Be aware; 4 into 1 is a style, not a specification. Each exhaust design has a flow and back pressure profile over the RPM range. A true, race designed, 4 into 1 will be tuned to boost scavenge effect in an upper RPM range. At lower RPMs, it will actually foul the flow of gasses due to neighboring cylinder firings entering the common pipe and reflecting back to other exhaust ports. It's all about timing and frequency. Racers only care about that sweet RPM spot for racing. Street bike drivers care about streetability. Many owners care about emulating the style of race bikes. Honda tried to placate them with the F model, and still made a streetable bike.
Honda well tuned the carbs for the exhaust delivered with the bike. It's really up to aftermarket manufacturers to either match the engineering specs of the original, or provide tuning data for the carbs to have them deliver air/fuel to the new exhaust requirements.
I think most mfgs. just make an exhaust that functions for the widest number of vehicles they fit. Or, they just punt the engineering off to the style buyers to figure out. It just doesn't matter to them that some may not have the skills to get the bike working as well as Honda parts do, once they have your money.
That's why I say that, if they did the engineering at all, they made them work with the highest popularity of bike model in existence, and that was the K model.
The stock F pipe was a high pressure pipe, for evening the back pressure spikes from neighboring cylinders at lower RPMs, quietness, and emissions reduction. This allows leaner tuning of the carbs, pretty much throughout the entire operating range. Aftermarket manufacturers sell to buyers who want a louder pipe and the "race" look, where desired HP gains can occur when you rev and flog the hell out of the motor. They aren't much looking for all RPM street performance like Honda did.
Anyway, the jetting setup for the K model is far more likely to work better for an aftermarket 4 into 1. They had the same engine, and the 4 into 1 exhaust is freer flowing with less back pressure than the Honda 4 into 1 system. Hence the change in carb tuning parameters.
IMO, the main jet is set for the maximum demands of the motor. As it has a bleed effect for other throttle positions, it's "leakage" effects tuning the needle and pilot jet. So, I set that first. Then move on to the slide needle selection, and pilot jet air bleed, for throttle response off idle (a 38 pilot jet pretty much works for all the 550s, PD carbs excepted). But, the pilot system does provide some bleed for other throttle settings, too. So, there may be some juggling in order to get the same ideal tuning that Honda originally provided for operation anywhere in the world.
It is all far more easy to reach end goal with an exhaust sniffing dyno that will provide you a fuel map for whatever assemblage of parts you wish to test and tune. Then you can address the specific carb section/RPM based on test results. Of course, stock parts don't need this extra attention. Just build it the way Honda did, and go ride.
Otherwise, a test and try can eventually get results, with a variable time in test and try mode. A test track and plug chops is a 2nd best alternative.
Cheers,
