The More/Less Air Test
The common Internet-recommended procedure of spraying stuff at the intake manifolds is not a very good test for vacuum leaks and not one pro mechanics use. The better way to distinguish between mechanical, ignition and fuel problems in the context of engine performance has long been known among industry professionals. It’s called the "less/more air" test. To do the test simply duct tape up half of the air filter. Many on the Internet advocate instead applying the choke partway, believing that is equal. But that is not correct. Using the choke presents problems which cloud the results. Note the performance with the duct tape, and then for good measure reverse the procedure, that is, remove the tape and add some extra air by propping open the airbox cover or similar. Don't just take the air filter out or the cover completely off—on most bikes this is too much change and will completely negate the test. Again, note the performance. If the performance symptom improves with either step, you can be about 60 percent sure the problem is on the intake (both air and fuel) side of the equation. Only 60 percent because ignition overlaps intake--they are interdependant. The next step then is to eliminate this ambiguity by confirming the ignition is good, leaving just intake. On the other hand, if the less/more air test results in no change, then the problem is neither intake or ignition, but likely mechanical or something else. But again, the result is 60 percent, so while keeping in mind mechanical, the lingering doubt about intake and ignition must still be put to rest by troubleshooting each. This is the professional troubleshooting method. And it works not only for solving performance issues, but also in rejetting carburetors to suit significant modifications.

Here are some real-life examples. A customer complained that after reinstalling his carburetors his engine would not rev out. On his model the air filter was one of the flat-shaped kind and not easy to duct tape so at my suggestion he wadded up a t-shirt and put that into the airbox instead. The bike then revved up normally. It was a simple matter to then discover he had left the intake manifold clamps loose and after correcting that, all was well. It was an intake issue.

On another one of my customers' bikes the engine ran very poorly. After duct-taping the air filter, throttle response was much better, again pointing to an intake issue. But it could still have been ignition. So to separate the two--intake and ignition--we made sure the carbs were clean, there was good fuel flow, proper air into the air filters, and good sealing at the manifolds. We then 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 porcelain. An ignition-caused misfire in other words. The partly drained voltage at the plug weakened the spark. The more/less air test we performed added mixture quantity, making up for the weakend combustion. Overlap, remember. Replacing the bad plug solved the issue.

More about this overlap of intake and ignition. Why aren't high performance ignition coils a big deal any more on bikes, as they were so many years ago? Is it because modern-day stock ignition coils are so much more powerful than before? No. In fact, they are weaker than they ever have been. High performance ignition coils have disappeared on newer machines, and have fallen into disfavor and become unnecessary on modern bikes--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. You think those tiny coil-over-plug ignition coils are stronger than the traditional large, high-tension ignition coils? They’re not. They far weaker. The point is, this fact serves to illustrate the interplay, the synergy, between intake and ignition. Virtually all of the symptoms of one also apply to the other, because they depend on each other. When ignition is adequate and does its job well, fewer demands are made on carburetion. When carburetion works properly, fewer demands are made on ignition. Accept this principle and troubleshooting becomes much more intelligent.

In both of these real-life scenarios it was important for us to not stop at the 60 percent intake, otherwise we would have gone off on a carburetor tangent, when in only one of the examples was intake the actual issue. And not only are these examples educational, but the more/less air test really works and professionals use it.

An important tip. No consumer-level test exists that will with 100 percent accuracy gauge an ignition system's contribution to engine performance. Not even Honda's oscillating dynamic coil tester, a great tool, or even a dedicated automotive ignition oscilloscope, both of which I have some considerable experience with. Because one perverse and not very scientific fact about ignition coils is that they can test good a half-dozen different ways (literally) and still not work their best, still negatively affect engine function. So unreliable and unrealistic are the factory's ignition coil resistance tests that one can chase their tail for months thinking their problem is carburetion, only to eventually stumble on an epiphany, if they ever do, after replacing the five-times-tested-good ignition coils on a whim and out of frustration. I was reminded of this a few years ago on two unrelated projects (one a 500T, the other a SOHC 750). Astonishingly, the bikes' "carb" troubles vanished. This happens every day somewhere in the vintage powersports world. Even seemingly perfectly functioning and conventionally-tested fifty-plus-year-old ignition coils when replaced have eliminated engine problems many times.