TOPIC: Mark 75

Hey Guys,

Its been a few months since i have been on here. I struggled with the MARK 75 all summer trying to make it run right, and decided i needed to take a break from working on it. Now that ive got my furnace hooked up in my workshop, i want to mess around with it some more. I am determined to figure this thing out!



Here are the symptoms the motor displayed much of the year:

Randomly the motor would miss-fire and run erratically. When this happened it would not die, although power was lost as it felt like it had stopped running one or two cylinders. also, when this happened starting in forward took longer, and starting in reverse was very hard if not impossible. Sometimes i could run for upwards of an hour before the problem would present itself, and sometimes it would only take minutes. I could not really isolate which cylinder / cylinders were dropping.


Here is what i did to try and remedy the problem:
-Rebuilt the powerhead. Compression at 160 across the board (new rings, seals, gaskets, and bearings)
-Rebuilt all three carbs (new floats & needles/seats)
-Rebuilt fuel pumps (new check valves, gaskets & diaphragms)
- new fuel lines (not factory crimp style, but used barb fittings and clamps)
- new impeller (pumps water well. doesnt seem to overheat)
- New spark plugs (probably 3 sets over the summer with no change)
- new ignition coils (both are new)
- Set point dwell with factory degree wheel (re-checked several times)

I was wondering if you guys had any ideas where i should start? Im in no hurry so i can take my time with the diagnosis. About the only thing i did not mess with is the wiring itself. I did notice some corroded wires that did not look 100%, so i tapped them up, but im wondering if i have an electrical short somewhere?

Any thoughts are appreciated.

Scott

distributer cap if it is a dark color greenish black grt a new one for sure. tan blue or gray are usually good.1 914 310 7086.

Hi Dave,

Thanks for the response. The Cap i have is actually a yellowish color. I assume this is the same as the tan?

If you still have the original resistors that step down the voltage, consider replacing them as ambient moisture can certainly cause them to be intermittent.
Have you checked or replaced the condensers.
Other than that, I would focus on the old wiring. Step by step of the level of resistance in the continuity of each wire might be enlightening. While doing these tests, be sure and move each wire and connectors on the ends to check for interruption.

Also,...Next time it happens be prepared with a jumper wire to short one set of points to ground at the terminal block on the port side water-jacket cover. Disconnect the Mercury Safety Switch prior
to this test.
If there is no change in the performance you have narrowed down the problem to being in that half of the ignition.
If the engine dies, you will want to focus on the other half of the ignition.
If both of these tests cause a noted change in performance, but the beast continues to run,...Please post for further.
doc F

Thanks for the comments doc f. I have not yet replaced the resistors but I did notice that one always seemed hotter than the other. They look alot to me like old Mopar resistors. Has anyone had any luck adapting them to these towers?

I also have not replaced the condensors and am thinking I will just to rule them out.
Looks like I have some work to do!

Attached is a link to a video I took late summer of ot running in the barn. Just for fun


www.google.com/url?sa=t&source=web&rct=j&url=https://www.youtube.com/watch%3Fv%3DqcDK9aFsZ8Y&ved=0ahUKEwiZ1sb0pd_JAhVnzIMKHbqRAJ8QtwIIRzAK&usg=AFQjCNHnzOTIhB_nyI48hsPTwsmUZKkxzQ&sig2=jIf7sjn4dpwHgUNtEc7Duw

Hi Scott,
Just as an FYI the lines you typed are so long that it boogers up the display when using an iPad
so I can't see your whole post, hit return more often? Probably a good question for the administrator.
Anyways.... So a few more thoughts to add to the excellent advice you have had.
Sounds like you combed through a rebuild nice work.
It sounds like issue shows up after run time so I'm thinking something gets hot and has reduced function
making it hotter still, electrical is a prime suspect for this. It also seems you had this before and after rebuild?
So first suspect what has not been touched.
Yes, replace the condensers, these will cause havoc and the old ones should be suspect.
Suspect the resistors, jump out at time of issue as Doc said but only to diagnose.
When replacing the condensers, double check your point faces and your timing as resistors and condensers
are there only to provide longevity to point life so, if it's one of these degrading then point faces may have
taken a hit.
If needed and you can't find some good original resistors, Mallory makes a decent ceramic one for auto
use. It's very close in size and mounting to what's on the motor now, Jegs or Summit or eBay.
wiring as doc said can be a mess. Insulation stinks and wires are hair like strands easily breaking,
so the fact that you have continuity means nothing, it could be just a few strands connecting.
that won't conduct enough amps for correct ignition operation, amp needs increase with RPM and heat.
In addition to distributor I would also look at the side terminal carbon in the dist. and the plastic. Same with rotor. I clean with auto spray
brake cleaner and look for cracks, that make great pathways for shorting when things warm up.
You can check ignition wires for shorts running motor at night, unlikely source but just thinking possibilities.

Lastly look at your control the switch and wiring. I might suggest once motor is running to directly jump 12 volts
from the larger batt cable on solenoid to the 12 volt feed on the resistors, this bypasses everything to do with the
control.
One thing Bob Grubb taught me was that the charging voltage goes up through the control and then
back to motor. The 75's are notorious for poor charging and your problem may simply be a battery slowly discharging
while running. A helpful improvement is to make a connection from the output of the rectifier right to the 12 volt cable
at the solenoid from the battery, this should be a permanent wire not a temporary jumper. The concern back in the
day was the rectifier would leak and discharge the battery when motor was not running so the ignition switch would break
the connection. The newer brown colored block style with a diode rectifier are better that the old "fin" ones in this regard
but I havenot had discharge issues with the fin ones using this jumper mod, but just be aware of the possibility.
Anyways food for thought....
Good luck!
Randy