All About the KACR

The Kawasaki Automatic Compression Release

You may have wondered about the mysterious KACR and how it works. This old page from Geocities, ca 2009, gives a nice description.

I searched the Internet for an explanation of "how" the KACR actually releases cylinder compression to make starting your engine easier, but none went into any useful detail other than to say a KACR holds an exhaust valve open when starting your engine. Since I had the rockers off the BUBF, I had a chance to look at the KACR assembly and turn the engine by hand to get a sense of how the KACR works.

The KACR is an elegant solution and works well unless the integral spring loosens (or you drop and damage the mechanism). Each BUBF cylinder has a KACR assembly that fits on the end of each camshaft next to the outside exhaust valve cam (located just "above" the exhaust port on the outside of the cylinder). A "pin" on the side of the hinged halves of the KACR rests in a notch in the exhaust cam lobe surface parallel to the camshaft in the "path" of the exhaust valve rocker arm when the valve is normally closed during the compression stroke.

At no or very low rpms, the pin protrudes just above the cam lobe face. When you hit the starter switch, the camshaft turns. When the exhaust valve rocker arm moves over the pin, the rocker arm is raised for a moment pushing open the exhaust valve. Holding the exhaust valve open for that moment reduces cylinder compression. Permitting the starter motor to turn the engine against reduced compression allows turning the engine easier. All this takes place as the spark plugs are firing normally and igniting the air/fuel mix.

As the engine catches, the camshaft turns faster. The spring holding the two halves of the KACR together (and holding the pin up in the path of the rocker-arm) is over come by centrifugal force and the two KACR halves move apart. As the KACR "opens", the "pin" moves down into the cam lobe notch no longer interrupting normal exhaust valve operation.

From the looks of the small spring and the few turns it normally takes for a 1500 engine to catch and run, engine rpms don't have to be very high to overcome the KACR spring. This is what I've observed and seems consistent with the service manual pictures and limited text.

(Posted at, original author unknown)

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