Lawn Mower Residual Gas Question

[Rich256]

Shouldn't be trying to grow grass in Vegas anyhow :-)

[Joseph Meehan]

The stuff that turns to gunk IS normal gas. :-)

[Chris Lewis]

According to Joseph Meehan :

Indeed.

Gasoline isn't a "pure" chemical. It's a mixture of hydrocarbons of various weights. Traces as light as propane or even methane, with traces as heavy as asphalt. Refining is nothing more than forcing the average to be roughly equivalent to somewhere around C7H16 or C8H18, reducing the "tails" of the distribution to a "reasonable" level and specified octane level, and having enough "lights" to give you enough vapor pressure.

Evaporation acts almost as "fractional distillation". The light parts evaporate preferentially, leaving the heavy/sticky parts behind.

Which means that "gumming up your carb" is not necessarily (or even largely) a chemical process, but is in fact a physical one.

With small engines (especially weed wackers and the like), running the motor dry and then putting it away is the worst possible thing you can do. The residue gas solidifies in place. If it's 2-stroke mixed gas, it's worse because the mixed oil is going to congeal and evaporate into a sticky obstructive mess.

Think of gasoline like corn or maple syrup. Sticky to begin with, it gets vastly worse if the water is allowed to evaporate.

[Joseph Meehan]

Gasoline not only suffers from evaporation as you well explained, but it also suffers from chemical reactions that can occur without any evaporation. The various hydrocarbons can and do tend to react among themselves, generally forming heaver - longer chains. This is the part of the process that stabilizers can help slow down. This part of the process generally takes a fairly long time.

[Lawrence Wasserman]

Your new Toro mower probably came with a new owner's manual that will recommend what to do.

[CanadianCowboy]

I let my mower run dry and it has never been the same. Seems like all the junk at the bottom of the tank clogged up the carberator inlet.

[Chris Lewis]

According to Joseph Meehan :

True enough, but with 2-cycle gas, it implies that even a stabilizer won't help (much) if you encourage evaporation (ie: running the engine dry). Stabilizer or not, the majority of the gasoline residue will evaporate, leaving the heavy ends and 2-stroke oil behind. Gasoline stabilizer cannot prevent that from turning into sticky/obstructive goo.

Particularly nasty in engines with small fuel lines and small fuel passages in their carbs (ie: weed wackers).

[Mark]

but the theory is that if you let it run dry, there is only a small amount of gas left in the carb, yes that will evaporate and leave a small amount of residue...

the alternative is leave the tank full and you then have an unlimited amount of gas to evaporate leaving a much larger amount of residue...

thats why I add Stabil then let it run dry. whatever small amount of gas is left still prbobaly evaporates but if there should be a large pool left somplace, the Stabil will slow down the degredation.

If the object of the game is to have the least amount of residue left, my bet is to have the least amount of gas avaialbe (i.e let it run drey) and have the gas that is left, stabilized. i.e. put stabil in before I let it run dry.

Mark

[Joseph Meehan]

In general I agree. In part, as you infer, it does depend on the specific engine. I have never had a problem, but others may. In most cases if you do either, you should be OK, over winter is not all that long. However on some engines, it could be a problem and that problem is more likely related to evaporation than the chemical changes staibilization helps

[Rich256]

I think putting in the Stabil is a waste of time. If you drain the tank and then run the engine until it dies there isn't enough left to leave any residue. There is no place to leave a "pool" at that point.

As others have pointed out in cool climates I have found no problem. When I lived in a warm climate I found reasons to start the mower occasionally so it was still not a problem. In cold climates the snow blower is the most likely to gum up if you don't drain the tank. I have been doing that for 30 years and that same engine still starts on the first pull each winter. I also use Mobil 1 5W30 oil in it. Never have drained or added to my lawn mower and it is on it's 25th year. Still starts on the first pull. But it is stored in a rather cold area that rarely gets much above freezing. The blower on the other hand experiences 90+ for months.

I guess the bottom line is to do whatever you feel safe with.

[Chris Lewis]

According to Mark :

Mixed 2-stroke gas is fairly viscous, and there'll be considerable residual gas, of which anywhere up to 1/16th of it is goo-forming oil.

Then, with small bore lines and carbs (ie: weed wackers), you could have gunkification within a few weeks. Especially on things like in-tank fuel filters where the elements will be saturated with gas no matter what you do in turns of running dry. Come spring, the elements are plugged.

It takes a very long time even for a (sealed) weed wacker tank to appreciably evaporate. Meanwhile, the volatiles are keeping the fuel from sludging up (and stabil would prolong that).

Stabil will not slow down evaporation. Once it's evaporated leaving two-stroke oil behind, stabil won't prevent it from becoming sticky muck.

On the last weedwacking day of the season, I simply stop my stihl and hang it on the wall. Full of gas. Come spring, it starts in 2 pulls. Just like it does in the summer when used every week.