Summer vs Winter gasoline

It was almost 50 degrees today and I wanted to go for a motorcycle ride. I had a tough time getting the bike started. This bike ran perfectly fine last Fall and after it warmed up a bit it was fine. I suspected that it had to do with the gasoline in the tank. I topped it off last Fall and added Stabil to it. That started me on this quest to find out a little bit more about gasoline.

Much of gasoline blending has to do with the amount of VOC's emitted (volatile organic compounds). It is a balance of trying to produce as little smog as possible while maintaining driveability.

Summer gasoline is refined/blended to make them less volatile. It doesn't boil off as easily. In the old days before seasonal blending and fuel injection, cars experienced vapor lock on hot days. Motorcycles and small engines without fuel injection can still experience that problem.

Gasoline for Winter is refined/blended to make them more volatile to allow for easier starts at lower temperatures.

How they accomplish this is by varying the amount of butane in the fuel. There is a scale RVP (Reid Vapor Pressure) that is used to determine how much butane a sample contains. The refiners either add more butane or debutane to remove butane from the fuel for the desired pressure.

RVP is measured by putting a sample of fuel in a closed container and then warming it to 100 degrees F and then measuring the pressure in the container. Summer gasoline with little butane may have a pressure of something like 7 or 8 PSI. Winter gasoline with more butane might have an RVP of 15 psi. Straight butane has a Reid number of around 50 psi at 100 degrees.

Most fuel injection operates at about 30 psi eliminating the problem of vapor lock.

I also learned that most of the refineries in Houston use common pipelines to pump their product. In my case, to a tank farm in central Ohio. They take an amount of fuel from those tanks equal to what they pump in at Houston and a tank truck will move it the last few miles to their filling stations. The notion that one brand is better than the other at least for the basic product before they add anything else is pretty much a myth.

Once I run this tank of gas out of my bike and replace it with some fuel blended for this month the bike will probably be fine. I was just happy to be behind motorcycle handlebars rather than the handlebars of my snowblower.

 

The Stabil instructions said to add 2 oz to 5 gallons of gas which is how I applied it. At the end of the summer season I top off all my gas gadgets (tractor, snowblower, power washer, generator, tiller, weed wacker and motorcycles) from that 5 gallon can. None have ever failed to start in the Spring.

The bike was a Honda Shadow. I pulled out the choke and pressed the starter button and it started immediately. I gradually began reducing the choke/fast idle but within seconds the front cylinder quit firing. I pulled a front plug which was wet with gas. I blew the plug off with compressed air and heated it with a Bernz-O-Matic and then replaced it. When I restarted the engine, it ran on both cylinders. I have never had a problem with this bike in warmer weather. I suspect that if I put seasonally correct (more volatile) gasoline in the bike it may have been fine.

 

This is a single carb motorcycle and the rear cylinder fired right up and ran just fine. The front cylinder ran fine after I removed the front plug, dried it with compressed air, heated it with a bernzomatic and then replaced it. I am pretty sure it is just related to the low temperature and the fuel not vaporizing like it normally would in warmer weather.