TOPIC: Mark 78 parts needed!

Hello,

Does anyone know were I can buy a new distributor cap + rotor for my Mark 78 motor?

We had a "test run" last week and the motor ran excellent on all 6 cilinders. With an inproper propeller it made 40 mls/hour on an Aristo craft Torpedo. Suddenly at low speed it seemed the motor was out of fuel but stayed running. Since than it ran on 3 cylinders.

We replaced the coils, points,condensers, spark plugs,timing belt and fuel pump bearing kits.We have strong spark on all cilinders but it still runs on 3. The other 3 spark plugs are soaking wet from fuel!
There is full "resistance" on both cables that come from the distributor cap to the coils.

We did following the instructions as described in the" big white Mercury outboards book prior to 1965".

Hope someone can advise what to do.

Thanks,Taco.

Taco


since you have strong spark on all cylinders and 3 plugs are soaked maybe check the needle & seat as well as the floats on the carbs, possibly one carb is flooding out the plugs? .......just a thought

edit: what 3 cylinders are not firing

What about your ballast resistors?? Are they corroded?? You say "strong spark" on all cylinders, is that a static test? You're probably loosing spark under load. I had to take my resistors apart and clean the connections........REALLY GOOD!! I also found one of the leads rubbed through to the block slightly, causing a short to ground. I would check those resistors and all related wiring before I did anything else! The resistance value should be 1.2-1.5 ohms. Been there........done that!

One more thing. You say you've replaced the points. Did you set the dwell and synch them?? Another problem area........

One more thing. You say you've replaced the points. Did you set the dwell and synch them?? Another problem area........

Taco I agree with this, & definitely maybe the issue, thats why I wanted to know what 3 cyl arent firing

Thanks guy`s for your advice! It seems that number 1,3 and 5 are not working.

I do not understand what is ment by :
Did you set the dwell and synch them ?? What is a dwell? synch = synchronice?

I found out that the lower carb looks " oily" at the outside. They were taken off a couple of months ago and cleaned and than rebuild with original Mercury carb kits done by the Mercury dealer.

We found a slight crack in the distributor cap and a repaired spot were the cap is mounted to the housing.

Regards, Taco.

When I asume that the resistors are the 2 large white ceramic pieces I think these are working well. We " rewound" the wires to avoid a short to ground. One resistor was broken. Not the spring but the ceramic and we replaced that for a new one that was on an Mark 75 motor. We also found out that no part from the mark 75 distributor fits the 78.

these 6's are touchy with the points & dwell setting and should be done by the book with the Dwell & sync plate
print this out but you will need to make a pointer Taco

Hi Mad props,
I have printed and it is pretty large. I think we have underestimated the importance of the points settings. I will go back to the Mercury dealer and let them do the job. There is only 2 weeks left for the classic boat race I want to run with the motor. So I can`t affort experiments anymore.
Thanks for your support!! Taco.

boatmaniac wrote:

When I asume that the resistors are the 2 large white ceramic pieces I think these are working well. We " rewound" the wires to avoid a short to ground. One resistor was broken. Not the spring but the ceramic and we replaced that for a new one that was on an Mark 75 motor. We also found out that no part from the mark 75 distributor fits the 78.

Cap, rotor and breaker plate are the same. It's the same firing order as well. Distributor drive is different. Resistors may "look" good, but you won't know unless you test them. Use a digital volt/ohm meter to see if you have continuity through the resistor. Check from the coil lead to the points lead, you should have around 2 ohms, 1.2-1.5ohms though the resistor alone.

Another thought. Are you sure you didn't break the carbon contact for the high tension lead? The side high tension lead must be removed before the cap can be removed. It's also easy to break the center one putting the cap back into position.........

Hello Marathon6,

I may have an older modell Mark 75 with another cap than the Mark 78. It has only 2 holes for mounting it and is dark red from color. The side connector with the carbon is slightly different to. If I mount this piece on the Mark 78 cap ,the cap will break on the inside.

Thanks to yours comments I am absolutely sure the points are not set and cynchronized correct.

Besides of that the boat ran pretty well on 3 cylinders! Probably because of the low weight of the Aristo Craft boat.

Regards,Taco.

should do 40-44 mph. been there done that!

Taco Vonk
it sure would be cool if you could post some photos of the classic race over there.........good luck!

boatmaniac wrote:

Hello Marathon6,

I may have an older modell Mark 75 with another cap than the Mark 78. It has only 2 holes for mounting it and is dark red from color. The side connector with the carbon is slightly different to. If I mount this piece on the Mark 78 cap ,the cap will break on the inside.

Thanks to yours comments I am absolutely sure the points are not set and cynchronized correct.

Besides of that the boat ran pretty well on 3 cylinders! Probably because of the low weight of the Aristo Craft boat.

Regards,Taco.

The part number is same for the 75 & the 78. I've interchanged them several times. I've also restored several of these motors. Mercury used the same cap for the inline sixes until the changeover to electronic ignition. Maybe look a little closer?? The later replacement caps came with 4 mounting tabs for different applications, but the caps will fit.

Hello guys,

Here`s an update about the solved problem with the Mark 78. Yesterday a friend of my neighbour came by with some tools and special equipment to check my Mark 78. I allready had replaced the distributor cap for a new one. It seemed that all spark plugs had strong sparks but not at the right time. After he removed the timing belt and turned the ignition a bit I had to start the motor and there was a fantastic "back fire". We had to change the wires to the coils as they were switched. At 2nd start the engine ran flawless and after a check with some flashlamp on all 6 cil spark wires he said it runs perfectly. Ready to race in Germany next week.

As we liked to find out what could have happened that the motor suddenly ran from 6 to 3 cilinders earlier on{ first message} I remembered that I tried the "kill switch" at low speed and after I re-started the engine it ran on only 3 cil. The timing belt was pretty worn and partly melted { the motor got on fire in his early life} at that time as the Mercury dealer told it would hold for some time and did not need to be re-placed.Probably the ignition wheel/coil? slipped in this old time belt a few "teeth" so that there was no spark on 3 cil./ or at the wrong moment?

Anyway I am very pleased the motor runs well now and I still have a few days to find a good propeller set-up for the race.

By the way: My Silver Arrow arrived in Amsterdam and does not look to bad after it sunk in the dock of Baltimore. See pic.

Best regards, Taco.

good news Taco Im glad there wasnt too much damage to that beauty