All across the internet, motorcycle forum members are annoyed with this single question;
Installed pods on XXX bike. Why doesn't run right?
Usually the bike has some form of Constant Velocity carb and nearly everyone says to put the airbox back on. Others just type
YOU WERE WARNED and include an angry face.
Naturally, this ominous advice was ignored. K&N cones were installed on some notoriously crappy 3 circuit mikuni cv carbs. Causing the bike to run like total garbage.
Knowing that pods would "lean out the mix" - bigger jets were ordered and installed. 6sigma provided the kit and some excellent tech articles. But something totally unexpected happened.
The bike was
Too Rich on the top end! Idle jetting was alright, midrange felt okay, but anywhere above 5500rpm would turn into a blubbery mess. Could not pass go, and already lost the $200 on parts and labor.
This was with 152.5 mains on a 250cc cylinder and the needle clip set to the leanest notch. Tried 150 mains, still had a rich bog at Wide Open Throttle. Floats were 3mm higher than stock, so that was brought back down. Still no dice. Could not make it to the redline without bogging. Midrange felt a little weaker, so the 152.5's were put back in.
Was under the assumption that maybe this WOT issue was from improper slide lift. So began some experimenting with different springs in the carbs. Going softer, while making the bike launch superbly, did not cure the trouble. This was where the local hardware store came through with a spring that was slightly stiffer, bigger around and little shorter than stock. Installing this stiffer spring gave more usable throttle, but could still bog at 6500-7000rpm. Following this improvement, went a little nutty and tried both the stock spring and the hardware store spring together. This proved to be too much, as the top end was still trouble, and now the mid range began to suffer. Removed the stock spring, and hit the books.
After staring at circuit diagrams of these mikuni cv's, reading about slide profiles to improve air flow, carb porting (and why not to do it), and dyno A/F ratio charts. Another "AHA!" moment came to mind.
Main Air Jet. A press fit brass jet linked to the emulsifier. One thing that had not been changed. Though 6sigma, in their wisdom, did provide a tiny tiny drill bit, that was just a smidge larger than this air hole. At this point, the entire weekend had been spent trying every configuration available. Depending on throttle position, the machine would accelerate smoothly. The tuning was close, just need a little more air at WOT. Determined to make this damn bike run, the main air jet got drilled.
Carbs back on, started up, headed down the driveway - bike starts to sputter. Crap. An unsmooth idle and a couple of backfires. The ignition was cutting out! Battery voltage 11.8 - No more test rides for the day.
Pulled it off the charger the next morning and gave it the kick. Idles nice, rev is a little slow through the low end. Fine tune the pilot screws and got that a bit snappier. Time to find out if drilling the main air jet did what I hoped it would.
Now the bike does 110. Ton Up!
Took a lot of black magic voodoo and frustrated smoke breaks. But the problem of rich bog at WOT is solved! Have a smooth wall of power from 2500rpm to Redline. As well as a new speed record for a custom cafe racer.
TL;DR -
Put pods on mikuni CV's and it sucked. Bigger jets helped, but could not hold throttle wide open without bogging.
Solved problem with over-rich jetting by installing stiffer slide springs and drilling the Main Air Jet one size larger.
Success
Hopefully this odd bit of agricultural engineering can encourage people to rethink their stance on pod filtering constant velocity carbs. It can be made to work with simple, off the shelf modifications!
