And a question: I assume pods lean up a mixture because they flow more freely. Would a 4-1 (replacing a 4-2) exhaust exacerbate this or help?
Um, sort of.
Pods (depending brand and manufacturing techniques) have less resistance to flow, which makes the pressure differential across the different filter membrane smaller. This effects the depth of the "vacuum" in the carb throats.
(Not all Pods are the same, btw. Just like all motorcycles are not the same.)
Each carburetor fuel delivery circuit behaves in a similar fashion to a straw in a liquid drink. If you create a small pressure differential at one end of the straw with your mouth, the atmospheric pressure on the surface of the liquid in the drink pushes the liquid up into the straw and into your mouth. The more pressure differential that occurs across the ends of the straw, the more fluid gets pushed through the straw to your mouth.
For the carb fuel delivery "straw" one end is in the carb throat and the other is in the fuel supply in the carb bowl. The bowls are vented so atmospheric pressure exists on the surface of the fuel and that pressure is pretty much the same at the supply end of the fuel jet tube (the straw). The other end of the fuel delivery "straw" is in the carb throat. When the filter membrane pressure differential across it becomes less, the carb throat pressure gets closer to outside atmospheric. The less differential pressure present across the orifice in the fuel delivery tube (the straw), the less fuel flows across that orifice.
In most cases, if you have not changed the volumetric efficiency of the engine mechanicals, the amount or volume of air passing through the carbs does not change. However, a filter membrane that allows less differential pressure will require a larger jet orifice to flow the same amount of fuel as before, just to maintain the same air fuel ratio as before the filter membrane change.
The end effect, is that pod filters often turn carbs that were tailored for a different filter membrane into lean delivery machines. Bigger fuel delivery orifices are then needed as compensation for the membrane change.
The stock exhaust was a 4 into 4, not a 4 into 2. So, there was already a back pressure change coincident with that change. The 77-78 550K exhaust is pretty quiet and the outlets are smaller on a per pipe basis than earlier 4 into 4s. This was part of the EPA regulatory issues impressed on this model of bike (both hydrocarbon and noise pollution). The increased back pressure reduces cylinder scavenging, leaving some unburned hydrocarbons in the chamber for the next power cycle, and reducing the fuel requirements for the next power stroke. The back pressure therefore allowed less fuel delivery for a correct mixture. Carbs were tuned to provide the correct mixture for the components installed.
A properly tuned 4 into 1 has an RPM or range of RPMs where the exhaust pulses help scavenge adjacent cylinders. This allows more oxygen into the chamber for the next power stroke, and usually more fuel needs to be delivered when "on the pipe" in order to keep the proper air fuel ratio.
A properly tuned 4 into 1 (race pipe with calculated pipe diameter and length) doesn't usually "come on" until the RPMs are quite high. Designed for racing engines, these pipes don't work until near and above labeled red line on the tach. Usually, this is best used on "built" engines that have been modified to survive these higher RPMs. Also note that race engines are frequently rebuilt (every race?) to keep worn parts from self destruction.
The down side of the 4 into 1 is that at lower RPMs ("off the pipe") the exhaust pulses from neighbor cylinders reflect back to other exhaust ports and actually interfere with the exhaust scavenging, making the engine far less efficient (than a 4 into 4, for example, and some 4 into 2s). 4 into 1's usually don't have good low and midrange power, where most street bikes operate most of the time.
Many 4 into 1's aren't tuned at all and are just cheap "form" examples sold to those who really don't understand what they are supposed to do, IMO. The Mac is one of these. Its a replacement to keep those with stingy pocketbooks, legal for the street. I don't believe there is any tuning engineering put into these systems.
From a jetting point of view, it all depends on the back pressure presented to the engine and in what RPM range it is being operated in. During scavenge boost, more fuel delivery is needed. During scavenge detriment, less fuel delivery is needed.
Simply stating it is a 4 into 1, does give a valid reference to a back pressure/boost map that can be used to predict a change from what once was.
You have made an engineering change. You can sniff/measure the exhaust to determine what is unburned under power. (such as a dyno)
Or, you can do "plug chops" to infer the combustion conditions your combination of parts demands, based on fuel deposits within the combustion chamber left on a the spark plugs.
Hope this helps...someone.
