Shop Talk | K.I.S.S. Theory

Tech Tip


Sometime in the 14th century, a Franciscan Friar named William of Ockham (Occam) formulated a principle so elegant that it remains to this day one of the foundations of scientific thought. The principle that old Willy came up is known as Occam’s Razor (or knife) and what it says is this: “Entities must not be multiplied beyond necessity.”

Boiled down, what that means is that all things being equal, the simplest explanation that solves a given problem is the best one, at least until something better comes along. The last part is emphasized because there are times when the simplest explanation isn’t necessarily the best, or fullest, but that’s a fine philosophical point.

Here’s how it applies to motorcycles. As a general rule, whenever you encounter a problem, check the easy stuff first. For example, if the bike cranks but won’t start, check the fuel before you pull out the compression tester. If you turn the key and the warning lights come on but the bike won’t crank, check the kill switch and make sure the bike isn’t in gear with the kickstand down before you replace the starter motor. If you turn on the key and everything‘s dim, check the battery voltage. If nothing at all comes on, suspect the fuse. No idle? Make sure the float bowl’s clean, and the injectors clean. Trust me on this, I’ve seen a lot more dirty float bowls and plugged injectors than I have warped carburetors or bad EFI computers.

I’m sure you can see where I’m going with this. I’ve worked as a mechanic of one sort or another for over forty years now, and in the vast majority of cases the solution to the problem, no matter how complicated, always turned out to be something simple, and by extension, the thought process that got me there always started with the simplest explanation that fit my facts.

I’m not nearly as articulate as Occam, so I’ve modified his phrase slightly; I call it the KISS theory, which stands for Keep It Simple Stupid. For me that works just fine.

What constitutes old fuel?

The rule of thumb is that properly stored gasoline has a shelf life of approximately 90 days before serious degradation sets in, but as always, there’s a catch or two. First, you never know old the gas you just purchased is. It may have left the refinery yesterday, or it could have been in storage for a month before you pumped it into your tank. Second, what constitutes “proper storage?” While there are several good answers, a half full motorcycle tank, parked in a non-temperature-controlled environment like your typical garage, isn’t one of them. The bottom line here is that, if you use your motorcycle on an infrequent basis, or leave it parked over the cold dark winter, a can of stabilizer is cheap insurance.


Engines need three things to run: Compression, spark and fuel. However if we want them to run properly, we have to attend to certain details, not the least of which is having the fuel delivered in the proper proportion, and therein lies the major problem for bikes that have been left to stew in their own juices.

Very often bikes that sit with stale fuel in them for any length of time will start, but refuse to run properly. If the bike is very difficult to start, won’t idle, or doesn’t respond to the throttle well, or maybe not at all, you can reasonably assume that the carburetor jets or injector orifices are at least partially blocked and require cleaning.

Normally, I’d tell you that if that’s the case, you’re going to have to bite the bullet and physically clean the parts in question, and quite frankly that’s probably going to be the most practical solution, especially if the bike uses carburetors, or has sat unprotected for an extended period of time.

However, as the quality of fuel has deteriorated, some very good fuel system detergents have come onto the market, so my first inclination, particularly if the problem isn’t severe, would be to try running a few ounces of a commonly available cleaner through the tank to see what happens. My guess would be that you have at least a 50/50 shot at curing the problem without resorting to pulling the fuel system apart and if I’m wrong, you’re no worse off.

Of course, if the bike refuses to start at all or won’t run once the choke is turned off, it’s unlikely that anything you pour into the tank is going to be of much help, but who knows? Maybe you’ll dodge the bullet.

Of course, the best way to dodge the fuel issue altogether is to use a conditioner, or drain the system before storage.