I've told this story before, somewhere on this site.
I have an exhaust gas analyzer, and a CB550 with 4 into 4 pipes. At the end of a tune-up service w carb sync., I put the exhaust sniffer into each pipe to measure the hydrocarbons. Using the the airscrew on each cylinder's carb, I turned them out until I read minimum hydrocarbons at each pipe. It took about 5 1/2 turns out to get it this way, which I thought odd. But, the engine purred as sweet as can be.
Time for a test ride. I got about 25 feet before I knew this was all wrong, as there was no engine pickup from twisting the throttle while the engine was under load. So, with a lot of coaxing I got back to the garage and turned each air screw in 2 turns. Another test, and it was better but anything more than about 1/4 throttle under load and all I got was engine wheeze.
So, I turned the screws in another turn, test, better, but needs more. I repeated this procedure until the throttle response and power were good when in gear at 1500 RPM and 1/2 travel on the throttle twist. And at 4000 RPM in gear any amount of throttle twist netted a power response with no wheeze or hesitation. This was certainly ridable and highly desireable for the street.
I then checked the final air screw setting. Honda called for 1 1/2 turns out from seated plus or minus 3/8 of a turn. Mine were at 1 5/8 turns.
Technical analysis:
The non-accelerator pump carbs rely on an over-rich idle mixture for engine pickup when the throttle is opened. The intake vacuum drops rapidly when this occurs. Loss of vacuum causes little fuel to be drawn from the jets and the engine gets mostly air. If the residual mixture from operating at idle is over rich, the extra air gets a power response. If it is too lean, a hesitation, at least until the air velocity in the carb throat rises to a level where the venturi effect can cause a pressure drop that pulls fuel through the jets.
So, the question is, when to deviate or optimize the air screw setting? The farther out the air screw setting, the better from the standpoint of not fouling plugs during idle, fuel economy, and minimum pollution from unburned hydrocarbons. However, too far out and the bike is a bucking beast on the street. So, turn them out as far as you can without making the throttle twist a butt puckering experience.
The air bleed screws compensate for the pressure drop in the carbs. The air filter ALSO effects the pressure drop in the carbs. A new filter with less restriction has less of a drop than one that is dirty and more restricted. So, the air bleed adjustment should be made with a brand new clean air filter. As it clogs up with particles, the throttle response from down low RPMs and load will actually improve and be more tolerant of large throttle changes. Of course, a clogged or restricted filter will make the entire operating range rich, as well. Hence frequent filter changes/cleaning.
Any changes to air filtration, such as pods, will almost certainly require a change to the air bleed screw settings (at least), as the stock Honda settings assume intake restriction and carb throat pressure drops similar to what the stock air filter provided.
I've heard some people proclaim that each carb can have a different ideal setting. However, if each cylinder is working at equal efficiency, and all parts in each carb are identical, I see no reason for this to be so. If you need different settings for different carbs, something is wrong and the unequal adjustments are a compensation, not a correction.
Cheers,
