View Full Version : Dockbuster operation

04-23-2008, 06:49 PM
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Our buddy Conrad from John Kwast's old Merc discussion board has really done his homework and has this old Merc running great. Kiekhaefer would be proud.

Watch closely as Conrad switches between reverse and forward ... look for the motion of the distributor.

The Merc Dockbusters use a special 6 cylinder distributor, that overcomes what would be the normal limitations of a 2 stroke 6 cylinder distributor, by actually being two 3 cylinder ignitions incorporated in unison. 2 coils, 2 ballast resistors, 2 sets of points, one distributor. Inside the distributor, on the rotor button, there are 2 contacts that meet the spark plug contacts. One feeds from the bottom center coil brush in the normal way, the other from an insulated slip ring on the side. Each feeds only 3 cylinders because of the way the points trigger the coils. The firing order is 1-6-4-2-5-3.

Back in the 1950s, lower unit technology wasn't yet developed to reliably handle the power of such an outboard, so Kiekhaefer simply followed the example of some other steam drive and marine drive propulsion systems and set up the now famous "Direct Reverse" system. With this system the lower unit gears stay engaged all the time. Reverse is achieved, by simply starting and running the motor counterclockwise (instead of the normal clockwise). At the helm a special control box controls the throttle and selects which direction the motor will start and run. The throttle handle moves through a Z shaped gate and actuates an array of micro switches to stop the motor and allow starting in the desired rotation. On the top of the handle is a button that you press with your thumb to actuate the starter motor itself. The "pistol grip" shaped handle and thumb button strongly resemble a World War II fighter plane control stick and machine gun actuator....great symbols of power in 1957.

There are 4 special parts on the motor for the Direct Reverse system:
First and most obvious is the special starter. This big clunky thing has 2 drive gears. At rest one is in the normal position below the starter ring gear on the flywheel, the other is on the extended end of the starter shaft above the starter ring gear. The starter also has 2 solenoids. For forward one solenoid engages the starter motor to run counter clockwise...the lower drive gear runs up its spiral ramp and engages the flywheel gear turning the crankshaft clockwise. For reverse the other solenoid engages the starter motor to run clockwise and the upper drive gear runs down its spiral ramp to engage the flywheel gear and turn the crankshaft counter clockwise.

The motor's second special part is throttle linkage that pushes the carbs OPEN at both extremes of the throttle cable's throw. This is necessary to increase motor speed as one pulls the pistol grip throttle farther back to go faster in reverse as well as the normal throttle opening for forward.

Special part number 3 is the Direct Reverse water pump. Regardless of which way the motor runs water must be pumped up to the powerhead. This is achieved with a water pump with two check valves. Each check valve is located a few degrees off of the water pump's "tight" side where the water is squeezed out of the pocket. The simple rubber check valve allows water to exit the chamber from whichever side it is being squeezed and not back down (into the pump) on the side where the impellor is opening the pocket to draw water in.

OK so now you are wondering what special part #4 is.....
In the distributor there must be some mechanism to throw the ignition to the other side of TDC to compensate for "BEFORE" and "AFTER" TDC changing places. This is achieved with slack in the distributor drive shaft. The upper part of the distributor shaft that is driven by the distributor drive pulley is not the same piece as the shaft with the cams for the points and the rotor button. The coupling between these 2 shafts has quite a bit of slack. This allows the timing slip over to the other side of top. I noticed that even after the age of the Direct Reverse was over, the distributors retained this slack in the drive system. Today this allows for parts interchanging with more ease across the years of the Kiekhaefer Distributor ignitions.

Since Mercury already had full gear shift lower units on the rest of it’s production engines at the time, the Mercury marketing boys needed a hook to explain this apparent step “backwards” in technology. They billed the Direct Reverse system as featuring a new “Silent Neutral”. Of course, if the motor failed to quickly restart when approaching your dock, this could also lead to a “Silent Reverse” and result in the operator coming into the dock with 5 more miles per hour than he has any idea what to do with. Deserved or not, these motors today are forever known as “Dockbusters” a term probably coined by “The Enemy” at OMC.

My feeling is that the Dockbuster's notorious reputation was most often undeserved; actually it was the owner's neglect of another part of the system that was fairly unreliable in the mid 1950s and needed lots of careful attention ... the starting battery that caused most of the trouble. Since around 1980 batteries have greatly improved and it is hard for most people to imagine how much trouble batteries used to be.

04-23-2008, 07:21 PM
My First "big Eng" Was A Merc 700 Dr...the Fairfield Merc Dealer
Told Me It Was A Fresh Overhaul...so I Got Scr## Twice...
But After I Put Six New Pistons And Other New Parts It Worked Great...it Was The Waterskier That Got The Worst End Of The Deal..
And To Make Things Even Comical...i Traded Up For A Merc 800 Dr..
But Along Came My Forth Child So I Sold The Whole Thing To Pay The Doctor Bill.....
P.s....the Mark 75 Needs A Lower Pitch Prop...as You Know If You Want Them To Go, You Got To Have Lots Of Rrrrr's

04-23-2008, 07:47 PM
First I Want To Say..mark 75...this Is A Great Video And Store..
The Average Jo Just Thought They Could Set The Points At .010 And It Would Go.....wrong...the Plastic Disc And Pointer Worked Great But The Best Way Was With Merc's Old Dwell/tach And The Elec Drill..you Could Get Those Babies Right At 90 Degrees Dwell In To Time At All..i Loved Working On The Old Ones..

04-23-2008, 07:51 PM
You are exactly correct. That trick stuff Merc only sold to dealers was easy to use and made them run RIGHT!