Octane question

[maowwg]

Just wondering

I store my saws with premixed fuel.

When I do a large job or even a medium size job, I mix a gallon of gas, typically 87 octane as I’ve read that there are fewer unhelpful additives in the lower octane fuel. When done I drain and run some premix as my last half tank.

Typically the saw starts with no issue. But I’ve seen a pattern where this and two other saws run out of gas with the first tank of 87 octane mix and then won’t start. A small adjustment to richen the low screw typically fixes this because the carburetor low end setting is lean when the saw is hot but even if it cools, it won’t start at that lower setting.

Is it as simple as a change in the fuel mix / octane is enough to make the difference between a good tune with snappy acceleration and not starting at all? We’re talking a quarter turn adjustment. Seems like it should run at 1 7/8 turns out vs not start and at 2 1/8 it shouldn’t be night and day better. Also the saw will restart when you shut it off and quickly restart at the leaner setting. It will start cold with a little bit of 3/4 to half choke at the leaner setting Bogs a bit until it’s run a minute then runs normally - trigger response is snappy.

The high setting seems to not be bothered at all.

Is this as simple as Octane or is there something else about hammering a saw cutting for a full tank straight and really heating it up that causes the adjustment to change or the engine to require more fuel to start even when it cools only for a chain sharping, fuel and B/C oil top off?

The whole thing doesn’t matter much for cutting firewood as once adjusted it’s a good light soft handled cutter for a modified Poulan 2550 and a quarter turn is easy to do. Just a question of curiosity

 

[Franny K]

Octane effects how much compression can be in the set up and if the flame kernel at the spark plug progresses evenly in all possible directions. Higher compression and more hot spots need higher octane or an even flame spread is not totally achieved. Most chainsaws seem to call for no more than 10% ethanol and 89 octane in the USA manual where octane is the average (R+M)/2. I generally try and get no ethanol which seems to be 89 to 91 but otherwise use 87 that is on site can't say I notice a difference. Kind of doubtful octane will effect what you describe.

I like the saw to start at idle setting but doing choke and then throttle, then release before pulling sets high idle. I do that not turn screws.

 

[Jasent]

One thing to remember. 10% ethanol fuel is only 90% fuel by volume. Non ethanol fuel has more fuel per ounce

 

[maowwg]

“I like the saw to start at idle setting but doing choke and then throttle, then release before pulling sets high idle. I do that not turn screws”

Not sure I understand. I set the choke. Pull a couple times expecting it to pop on the first or second pull. Then choke in after 3 to avoid flooding and pull with the choke in for three or fewer pulls. It’s running by then or I’m not happy with the settings. Then I punch trigger to release high idle.
Are you saying one should start with half or 3/4 choke and low idle?

 

[Stihl99]

Why would you run anything less than 93 octane anyway?
The owners' manuals on two cycle equipment has been written for years minimum 89 octane, but then you have the complainers about their equipment when they run the cheap ****** 87 octane gas.

 

[Franny K]

Are you saying one should start with half or 3/4 choke and low idle?

I was discussing already warmed up, shut off and then restarting which post 1 seems about. Setting high idle for restarting.

I don't think my current saws have springs on the carb adjusting needles and would think excessive adjusting will lead to not staying in place.

Why would you run anything less than 93 octane anyway?

Because the engine is not of high enough compression ratio to need it. From post 1 "as I’ve read that there are fewer unhelpful additives in the lower octane fuel" I have heard this as well. Years ago a friend of mine bought a new Harley Sportster and that was his instructions use 89 or mid grade as high test has more unwanted stuff.

No ethanol recreational fuel at a pump often right next to the off road diesel is not over 91 octane rating in my travels.

 

[Stihl99]

I was discussing already warmed up, shut off and then restarting which post 1 seems about. Setting high idle for restarting.

I don't think my current saws have springs on the carb adjusting needles and would think excessive adjusting will lead to not staying in place.





Because the engine is not of high enough compression ratio to need it. From post 1 "as I’ve read that there are fewer unhelpful additives in the lower octane fuel" I have heard this as well. Years ago a friend of mine bought a new Harley Sportster and that was his instructions use 89 or mid grade as high test has more unwanted stuff.

No ethanol recreational fuel at a pump often right next to the off road diesel is not over 91 octane rating in my travels.

I would love to find a station around here that does not sell ethanol fuel, but alas 93 with 10% is our best option here.

 

[John Walsh]

I would love to find a station around here that does not sell ethanol fuel, but alas 93 with 10% is our best option here.

Have you tried the www.puregas.com website? They ask for a zip code to find station(s) near you.

 

[sean donato]

It has nothing to do with the ethanol content, and likely more to do with running Alkylate fuel (the canned fuel) vs regular gas. Also it's a stock saw, even modified saws barley have a compression ratio high enough to warrant anything other then pump high test, and 87 vs 89 isn't going to make much if any of a difference in this case.

 

[bwalker]

What the original posted is talking about is volatility of the fuel. This is always a trade off. Too much volatility and you vapor lock in warm weather. Not enough and the engine is hard to start.
These days the EPA in their infinite wisdom and under the guise of lowering evaporative hydro carbon emmissions places strict controls on volatility which is measured by the Reid Vapor Pressure test. RVP numbers change several times per year based on the season. The RVP number is the same for 87 or 91, E free or E10.
Canned fuels don't have to abide by RVP and are more stable so afterbeing stored for long time they still have the necessary light ends to light the engine off easily.
When the OP switches back 87 its no suprise he has to fool with the carb settings. Less volatility of pump gas would require a richer carb setting on the LS for certain.
This has nothing to do with octane number at all. It's all about volatility.