mark 75 issues

[motoscott909]

Hey all,

Ive been working on my Mark 75 for a while now, and thought that i had it running good, but it started acting up again and i’m not sure where to go from here?

The issue is that the motor will run absolutely great, and then will randomly lose power and the throttle response is very slow. I ran the motor for an entire day last week on vacation and it ran perfectly pulling tubes all day with great power.The next day after about a half hour the motor seemed to lose some power . It still seems to be running on all 6, but the throttle response is very poor and top speed drops about 4MPH, and getting on plane is much much slower. For the life of me i cannot figure out what is going on, as everything seems fine. Randomly the motor will kick back in and run like a madman again. There is just no rhyme or reason to when it decides to lose power.

Here is a list of everything i have don thus far.

-Powerhead rebuild (new rings, bearings, seals, gaskets)
– Carbs rebuild / cleaning (new floats, seats, gaskets)
– Fuel pumps rebuild (new check valves and gaskets / diaphragms)
– all new fuel lines & new tank line / primer bulb
– Both ignition coils replaced
– New Impeller
– New spark plugs
– set point dwell with factory synch plate (re-checked at least twice)

anyone have any ideas? ‘Im really not sure what else to do here.

[bill-russell]

I once had a mark 75 that I ran quite often. When it would begin to run stupid,a new set of plugs would usually cure mine. Might be worth a try. Bill

[frankr]

Just wondering, why all the powerhead parts replaced? Rust???? If so, maybe it is ingesting water into a cylinder or two. Just a wild guess.

Hey, I always had a hard time making those things run properly too. But I know I just don’t have the touch. Or not smart enough.

[motoscott909]

Hey all,

I took the powerhead apart because it had been sitting since about 1970 and some of the rings were stuck. The rest of the internals looked very nice. I split the rod caps to inspect the journals and they looked brand new along with the rollers. I replaced the upper and lower seals along with the upper and lower main bearings only because i was in there and didn’t want to have to do it again.

Frank- I think that your idea about water ingestion is very good, because honestly im not sure what else it could be? Cylinder #3 spark plug always looks "wetter" than all of the others, which really makes no sense to me, because cylinders 3&4 share one carb. Cylinder #4 is always golden brown along with the others.

I guess my next question is, How do i determine if it is indeed ingesting water, and where could it possibly be coming from? When i had the powerhead apart ,i replaced all the gaskets including the top cover and the large exhaust plate cover. The inner exhaust plate did seem sort of "warped" but i figured it would flatten out when i tightened the exhaust cover bolts.

Thanks again for any input!

Scott

[dave-bernard]

Is #3 on the fuel pump ? if so might be leaking past the diaphragm.

[motoscott909]

hey Dave,

if i’m not mistaken, the top fuel pump is actually between 3&4. I could be wrong, but i’m thinking it would affect both cylinders?

I did put new diaphragms in the pumps when i rebuilt the powerhead, but im not saying one of them didn’t have a pinhole. I guess i never checked and assumed they were okay.

One other thing that i noticed is that in order to get cylinders 3&4 to idle correctly i can only have the low speed needle out about 3/8 of a turn. The other two carbs low speeds are set at around 1 1/8 turns out. I wonder if the new diaphragm i put in is faulty. Ive been through the carbs so many times that i dont think i could have missed anything.

[frankr]

I was thinking inner exhaust cover plate, but it seems you got there before I did.

[dave-bernard]

it will affect only on cyl. check or replace the fuel pump diaphragm. cheap enough.

[jeff-register]

How about your tank? Your pick up could be getting clogged & after a run the clog would fall off until it was sucked up again. Can you blow backwards thru the hose, then dump the tank to see what you find. What do you mean by wetter plug? I do have a tech tip to get the exhaust plate assembled without leaks. Did you hold the inner cover up to light looking for cracks?

[motoscott909]

HI guys,

Thanks for all the input. Its been so darn hot here i haven’t wanted to get out in the barn to work on the Mark 75. last night it cooled off to about 85 degrees so i went out to see what i could find. I took the bottom fuel pump apart and all seemed fine. On the top fuel pump i noticed an ever so slight tear in the gasket (probably a manufacturing flaw) but the diaphragm was 100% fine. Im going to replace the gasket to be safe, but my guess is that this is not my problem.

I didn’t get a change to look into the fuel tank, but i will try to do that tonight if it stays below 90 degrees. One other thing that i have noticed since i replaced the primer bulb and line,which i didn’t mention before, is that when i squeeze the primer bulb it stays collapsed for a few seconds before it lets you give it another pump. With my old OEM merc bulb it never did this. As fast as i could pump, the bulb would allow it. The new bulb is an aftermarket ATWOOD brand (all my local marina had). Has anyone else experienced anything like that? Thinking about getting a good OEM Merc bulb again.

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