TOPIC: More mark 75 issues

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 im 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 after about a half hour. 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 i 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.

Don't get angry with me, but have you checked the vent on your gas tank? It's easy to forget to open it after refueling or whatever, it happens to the best of us!

Sounds like you have the rest of the bases covered, just trying to throw some ideas out there.......

Hi mark.

i was thinking the same thing when it first started happening and so i tired running it without the gas gap to see if it improved, which it did not

Maybe recheck the pumps to see if a new problem has developed inside since the rebuild

I had a similar issue a few years back where she was running like a champ and then over night it ran crappy, especially try to get up on plan. One difference in the behavior though is that mine idled really bad.

Discovered that one of my brand new fuel pump diaphragms had split. Wasn't even half a season on it. Fortunately I had saved the ones I took out. Popped that one back in and problem solved.