Why Carburetor Bowl Vents Require Attention
Think these are innocuous little vents that don't affect overall fuel enrichment? Will exposing them to non-atmospheric pressure conditions affect fuel delivery? Maybe some of the following will help point out their importance.
Simplistically described, a carburetor provides fuel as a function of its delivery system (passages and jets) that relies heavily on the difference in pressure between fuel bowls and throats. If for some reason bowl pressure is caused to deviate very far from atmospheric pressure, especially if this deviation varies over a range of engine or vehicle speed, net fuel delivery can become influenced by a condition other than delivery passage sizes. You'd actually like for bowl pressure to be constant throughout all conditions of engine rpm and vehicle movement, as near to atmospheric pressure as possible. That's in a perfect world. In reality, making an effort to not create pressure fluctuations at the point of bowl vent entry is a fundamental requirement to allowing stable fuel delivery. Exposing vents to irregular pressure conditions or pulses should be avoided wherever possible, even if you only use the tried-and-true method of joining primary and secondary bowl vents with a section of hose and providing a small vent-hole around the mid-point of the hose.
Is Valve "Float" Really Float?
Maybe this is an issue of semantics. Do valves really experience "float," or does the condition amount to a lack of valve control? Regardless of how the issue may be defined, you can boil it down to two primary periods during valve motion when it becomes problematic. One has been termed valve "pitch," when camshaft followers are briefly separated from the nose of a camshaft lobe, particularly at higher rpm. The other occurs at the instant of valve seating when a lack of motion control is lost to what we'll call "bounce" that can immediately follow the failed initial seating. In either case, barring mechanical contact between a valve head and piston crown, valve-springs face immediate damage.
Years ago, well in advance of when commercially-available valvetrain "spin" machines became available, we built our own at Edelbrock. It was during a rocker arm design and development project and we were doing our best to examine as many dynamics variables as possible. What we quickly learned was that an unstable valvetrain (for whatever reason) could cause spring damage prior to any audible evidence of the problem. And, in fact, when the problem became heard, chances were good that damage had already occurred. We also discovered that the subsequent loss of valve-spring pressure did not transpire in a linear fashion. In other words, when we created and repeated the conditions of valve motion instability (on our spin device), spring pressure would decrease by multiple times what had been lost from the previous such test.
As a result, each period of instability caused the same condition to occur at a lower rpm, during subsequent run-ups of the machine as a further sign of rapid spring pressure decay. Of course, today valvetrain component design and the tools by which they are configured are far more sophisticated. But the problem can still be created by racers and engine builders, virtually independent of how well the camshaft and valvetrain component manufacturers do their jobs.
Small Holes in Header Pipes Near Their Cylinder Head Flange
Years ago, Smokey was doing this little trick, causing broad speculation about his reason for doing so. All sorts of theories abounded, some so complex it was difficult to separate fact from fiction from speculation. One frequently-heard reason was he'd discovered some innovative way to control reversion, if you can believe that . . . and on and on. During one particular late-night engine dyno session in his shop, I asked him point-blank if it was a quick way of seeing if any cylinders were misfiring. His answer? "Yep. But I like being told all the theories because it ain't often I get to hear from geniuses."