This is a photo of collected CBX/CB1100F/CX500/GL1100 pilot screw limiter flags, about half of what I actually have. I need to throw most of them out, and keep just a few for customers who ask for them. Very few do. There's no good reason for them.
I wrote previously about the technical aspects as well as the history of the carburetor pilot screw. What I want to explore today is how important, how impactive this lowly part is, in terms of achieving statisfying engine tune.
The pilot screw is part of the idle circuit. The idle circuit is the most important passage in the carburetor. This circuit has the most branches, the most appendages (the air cut valve, the bypass ports, the idle are bleed, and more), is the circuit most responsible for good low speed performance, and of course is the critical circuit in terms of starting and idling. And with the engine having its *least* efficient combustion at an idle, the idle circuit is the part of the carburetor most responsible for fine tuning, and the only conventionally adjustable circuit for that reason. It *has* to be adjustable. Even very slight changes in conditions demand this, which is why emissions regulations never could completely eliminate the pilot screw. 1
You already know that the feds required powersports OEMs to get coy about the pilot screw beginning with the 1978 model year. Keihin, the maker of Honda's carbs, began by putting pressed-on plastic stop-caps on the screws so as to discourage inexpert fiddling. They were easily removed. This in later years morphed into glued-on aluminum limiters and finally, in the carburetor's final stand before fuel injection took over, the screw was buried so deeply in the carb casting no one could find it, and to make sure, covered with a tiny manhole cover, making it completely invisible. Out of sight, out of mind. And a lot of subterfuge.
What many do not realize is that this devilment was for the sake of the end-user, to thwart injudicious and ill-advised adjustment in the name of emissions. It was not however intended to slow down the expert, the professional mechanic who would continue to adjust this screw as he always had. 2 Because there was and is no sanction on either Honda's part or the government's against dealers and other pros doing what was necessary. Honda even proscribed adjustment methodology and made available new replacement screws and new hidey caps and covers. 3
Among carb makers, Keihin led the emissions front with their absolutely astonishing 1 1/2 turn specification for the pilot screw setting on most of their carburetors. So adversely lean was this that in some cases the setting can actually be doubled before carbon monoxide (CO) levels reach federal limits. Go figure.
Why adjust the pilot screw to a richer setting than stock? A few reasons. One, on many carb models a slightly over-rich idle mixture greatly improves throttle response. This is the case with many of Honda's early inline fours, and also the GL1000, whose carbs are actually slightly too large and thus make the engine's intake air speed sluggish. A larger pilot setting acts like a much-needed accelerator pump in the GL1000's case. 4 Another benefit of richened pilot screw settings is realized in 40-50-year old bikes whose engines invariably have lost 20 percent or more of their cylinder compression. A greater pilot adjustment compensates for lost fuel efficiency and enhances these engines' idle smoothness. 5 Finally, even very recent Keihins such as the semi-downdraft VD and VP series, the latter being the final production carb Keihin would produce, function better with one-half turn more pilot screw than they shipped with. In short, make GL1000s 3 turns, GL1100s 2 1/2 turns, and GL1200s, GL1500s and Valkyries 3 turns. As long as the engine is stock and sound, and the carbs properly cleaned, assembled and set up, this will work well. 6
A reminder from a previous article. The air-only pilot screws on Honda's early SOHC fours and the air/fuel screws on the Gold Wings work differently. In addition to having opposite rotational references (air screws = in for richer, air/fuel screws = out for richer), the two screw types are also very differently shaped. The air screw's blunt tip means a tiny adjustment makes a large change, while the air/fuel screw's very sharp tip can be turned much more before effecting very much change. 7
One more thing. There is an important connection between throttle sync and pilot screw adjustment. Pilot screw setting affects sync. Therefore, the pilot screws must be adjusted first, before the throttles are synchronized. 8
1 Though the EPA tried. As for the starting importance, that does not apply to the GL1200, GL1500 and Valkyrie, which have stand-alone starting circuits and do not rely on the idle jet for starting.
2 It is actually an emotional moment when in carb rebuilding I spot a still-intact pilot screw stop flag on a Keihin carb in for rebuild. Think of it. It means that however many owners the bike has had, whatever kind of attention the bike did or didn't receive, in its 40-plus years of existence and use, from the moment it was removed from the shipping crate til now, the machine not once received a professionally-performed maintenance service. "Tune-up", if you prefer that word. It's a tangible thing --I can actually feel the impact of the decades of neglect. Very sad. And it shouldn't have happened.
3 New pilot screws for the GL1100 come with not-yet-installed aluminum flags, and the miniature manhole covers for the GL1200 and GL1500 can be purchased by part number, if you want them. Why you would escapes me.
4 A setting of 3 turns out, together with appropriate tuning, works so well that at that point, even an accelerator pump would not be an advantage.
5 Contrary to popular opinion and typical Internet forum fallacy, more compression makes carburetion work better (effectively richer) and less compression makes carburetion struggle (act leaner).
6 In the rare event that any of these bikes have had professionally performed top end rebuild work and thus enjoy factory-fresh cylinder compression, remove one-half turn from each of these suggestions.
7 An episode I had with my EGA this past week reminded me how very sensitive air-only pilot screws are. On my early Honda four, as you near the optimum setting, one-sixteenth turn of the screw makes a half-percent difference in CO (carbon monoxide). When the target is 3 percent, that is a lot.
8 Pilot screw affects sync, but not the other way around.
