YICS and other afkos

ER Classics Desktop 2022

At the beginning of the eighties, the idea was to run combustion engines cleaner and more economically. It seemed as if they already knew that the petrol here would cost almost 2 euros. Suzuki was working on the Twin Swirl Combustion Chambers, Honda successfully experimented with the CVCC, or Compound Vortex Controlled Combustion and that was the start of a whole lot of cleverness with Yamaha's YICS that now leaves us with a pile of abbreviations. Kewl!

Here in the garage is now an eighties Yamaha with YICS

What was that again? YICS stands for the Yamaha Induction Control System and is a performance and fuel efficiency enhancing cylinder head design. The system promotes a better and more complete 'filling' and 'mixing' of the combustion chamber. And thus a better combustion efficiency via a potted chamber with a diameter of 10 mm. In the rear top edge of the cylinders connected to special passages in the cylinder head plus a mixture case where a chunk of mixture can be stored between the power strokes. This chamber in the head is sealed with a small hex bolt ("plug") and a sealing washer at each end. And inside, chamber has 4 individual small holes (or "sub-inlet ports," as Yamaha calls them) leading to the cylinder head's main intake tract.

The factory says it works fine

According to factory marketing gurus, the end result of this controlled combustion chamber turbulence is faster, more complete combustion of the air-fuel charge. And so all (well, at least 'more') of the power contained in the fuel is released with each power stroke of the engine.
Note that in all of this discussion by Yamaha, the underlying theme is 'greater fuel economy', and NOT one of more power or performance, although the two go hand-in-hand.
YICS first appeared on several models from 1981 and was used until 1986.

The conclusion?

YICS is undoubtedly a good invention and probably offers a limited gain in fuel economy and performance, just as the factory claims. It adds an extra step and a special tool required to the engine synchronization procedure, as a YICS pass blanking or block-off tool must be used when performing a synchronization. It's also not a bad idea to clean the small YICS chamber port holes every now and then and replace the YICS grommet sealing washers (scrabble!) every now and then. But otherwise it's a pretty simple and hassle-free system because, as Yamaha likes to say, “We did all this without adding any moving parts!”

In many old forums, by the way, the discussions are about how to get rid of the whole system, how to work on it without special tools. We also found another genius who had circumvented the entire carburetion system by creating a self-invented injection system. That led to a massive backfire, singed eyebrows and a persistent whistle in the right ear for days.

A box with an ugly lid

High between the cylinders of the XZ is a triangular 'box' with a (torn) plastic lid. It says YICS on it. There is a thick, peeling layer of chrome on the plastic. That's the understandable part of the concept where any snake shop porosity or other false air conditions can apparently cause convincing problems. In the meantime, the XZ has started up here. And it runs very nicely. Yamaha? That is quality with surprising frayed edges at the time. Surprisingly, Joost Woesthoff has a lot of XZ 550 parts. So there will in any case be a nice chrome lid for the in-between pant.

YICS 001.preview YICS Split

Simple: The in-between panting box


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  1. Kewl Dolf Peters. Unfortunately I don't know what Kewl stands for. Better is: hello Dolf Peters. What does Kewl mean?

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