YICS

YICS stands for the Yamaha Induction Control System and is a performance and fuel-efficiency-enhancing cylinder head design that promotes better and more complete combustion chamber "filling" and "mixing", and thus better combustion efficiency via a cast-in, full-length, 10mm diameter chamber within the back top edge of the cylinder jugs connected to special passageways within the cylinder head. This chamber is sealed with a small hex-headed bolt ("plug") and sealing washer at each end, and within the chamber has 4 individual tiny holes (or "sub-intake ports", as Yamaha calls them) that lead up into the cylinder head's main intake passageway.

YICS made it's first appearance on various 1981 models, and was used until 1986. Interestingly, about the same time that Yamaha was developing (1978-79) and implementing (1981) this system for their engines, Honda (the auto company) had recently introduced their "3-valves-per-cylinder" concept on their CVCC engine design (which stands for Compound Vortex Controlled Combustion). This system used an ultra-rich fuel mixture, which entered into a very small, segregated combustion chamber which enveloped the spark plug, and via a separate intake valve, received this very rich fuel mixture from a separate venturi from a common carburetor.
The main fuel charge was ultra-lean, and entered the main combustion chamber thru a much smaller 2nd intake valve.
The spark plug ignited the tiny ultra-rich mixture, which then propagated outward through a small passageway from the small "rich-mixture" combustion chamber into the much larger main (lean-mixture) cylinder chamber, and it was this traveling, swirling wall of flame which actually ignited the main (lean) fuel mixture. The "swirl" initiated by the rich fuel mixture flame front into the main combustion chamber closely mimics the purpose of the YICS system as described below.
Honda was so successful with this development, that they were able to exceed US emissions standards with these CVCC engines by a full 3 years or so, and were the only US-sold automobile that did not employ the use of catalytic converters in order to meet emissions requirements.
Here is how Yamaha describes their YICS system:
"The YICS consists of 2 separate chambers, each connected, to a port on each cylinder just behind the intake valve. As the piston moves down on the intake stroke, the cylinder fills with a fresh air/fuel charge. A vacuum is also created in the YICS chamber at the same time. When the intake valve closes, part of the charge is drawn into the YICS. When the intake valve reopens the mixture in the YICS shoots back out through an angled tube into the cylinder, mixing with and swirling the main intake charge. The swirling charge is then compressed and ignited, burning more completely and producing more power than that of a conventional engine."
These YICS sub-intake ports have approximately 1/4th the cross-sectional area of the main intake port runners. These smaller YICS ports tie into and enter the main runners just above the intake valve seat in the combustion chamber and at such an angle that their air/fuel "charge" is directed around the outside walls of the cylinder/cylinder head/combustion chamber, which results in a swirling effect as the mixture is drawn in and then compressed via the piston.

These "sub-intake ports" are connected via the YICS cylinder head passage. Since only one cylinder is on the (downward) intake stroke at any one time, the YICS sub-intake port FOR THAT CYLINDER also draws in some air/fuel mixture charge from the other three sub-intake ports and carbs. When a piston is moving down in the cylinder bore during its intake stroke, the vacuum created operates on both the main intake port (for that cylinder) AND on all the other sub-intake (YICS) ports. But since the area of the sub-intake port is so much smaller than the main intake port, this sub-mixture moves through the YICS passage/port almost 4 times faster than through the main intake port. Thus the cylinder "filling" and "swirling" effects.

According the factory marketing gurus, the end result of this controlled turbulence in the combustion chamber is quicker, more complete burning of the air-fuel charge, and thus, all (well, at least "more") of the power contained within the fuel is liberated on every power stroke of the engine.

Note that in all of this discussion by Yamaha, that the underlying theme is "greater fuel efficiency", and NOT one of more power or performance - although the two do go hand-in-hand.

Perhaps it is telling that the only difference between the 1980-81 XJ650 Maxim carbs (non-YICS) and the 1982-83 XJ650 Maxim carbs (YICS) is that the air jets were increased in size by almost 15% on the YICS versions resulting in a leaner fuel mixture without any decrease in power. A leaner overall fuel mixture, just as Honda discovered with their CVCC system, results in better fuel economy with the same amount of power output.

Things Go Better With YICS, At Least On These Models:
YICS was used on:
* all XJ550 models
* all 1982-up XJ650 Maxim and Turbo models
* all XJ700 non-X models
* all XJ750 non-X models
* XJ900RK
* XJ1100 models.

Strangely, the 1980-81 XJ650 Maxim and XJ650 Midnight Maxim models, as well as the 1982 XJ650RJ Seca models, did NOT use the YICS system. It has also been reported by some owners that some very early model 1982 XJ650 Maxim models did not use the YICS system.

YICS-engined bikes have front lower (oil pump) covers on both sides that features a red, rectangular "YICS" emblem in them below the cast-in YAMAHA wording. Non-YICS models have the oil pump covers with only the YAMAHA wording (or no wording at all) cast into the cover.

YICS-engined bikes also use a valve cover that has the word YICS cast into the top surface, while non-YICS models are blank or are cast "YAMAHA" (or "YAMAHA 5VALVE" on 700-X/750-X models) on the valve cover.

Here is how we describe their YICS system:
YICS is probably a good feature, and probably does provide some moderate advantage in fuel economy and performance, just as the factory claims. It adds an additional step.....and tool.....to the engine synch procedure, as a YICS passage "blanking" or block-off tool must be used when doing a synch, and it's also not a bad idea to clean out the tiny YICS chamber port holes every once in a while, and to replace the YICS passage bolt Sealing Washers every so often; but other than that, it's a pretty simple and problem-free system; because, as Yamaha likes to say, "we did all this without adding any moving parts!"