Using AVgas in your small engines

subcamper wrote:

Kemahsabe wrote:
The owner of a local outboard motor repair shop told a group of us that we'll have fewer fuel-based problems in small engines using 100LL. A member of the group has had a lot of small engine problems at his remote ranch in west Texas. So he went to the local airport and bought 5 gallons of av gas.

None of the engines he tried ran right, or would run at all, on that gas. He had to drain it out and replace it with plain old auto gas. Everything started and ran fine. No idea why that would happen. Plan to ask the outboard motor expert next time I see him.

Despite rumors to the contrary, high-octane fuel is not "more powerful". The higher the octane the HARDER it is for the fuel to ignite. High-octane fuel is needed in higher-compression engines to prevent pre-ignition ("ping"). In other words, the gas ignites too soon, before TDC which causes the piston to push down while it is still going up toward TDC (bad situation). Also, engines using high-octane fuel can advance the timing to get more power (and sometimes better fuel economy).

Small engines will often not run on high-octane fuel because there is not enough compression to ignite the fuel. A friend bought a new Sears lawnmower a few years ago and it would not start. He called me over and I checked everything (spark, fuel going to carb, etc) but it stoill would not start. Finally, he said he got some premium gas and put it in when he bought it. We dumped the premium and put in regular and it started and ran fine.

Steve