Reed Blocks

By Chuck Petersen

A few years ago I was sitting in my shop chamfering the ports on a 4 cylinder Merc when a friend stopped by. Her question: “what are you doing to that main gut piece ?” Well, it’s true! One of the inner “guts” that some outboarders have never seen are the reed blocks and reeds that make up a key element in the fuel transfer system. While the carburetors mix the fuel/air charge, this mixture must be pulled through a set of reeds or flaps to enter the crankcase on its way to the combustion chamber at the top of each cylinder. The reed valves control the amount of air and fuel allowed into each cylinder for proper combustion. Each cylinder has its own set. The reeds are attached to a block or shell that sits directly behind the carbs in the crankcase. One end of each reed is held down by a bolt and maximum opening is limited by an arm or reed stop extending an angle toward the open or free end. As each piston moves up on a compression stroke, the vacuum created draws in a fresh charge of fuel and air through the reeds. During combustion the reverse pressure caused by the piston moving downward holds the reed valve closed until the cycle repeats.

On the older Merc (pre 1960) models I work on the blocks are machined bronze castings with fairly small openings for the reeds used. Newer models went to aluminum castings, probably for budgetary reasons. The opening sizes were increased as well. This presents a problem as the reeds tend to chip away over time, leading to piston and/or bore damage. Competition motors today use fiberglass or plastic reeds. These break over time but don’t hurt the motor when they do.

Some tips on assembly:
1) Be sure reeds sit flat on openings before and after tightening.
2) Carefully line up stops over the center of each reed.
3) Be sure reeds are lined up straight and cover the opening completely.
4) Be sure openings in block are smooth and free of burs.
5) Apply LocTite to all screws and mounting bolts. How can you tell if reeds are worn or chipped? Poor idle quality, lack of power and rpm loss are key indicators.

Today, reed blocks are more sophisticated versions of an old idea. Reed technology has improved by using better materials and two stage or overlap designs. Boyesen Engineering in Pennsylvania has built a business around supplying specialty reeds. In closing let me say that one thing I have learned from builders of race engines is that no small detail can be over looked. I have seen more time and patience used on setting up a good set of reed cages than might meet the eye of the casual observer.

Hello to my friends at BSLOL. Hope the snow melts soon!!