15-08-13, 03:27 PM
(15-08-13, 09:50 AM)solorider link Wrote: I think the idea stems back to the late 70s and early 80s when Yamaha did link the intakes with a passage on some of their bikes called YICS, it was there to improve efficiency in the midrange, and it did work, but I suppose as emissions regs took over and bikes got more efficient and the cost of extra machining the system was removed, I know when I blanked the system off on one bike I had it was noticeable, with a slight power dip in the midrange, but then the passage was larger than the fuel pipe I was using, I believe the passage was around 8-10mm and it was in the cylinder head to aid swirl in the combustion chamber.
That takes me back. I still have a genuine YICS service tool. It was used to blank off the passages you mention in the head so the carbs could be balanced.
I tried the pipe mod on my xs1100 and I noticed no difference. It is certainly an old idea.