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Small Engine Troubles II: Fuel

As a sequel to “Small Engine Troubles: Help!”, this post is all about fuel.

Today’s pump gasoline contains 10% ethanol.  As such it offers a mix of benefits and drawbacks in automotive use, and nothing but trouble as fuel for small engines.  The main issue becomes shelf life, as gasoline-ethanol blends turn stale more quickly than pure gasoline does.

In most small engines, both the fuel tank and carburetor are vented.  This means that fuel left in them can continuously evaporate, oxidize and absorb moisture.  Evaporation is a problem because it’s the lighter, more volatile molecules that leave first.  What remains are the heavier compounds which will still burn, but hard starting results.  Then oxidation, the true fiend takes its toll.  Fuel reacts over time with oxygen to create new compounds that I will just call gum.  At this point we have a real fuel and engine problem.  Third, the ethanol in modern gasoline is hydrophilic, as it will draw moisture out of the air.  Initially this can be a good thing, since keeping water in suspension allows it to be harmlessly burned away as fuel is consumed.  Fuel line icing in cars is almost unheard of these days because of this.  In a stored small engine however this water keeps accumulating, eventually just adding more contamination to the gum already forming.  A carburetor has small passageways for fuel that slowly become clogged with this gum.  When fuel no longer flows, the only solution is to remove the carburetor and thoroughly clean it.

So what’s the bottom line?  We need to keep fuel as fresh as possible and not store it for long periods in a small engine.  There are some things we can do to make that happen.

I’m sold on using gasoline stabilizers.  Before taking my empty gas cans to the station, I put a dose of stabilizer in each one.  These products help keep fuel fresh longer.  They do not restore bad fuel though, so you must mix them with fresh gas when you fill up for them to work.  Another thing we can do is use gas cans that seal tightly.  This minimizes oxidation and condensation issues.  Lastly, perform a maintenance session on equipment at the end of their usage season.  Snow blowers for example should be put away in the spring.  Drain all the fuel out of both the tank and the carburetor.  Change the oil.  Adjust the scraper, belts and skids, etc.  Next fall when it snows just add fresh fuel and it will start right up.  Yard equipment usually needs more than just annual maintenance, but make sure one session happens in late fall so all the fuel is out over the winter.

This may sound like a lot of work, but not doing it will eventually require a carburetor rebuild.

Here’s a link to a post I made on how to clean a chainsaw carburetor:

Homelite Chainsaw Carburetor Cleaning

The process works pretty much the same for any small engine regardless of carb design.




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Posted 2014-05-16T14:43:03+0000  by Chris_HD_CHI Chris_HD_CHI

We had an running issues with a new mower we had bought from a local dealer who blamed the problem on fuel.  While I'm not entirely convinced fuel was the sole cause the problems he did offer a few suggestions.  These would also apply to any gasoline powered piece of equipment
  • As you mentioned, use a fuel stablizer
  • Buy only enough fuel that you will use in a month.  For some people that may only be a gallon.
  • If you have any fuel left at the end of the month, dump in your car and buy fresh.
  • As you you also mentioned, it's critical that any remaining fuel be emptied from the equipment at the end of the season.  Nothing will cause more carburetor problems than leaving gas in a mower over the winter. 

Part of the problem is the higher government mandated emissions requirements on new lawn equipment.   It's a lot easier to get a small engine to run with a slightly richer fuel/air mixture than it is a lean mixture.
Posted 2014-05-17T11:51:11+0000  by Adam444

Hey Adam444.  Thanks for the response.

Engine running issues with brand new equipment can happen, but if bad fuel is the issue, then flushing out and using fresh fuel should fix it.  If this is a poorly running engine due to emissions regulations, then the manufacturer did something wrong that needs correction.  The choke or priming system should allow for easy starts, and even a lean running new engine should perform well.  Unfortunately the fix is unlikely to be easy, as small engine carburetors are not adjustable for fuel/air mixtures as they once were.

My old air-cooled motorcycle was factory set to run on the lean side of normal for emissions reasons at a time when gasoline did not have ethanol added.  When it was new it ran great.  Over time as ethanol blended fuel became established, I had to seek out stations that still sold pure gasoline.  The ethanol blends leaned out an already lean running engine to the point where the bike was stalling, overheating and approaching unrideable.  Eventually there were no stations selling pure gas, and I had to remove and re-jet the 4 carburetors in order to restore low to mid throttle performance.  This issue was not due to the manufacturer selling poorly running engines, but to the change in fuel chemistry over time.  Modern fuel injected engines can compensate automatically for these changes, but small engines still have very simple carburetors.  To be honest I am not sure how your mower engine performance will be remedied.  Quite simply, a small engine sold today should be set up to properly run on a 10% ethanol/90% gasoline blend.  If the government changes fuel requirements to let’s say 15% ethanol, then the issue becomes the same as I had with my motorcycle.

As far as storing gasoline, if you use a fuel stabilizer then one month is very conservative.  While I may well go through my gas can fuel each month in the summer, some equipment sits for longer than that during the season.  Examples include a chipper/shredder that I use in the spring and fall.  Rarely would I use this machine in the summer.  I put a fuel petcock in the gas line.  When I am done using the shredder, I close the petcock and run the gas out of the carburetor.  Fuel stays in the tank until the next time I use it.  This machine is 18 years old and I have not had a carburetor problem with it yet.  Without fuel stabilizer I believe this machine would have clogged up many times over the years.  I think the one month rule applies to non-stabilized fuel, and that should be a reasonable time frame.  Disposing of older fuel that is still good by pouring it into a car tank is a good idea.  You can even do this with the small amounts of 2-cycle fuel left over as long as the vehicle tank is already more than half full and can dilute the oil/gas mixture.

Nothing will cause more carburetor problems than leaving gas in a mower over the winter.

Absolutely, and that really is the issue.  



Posted 2014-05-20T16:39:00+0000  by Chris_HD_CHI
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