The 60/40 Rule
There is a troubleshooting technique known in the industry as the "less air/more air" test. Relied on by generations of home-grown mechanics 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. 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.
But why only 60 percent? The reason is important and it is what I want to emphasize in this article. Even with the high probability of a fuel system issue when the less air test shows positive, 40 percent of the time it's electrical, i.e. ignition. The reason is simply that air/fuel mixture and spark are interdependant. 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. 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.
Let's see the 60/40 rule in action. I was helping someone troubleshoot their bike. 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 the 60/40 rule. After making sure the carbs were clean, there was good fuel flow to them, the air filters sealed well and were unobstructed, and there was a seal at the manifolds, we turned to the ignition system and 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 the poor spark. This is a classic example of the interdependedness of fuel and ignition, and had we not applied the 60/40 rule we would have gone off on a carburetor tangent, when there was nothing at all wrong with the carburetors. Remember, just because more fuel gives a positive result does not mean the problem has to be fuel! More of one makes up for less of the other. Don't forget the 60/40 rule.