There is a troubleshooting technique known in powersports as the "less air/more air" test. Relied on by generations of home-grown mechanics, promoted by Yamaha in its 1980s-era dealer technical training, and popularized in recent years by the customer service dept. at Dynojet, it is an easy way to divide the probable cause of an issue into the areas of fuel, electrical and mechanical. It's a classic test, and very effective if done correctly, which is what this article is all about.
To do the test simply duct tape up half of the air filter. This is the Dynojet version. Yamaha taught applying the choke partway. However, the choke method has problems which cloud the results, so the duct tape method is better. Note the performance with the duct tape, and then for good measure reverse the procedure, that is, remove the tape and add some air by propping open the airbox cover or similar. Don't just take the air filter out -- in many bikes this is too much change. Again, note the performance. If the performance symptom did not improve in either case, you can be about 60 percent sure the cause of the symptom is not an intake issue.
Note I say "intake" not "fuel," so I can communicate that intake is not just fuel, air is also included, i.e. intake manifold vacuum leaks, altered air filters, blocked carburetor air bleeds, etc. I once encouraged a mechanic to do this test and he found loose intake manifold clamps. Without the duct tape the engine would not rev, with it, it would. Tightening the overlooked intake clamps duplicated the result of the tape and solved the symptom. The point is, you must keep in mind air is included, thus we properly say "intake," meaning everything from the gas cap to the exhaust pipe outlet. Everything in between. This is important to remember. A non-venting fuel tank or a carboned up or rusted up muffler are both common issues and both part of the intake system.
So a positive result means a 60 percent surety of an intake issue. But why only 60 percent? The reason is important and it is what I want to emphasize in this article. MOST (60 percent is over half) of the time it will indeed be an intake problem. But, the remaining 40 percent of probability involves electrical, i.e. ignition. That is, 40 percent of the time it will not be intake but ignition. Here's why. The nature of combustion is such that air/fuel mixture and spark are interdependant. That is, each needs the other. The air/fuel mixture works best when the spark is vigorous, and the spark is most effective when the air/fuel mixture is potently metered. They each need each other. But what this really means is that if either is weak, the other has to compensate to maintain good combustion. If spark is weak, richer mixture is needed to make up for it. If mixture is weak, spark has to be stronger to compensate. They are inverse, or opposites.
Ignition and fuel are opposites. This is a well established fact that explains much of how engines are designed and how they operate. The result of this fact can be viewed on an exhaust gas analyzer, and even without such sophisticated tools, it can readily be observed in action. For example, think about this. Why aren't high performance ignition coils a big deal any more on bikes, as they were 30 or more years ago. Is it because modern ignitions are so much better than before? No, not at all! In fact, they are weaker than before. (More on this in a moment.) High performance ignitions have fallen into disfavor, have in fact become unnecessary, in modern engines because of fuel injection. Fuel injection is so much more precise, deliberate and finely controlled that mongo ignition is no longer an advantage. Better fueling has eliminated the need for superlative ignition. Remember, opposites. This also explains how modern bikes can get by with the really low-powered ignitions they have now. You think the coil-over-plug ignition coil is stronger than the traditional high tension ignition coil? Not at all. It is far weaker.
Let's see the 60/40 rule in action. I was helping someone troubleshoot their bike the other day. We used the less air test and found its result dramatically positive. The bike revved much better with the duct tape. So, a carburetion problem, right? No, not necessarily. Remember, a positive result means 60 percent likely an intake issue, 40 percent an ignition issue. So, after making sure the carbs were clean, there was good fuel flow to the carburetors, proper air into the air filters and good sealing at the manifolds, we turned to the ignition system and eventually found that one of the bike's spark plugs was firing, not at its electrode but down inside the plug, partway down the insulator, at a crack in the insulator. A misfire in other words. The partly drained voltage weakened the spark. The less air test we performed added mixture quantity, making up for that. A classic result. More recently, one of my carburetor customers was experiencing a fouling problem. The more air test improved the symptom, lightening the carbon deposit on the spark plug. The customer was encouraged to think outside the box, and this led him to replace his aftermarket ignition points with stock high quality points. The problem disappeared. No more fouling. In each of these scenarios it was important for us to not stop at the 60 percent carburetor but also remember the ignition by applying the 60/40 rule. Otherwise we would have gone off on a carburetor tangent, when there was nothing at all wrong with the carburetors. This is a perfect illustration of the balance, the tension, between ignition and fuel, and the main point of this article. DON'T FORGET THE 60/40 RULE! Just because more fuel gives a positive result does not mean the problem may not be ignition, not fuel. More of one makes up for less of the other. Don't forget the 60/40 rule. I try to teach my customers this when we discuss diagnostics.
Just as importantly, this real-world troubleshooing episode also serves as an example of how troubleshooting really goes on inside your head, not just in your hands. It is more important what you make of the results of a test, than that you do the test in the first place, as important as that is. Remember the 60/40 rule.