What was the reason behind McCulloch doing away with using the third port and changing the castings on the 91 and later blocks. ??

McCulloch Engines and the third port

The early Mac blocks had the third port that was controlled by the piston position and was to become the most single important peformance increase, thanks to a man by the name of Vince Petruzzi. Vince discovered that by machining those three ports up to the height of the other six intakes, and removing the rib in the block under the intake manifold, we now had much more intake area. Thus--the "nine port" came to be. Vince's discovery completely blew away the competition! Soon, engine builders started coming with many variations, in an attempt to even out the playing field. McCulloch soon realized that this modification was significant, and the castings were changed. First with a large "window" type port, with a removable cover, and then the two "finger port" models were introduced, which remained with all the Mac kart engines up to the end of production. You hear very little about Vince Petruzzi and all he did for karting!

The cylinder below is off a PM 105 or  Super Pro 105 - 125 and shows that there is no third port or boost ports

This is an old grabbed cylinder i cut the head off and it allows viewing of the decompressor port layout and the blank back wall of the PM 105 and SP 105, 125 cylinders.

This page should be titled Evolution as above and to you right is the third port layout a port layout used by McCulloch for many many years till the Karters needed more power enter Vince Petruzzi who showed how to achieve the desired improvement by lifting the ports and doing away with the need for the side channel that Mac had been using. Once this mod had been used Mac soon changed their production to follow suit and the first port changes were seen in the picture below on the 91b which had the removable port window but again Mac upped the game and the boost ports were case direct into the cylinder as in the 91b/1 and later engines.