1994 Merc Ignition Issue Any assistance PLEASE

[jdsample]

Mid summer last season my 5.7 Merc would start and idle perfectly. I would take of and run the boat at cruising speeds for approximately 30 minutes and when returning to the dock and coming off the throttle and entering the no wake zone the motor would start to break up. It would run horrible but would not stall. I would pull up to the dock and shut it down, wait about 1 minute and restart it and it would run PERFECTLY again. The issue continued to worsen to the point it was starting to break up on plane and the boat would loose power. I took everyone's advice and started with all the easy maintenance items. I replaced plugs, wires, cap, rotor, and the coil too. We also removed and cleaned every ground on the motor. Ran the boat again....absolutely no change. While the motor was running very poor I had a mechanic with me that adjusted the carb to make sure it was not a fuel issue...definitely not a fuel issue, carb working as designed and nothing sticking. I preceded to throw a hail mary and I then replaced the ignition pick up that is on top of the distributor(under the cap) and I also replaced the ignition control module....the issue was finally gone!!! Boat has run perfectly for about 3 months....UNTIL

I ran the boat yesterday for 45 minutes and came off plane and into a no wake zone and it started to break up again and would idle but definitely did not want to. If i hit the gas, the boat would take off and jump right back up on plane. Anyone have any ideas??? No fouled plugs, good compression, Im at a loss here...

 

[Bt Doctur]

What is the engine temp, the riser temp and the voltage reading on the gauge

 

[alldodge]

had a mechanic with me that adjusted the carb to make sure it was not a fuel issue...definitely not a fuel issue, carb working as designed and nothing sticking.

With info provided I think you have a fuel issue

 

[jdsample]

Engine temp is staying exactly at 170 degrees. I have not checked riser temps. The volt readings are showing around 13.5 volts when cruising. I should have said in the original post that when the carb was being checked we did have a fuel pressure gauge on it and the pressure was right where it needed to be. I have also checked the fuel and do not have an contaminants. I did notice something else strange and really didn't think anything of it, but I guess I should have. The tach appeared to be off by about 400 rpms. I was told to disconnect the tach and run the boat and see it the problem goes away. I will be doing this Saturday morning. The tach could cause that big of a problem? Could it take out my Ignition Control Module again? I don't think this is the problem, but ill try anything to find the issue!

 

[alldodge]

If the Tach is shorting out it will kill the motor or make it misfire

 

[jdsample]

UPDATE- removed the wire on the back of the tach. Ran the boat at 3200 rpm for approximately 25 minutes, brought boat down to a low rpm speed- no wake speed. Engine was trying to stall and overall running poorly. Right away checked to see if the ignition control module was hot, it was only warm to the touch. I shut the motor down. Restarted it within 5 to 10 seconds and it runs perfectly. Ran the boat for approximately 10 minutes in a no wake zone and no issues. We stopped and had lunch for about an hour. We then took the exact same trip back to home port. The boat ran perfectly. When I came to the no wake zone at my home port- this time, the boat ran perfectly...did not sputter or act like it was going to stall. I have no clue...any help????

 

[alldodge]

Still sounding like a fuel issue. Could be you have a dirty carb or water in the fuel, and your running it enough to get it cleaned out.

Adjusting the carb while motor is running only changes the idle mixture. For a short time you had issues at higher rpm, but then this went away, now it sticks at the idle speeds if it does it at all.