Is filter oil really necessary?
It is if you are using a foam element and want to keep dust from eroding cylinder walls.
You don't need bigger pilot jets when you switch to pods. At idle your engine flows very little air, much less than the stock airbox is capable of supplying, so you are not going to flow any more air with pods.
This is an absolutely correct conclusion. But, the entry statement is not supported by it.
Pods do NOTHING to increase air flow over any other filter at idle speeds. The amount of air flow is determined by the engine displacement, RPM, and the efficiency of the valve train components in the SOHC4.
A filter media change effects the depth of the pressure drop across the filter media (comparing one filter type to another). And, it is the depth of that pressure drop in the carb throat that determines how much fuel can be drawn through a given fuel metering orifice diameter.
If you are using pods that specify oiling, the manufacturer has determined that oil is an integral part of the filter design. Further, oil in a foam filter has an impact on it's cleaning and restrictive (pressure drop) properties, as in effect, it further narrows the openings through which inlet air passes. Oil viscosity matters.
For the 78 PD carbs, the idle mixture screw meters the total air/fuel mixture provided to the carb throats from the slow speed system. At some adjustment point, the IMS is open as far as it will go, then the slow jet and slow air jet orifices determines the air fuel mix. The IMS will become less effectual as it approaches maximum adjustment range. If you need a richer idle/slow speed mixture than what an open IMS can provide, then you WILL need to obtain a larger slow jet orifice diameter.
You can observe these fundamental principles using your very own mouth. When you suck on a straw you create a partial vacuum in you mouth. You can feel the depth of vacuum vary in your mouth and how the straw diameter effects the difficulty in obtaining a given vacuum. Further, you can draw more water into your mouth using a larger diameter straw, than a small diameter straw in a given time period using the same level of partial vacuum.
The stock carb orifices were specified by Honda engineers while using the pressure drop qualities of the stock air box/filter media qualities, and the needs of the engine, given the cylinder scavenge characteristics of the stock exhaust system and it's exit orifice diameter. Poor exhaust scavenging means less oxygen available in the cylinder, and less fuel is then required for a proper burn.
So, for the 77- 78 engine, the exhausts had lower scavenge than previous models. This tended to make carb jets smaller (so less hydrocarbons made it into the air, an EPA design goal). It follows that the carb jets would have to be adjusted to provide more fuel to the engine to keep pace with more oxygen associated with a more open exhaust.
This is a separate factor from carb throat pressure, which is directly impacted by the air filter arrangement. And, carb throat pressure determines the amount of fuel drawn through the carbs metering orifices.
Simple, eh?
