolling. My ’77 55hp Johnson ran super smooth at 1000+ rpm, but had the hippy hippy shakes increasingly worse as I slowed below 1000 rpm.

I was running 89 octane non-ethanol mogas mixed 50:1 with std evinrude 2-cycle oil along with a few cap fulls of seafoam mixed in a 6-gallan tank. Shame on me, but I had about a half tank left over from last year that I purchased on a lake a few hundred miles from my home that was non-ethanol so I used it up this spring with great results.

I cannot buy non-ethanol fuel in my area so I thought I would give avgas a try. There are so many mixed reviews as to its value, but most who used it said it was the same or better performance. So……, I filled my empty tank with it and mixed in some penzoil 2-cycle full synthetic at 45:1.

After cleaning out the fuel lines, and without making any adjustments to the carburetor or jetting, no changes to the spark plug temp, and no changes to the timing, I proceeded to the lake without even trying it in the driveway with the earmuffs. I was hoping this magic elixir was going to be the end to all my worries…., non-ethanol and a slower burn rate should help low rpm, with only minor deterioration in high rpm performance….

Starting – I choked it and it popped off after a few revolutions and then died. Then no choke it fired and ran after a couple more revolutions. Not much difference except that it would have kept running the first try.

Idling – In neutral it seemed to run the same as before. It ran at 800 rpm in gear, but I wasn’t sure if the propulsion was due to the spinning propeller or if the skeg was acting as a tail fin because the motor was shaking so badly.

Throttle response – Previously there was no delay. With the avgas, I could easily stall it if I opened the throttle too fast.

WOT – My peak rpm used to be 5800 rpm, with the avgas, 5200 rpm and it didn’t run nowhere near as smooth as it used to. In fact, it really wasn’t wot, because that would bog down the motor so I had to back it off just a little to get it to run.

I hate the thought of running ethanol in this motor but this avgas experiment was a complete failure. I am sure I could tune it to optimize idle or WOT, but I would sacrifice way too much of one for the other to make it worth it.

I’d be glad to hear some feedback of other experiences or suggestions for both ethanol &/or avgas.

 

[Faztbullet]

Av100 has a lower ppecific gravity than pump gas (density) means you actually run leaner with the same jetting, Thats why it ran like crap as idle mixture and main jetting needs increased in size. Some of these avation fuel are oxygenated which makes em run lean. Since its slowing burning the timing will need to be raised... Below is from old post..


Sunoco

AV Gas has a MOR (motor octane rating) of 96, R+M/2 rating of 100, and ROM (Research Octane Rating) of 106.

AV Gas is lighter than racing gasoline thus more fuel/larger jetting is required. Jetted correctly you should not experience a lean burn at WOT.

Shelf life is NOT better. The reason pump gas won't last as long is because street gas has extremely lightend hydrocarbons to help your car start and idle. Racing Gasoline does not have these light end hydrocarbons needed for idle and starting, hence the reason race motors start and idle poorly.

Av Gas is NOT designed for low RPM motors. AV Gas is designed to not detonate/preignite causing detination. This would be the same design as race fuel. If you compare the "distillation curve" of AV Gas to Race Gas, you will find they are almost identical. The "distillation curve" controls the speed of burn across the combustion chamber.

The LEAD (TEL) added to AV Gas is to increase the octane rating only. All heads these days have harden valve seats. There is no need for lubrication of the valve seats. All engines have come with harden seats since the late 60's.

AV Gas is not formulated for High Altitude. and will have very little, if not any performance differences vs racing gasoline. On the other hand, commercial grade fuels (87, 89, 92) will definitely enhance your performance due to the commercial fuel being oxygenated. The Oxygen enhancers added to commercial fuel is only for California Smog laws.

Advancing timing on your motor will definitely help with AV Gas and Race Gas due to its slow burn characteristics. On the other hand, be careful if your running commercial grade gasoline, more timing can cause detonation/preignition quit quickly.

AV Gas does not go BAD faster. It is extremely consistent. The MOR is only 96, whereas Sunoco Purple or VP C12 is 104. A rating of 96 is good for up to 10:1 on Steel heads and 12:1 on Aluminum heads with water cooling. Air cooled motors run much hotter.

Remember this...OCTANE is a measure of a fuels ability to resist detonation/preignition. The higher the Octane number, the slower the fuel burns. Technically speaking 87 Octane fuel will develop more power than 118 Octane fuel. With this said, you should see gains in throttle response and HP by mixing commercial fuel and AV Gas/Race Gas. You now have some light end Hydrocarbons for throttle response and heavy hydrocarbons/TEL for detonation resistance.

Bottom line... use the most consistent fuel you can find and create horsepower by moving as much air as possible though the combustion chamber.

 

[Outsider]

Your problem isn't your fuel, your idle should be down around 5-600. Trouble shoot the real problem ...

 

[GA_Boater]

AV Gas is not formulated for High Altitude.

That's an interesting statement in a write up about aviation fuel. Curious why Sunoco would say that.

 

[octofoil]

Your problem isn't your fuel, your idle should be down around 5-600. Trouble shoot the real problem ...

Judging by the way it has ran in the past, I am of the thought it is the idle circuit on the carburetor itself. Whoever rebuilt it before I got it damaged the "straw" that goes to the idle jet. I don't want to mess with that until I can find replacement carbs for backups as trying to pull a brass tube from an alum casting can get tricky.

The only reason I tried Avgas was because there is no place local that sells non-ethanol. But...., here is a little update: I ran about a 1/2 tank when I used the Avgas mix. So, on a recent trip I filled the remainder at the pump with what was supposed to be non-ethanol 93 octane and instead of using synthetic, I used regular 2-cycle oil as that was the only thing I could find. It ran HORRID. It was very tough to start, wouldn't idle below 900rpm, and I had to really be careful not to kill the motor shifting it into gear. Then, I could never get to WOT or it would spit and sputter and die. Max rpm was about 4800 rpm and even then it ran terrible.

Its not timing, its not spark, its not compression, and the only thing that made a difference since I've had it was when I rebuilt the carbs and found 89 octane non-ethanol and used OMC 2-cycle oil. Still had poor idle, but anything over 1000 rpm was super smooth.

I'm about ready to give up. I missed a couple of carbs on ebay. Wishing I knew what else would bolt up as I can build my own linkages and fab anything else I need (except a new carb body).

It seems to me that fuel is the issue, or at least a major symptom that I cannot find a root cause to address.