Is it timing, electric, or gas problems? 95 4.3 merc

[KBstewart]

Hello guys, I have tried searching but haven't found much that has my full problem in it. So here goes. I put a new/rebuilt 4.3l merc in my 95 stingray about 4 years ago. have logged many hours since. When I originally timed the boat I did it in the driveway with muffs only, never on the water, but I have not had any issues with it since. For some reason this year after running the motor for a period of time, say 1/2 hr or more, once the motor is completely heat soaked. It will start popping at around 4k and wont be able to get any higher in rpms, and when this starts happening, trying to trim up cuts the power to the motor considerably. Now when cold the boat runs absolutely great, i can trim up and have no issues with cutting the motor power, only when hot. The longer I run the boat the worse it gets, till it wont even get up on plane. The pop I hear sounds like an ignition problem, not loading up and backfiring. Here's what I have done to try and remedy the problem. I have cleaned all power and ground contacts, to and from motor and trim motor, replaced coil, replaced cap and rotor, and new fuel filter (the old one only had 1 hr on it). I have no visible arcing on plug wires, i have 12.5 volts on new battery before and after i get off water. I have 13.5 volts from alternator at idle. I don't know whether I should check timing after it gets fully heat soaked, or if my alternator is possibly breaking up at high rpms once it gets hot, or if the electronic ignition module is going bad. I don't want to just keep throwing parts at it. The motor cutting out with the trim thing is what has me stumped. I am generally good with motors and diagnostics, but this is baffling me. Thanks in advance.
Kyle from Upstate ny

 

[alldodge]

Howdy

Is it a 2 or 4 barrel?
The pop at high rpm leads me to running lean. This issue could be from a weak fuel pump, dirty carb or restricted fuel system.

Being a 95 it should have an electric fuel pump, but please verify?

Does great when cold and bad when hot could also be a bad connection on the distributor or motor

 

[KBstewart]

AllDodge: its a 4 barrel, electric fuel pump, and have gone through my connections. the pop starts at high rpms, then gradually works down in the rpm range till it wont even be able to get on plane. runs great for the first 1/2 hr- hour. once it starts doing this the trim will cut about half the motor power. i cleaned contacts in distributor when i put the new cap and rotor on. runs great at idle while in gear, and will get full rpms when in neutral.

 

[KBstewart]

also dont know if this helps but i put seafoam through it occasionally throughout the course of the boating season

 

[alldodge]

Thinking you have a fuel issue.
Start by removing the fuel filter and dumping the contents into something clear, like a jar or baggie. Let us know what you find.
Next pull the plugs and see what they look like and note the color and/or deposits if seen.

Then suggest getting a vacuum fuel pump gauge and put it inline between the fuel pump and the carb..

 

[Rick Stephens]

Might be:

• Coil shorting when hot.
• Fuel pump changing output when hot.
• Automatic advance no longer working when the module gets hot.
• Air leak on intake when hot.

 

[KBstewart]

AllDodge: Plugs are a nice dark tan with very slight black buildup around the inside. changed fuel filter before my last trip out fuel filter had absolutely nothing in it but gas, no water or any sediment whatsoever.
Rick: Have replaced the coil as that was the first thing that came to mind.

I did notice one thing while in the engine compartment that I had missed prior, on the back of alternator the ground wire was missing the nut. didnt notice it before since it is so conveniently tucked behind the coolant hose. Fingers are crossed that that may be it. Taking it out tomorrow to test it out. will throw the timing light on to check its advance. and maybe spray some starting fluid around base of carb for air leaks if the wire was not my problem.

Thanks for the replies

 

[Scott Danforth]

What does the cap and rotor look like?

 

[NHGuy]

Weird that an electrical load from the trim will do that. Maybe the power is sketchy and the alternator loads the engine. Sure sounds like fuel but does the electrical gauge report 14.3 while running? You said you load tested the battery?
What about checking the carburetor inlet screen/filter. You could try opening the gas cap when the engine starts to mess up, listen for a rush of incoming air as if the fuel tank vent is clogged..