The engine is a K3, and since switching to pods the bike wants to bog at higher speeds. The mechanic at the shop said with some kind of conviction that the pods weren't capable of creating the kind of air pressure that the carbs need to get the mixture through at the proper pace, and recommended that I re-install the air-box.
Do any of the vets have anything comments on whether or not there's anything to the air pressure idea?
The mechanic didn't explain it well, but he is essentially correct.
A little background:
Draw on your experience with a straw in a glass of water. Liquid doesn't flow up the straw until you apply negative pressure (suction) on the straw with your mouth. The more negative pressure you apply, the more volume of liquid you are able to extract from the glass.
The slow and main jet are, in essence, tubes with an orifice in it. One end of the tube is immersed in fuel (the float bowl reservior of the carburetor) and the other end is exposed to the carburetor throat. Each intake stroke of the cylinder creates negative pressure in the carb throat, and that negative pressure pulls fuel through the jet orifices. The deeper the negative pressure, the more fuel is pulled through the fuel jets.
The carb throat is part of yet another tube that goes from the air filter to the cylinder intake valve.
When the intake stroke occurs, the depth of the negative pressure within the tube is related to the restriction offered to its passage. You have to suck harder on a small tube to inhale the same amount of air as you could get from a larger tube. When you exercise, you breathe through your open mouth rather than just your nose, as it allows you to fill you lungs easier. The mouth passage offers less restriction to the inrush of air. The filter media offers a restriction to air passage in the carb throat tube, and the more restricted the entrance becomes, the deeper the negative pressure gets.
The stock filter and air box is generally thought to be more restrictive than individual pod type filters, partly because the stock filter traps smaller particles than the pod types do. The factory selected fuel metering jets based on the depth of negative carb throat pressure that is achieved with the stock air filter. Reducing the depth of negative throat pressure with an aftermarket filter that offers less resistance to airflow also makes the factory selected fuel metering jet orifices flow less fuel.
This is why changing to pod type air filters, often requires larger fuel metering jets. If you don't want to change metering jets, then restore the factory designed filter and air box.
Finding the correct jets and needle positions for pods can take time, trial and error, or a Dyno run or two to get all throttle positions at proper mixture. The cheapest and fastest path to getting the bike to run correctly is to put the stock filter and air box back on, as the Honda factory has already made the correct jetting selections that enable the bike to run correctly. I'm sure if you wanted to pay your mechanic for 10-40 hours of work experimenting with your carbs and tracking down the correct parts, he might be persuaded to rejet your carbs for you. But, I think he offered you the cheapest solution to your current problem.
If you are determined to run pods, turn your air screws in about 1/8 to 1/4 turn, raise the throttle valve needles one clip position, and increase your main jet by 005 or 010 larger than what you have now. (They are stamped on the jet body). Vacuum synchronize the carbs and drive it to see if you've cured the "bog" problem. You might also learn to read spark plug deposits, as they are your window to combustion conditions within the cylinder.
You can get the dial vacuum guages as a set from JCWhitney.
Cheers,
