1984 350 mercruiser running rough

[lx.crowell]

This is my uncles boat it is a 1987 chris craft scropion with the 350 mercruiser. The other day had it out and it started to dog out, wide open the motor was acting like it was on 1/2 throttle, we assumed it was starving of gas so replaced the fuel filter/seperator and my uncle being my uncle decied to replace the plugs also. When we took it out again it was worse had a misfire like crazy and when you would try and plane it would backfire and it sounded like a lawn mower. Took it out put new wires on it tryed it no better, put a new coil cap and rotor on a little better but still had a horrible miss. To me sounded like a preignition checked the timing it was at about 10 BTDC at 800 RPMs. Also did check the firing order and that was corrrect, not sure where else to go now but into the carb.

 

[alldodge]

This is my uncles boat it is a 1987 chris craft scropion with the 350 mercruiser. The other day had it out and it started to dog out, wide open the motor was acting like it was on 1/2 throttle, we assumed it was starving of gas so replaced the fuel filter/seperator and my uncle being my uncle decied to replace the plugs also. When we took it out again it was worse had a misfire like crazy and when you would try and plane it would backfire and it sounded like a lawn mower. Took it out put new wires on it tryed it no better, put a new coil cap and rotor on a little better but still had a horrible miss. To me sounded like a preignition checked the timing it was at about 10 BTDC at 800 RPMs. Also did check the firing order and that was corrrect, not sure where else to go now but into the carb.

Sounds like it's running real lean and back firing from that. You could check your fuel pump pressure (4-7 psi) but I'll guess it's the carb. Could also be bad fuel, and putting anything like sta-bill and the like will not help if the fuel is bad. Smell it, and if it doesn't smell like gas, drain it. Compression test could also provide input. If everything checks out need to pull the carb and rebuild it.

 

[garbageguy]

From your description, sounds like it ran worse after plugs replaced. May not be the answer, but easy to check if wires all going to the correct plugs. You might also check the screen at the carb inlet.