I heard a great disturbance in the Force...
I have a CycleX streetfighter motor built by Ken himself. It has 849cc big bore pistons together with all the goodies Ken builds in his Streetfighter package. I struggled for 3 years to run a twin Mikuni set up but finally abandoned it for a set of PD41A carbs I bought at a flee market. I installed the carbs on the motor with a set of K&N pods and worked that summer to dial in the jets and settings. I used a spreadsheet to collect and display data from each tuning and ride session. I even spent money on a couple of hours on a local dyno to be sure air/fuel ratios were dialed in as close as I could get with the set up.
On the road there were days it ran like a raped ape and some days I could feel a flutter or studder at a particular throttle position. I couldn't understand it, the barimetric pressures were roughly the same, temperatures the same and even humidity. Then one thing started to show up in the data, the wind. On a windy day the flutter/stumbles became more prevelent. One day in particular the wind was extremely high, cross gusting to the direction of travel. The bike suffered, but I was too far from home to simply get out of it. I feathered the throttle, road with knees tight to the tank, shifted gears to stay away from that throttle range. Having a long ride in the wind with it hitting me across wide open fields or catching me as I came out of protected areas the guts allowed me to experiment and make observations. The most alarming fact was as I rode along and the bike settled in holding its own pulling well with no noticeable faultering I would see approaching riders and removed my left hand off the grip to acknowledge their wave the bike coughed and fluttered until I put my hand back. I started to test the conditions and moved my hand off the bars and as the bike faultered I adjusted the choke to see if I could determine whether it was a lean or rich condition being experienced. But I couldn't sense enough difference to come up with a better understanding.
I got home and waited until the bike cooled down and pulled the plugs to see if there was any significant indicator of extreme mixture evidence, nothing tan colored four plugs. I slept on it but the thought of air flow disruption from the movement of my hand stuck in my head all night. A side note at this point, years before with another bike I had set up a Tornado by Thunder filter on a VTwin. That filter came with concial chrome cover that shrouded the filter by 270 degrees. The filter was designed to point into the wind in the direction of travel and I eventually found that the perfect set up had the open section of the filter facing inward towards the cylinder jugs. Some of the evidence that I looked at when I tried to perfect the set up was that when the open part of the filter was in the un-obstructed airflow the bike would return extremely poor fuel mileage. This led me to believe that the air turbulence was actually creating a very rich situation, not because of jet settings but because air flow in the carburator throat is being distrupted.
Enough on that, back to my '75 CB750F. The next morning I woke and drew up a design for an aluminum shroud that would provide cross-wind turbulance for the four pod filters.
You can see in the pics the shroud is just that a barrier that protects the side of the pod from air turbulence. I have since installing it, ridden the bike on windy days, still days and opened my knees to catch the air, moved my hands off the bars and not had the motor faulter as it did before. The shroud is completely open at the back end and the front is wrapped tight around the outer two pods. There is no flat plate that stops air from flowing in and around between each pod so no low pressure zone is created in the shroud, it just smooths out the air reducing turbulence over the filters.
I did something similar on a CB450 I built, I made two cylindrical tubes of thin aluminum the diameter of the carb horn and an 1" shorter than the length of the pod filters. The cylindrical tubes slide on the carb horn, then the pod slid over the tube and clamped to the carb horn holding both filter and tube in place. The tube acted as a crude velocity stack covered by the filter. Because the tube didn't touch the fiter material the carb could still draw air through the whole filter but pulled it straight down the tube into the throat. The tube prevented side air turbulence from affecting the air flow down the tube and the carb throat. Just a different way but it worked. I could have tried the same with the CB750 but I felt making four identical tubes and positioning them perfectly was going to be near impossible so I created the outer shroud.
Sorry I am remis
Pilot Jets: 35 (Keihin)
Mains: 125 (Keihin)
PMS Screws 2.5 turns out
PD41A Carbs
K&N Pods
DR8EIX NGK plugs
