1983 mercruiser 470 runs bad after 1/2 hour

[firemanlowden]

Just got this boat 2 weeks ago and runs great for about 30 min then bogs down at high rpm and can only run at a low rpm to keep it from backfireing though the carb and stalling. I cleaned the carb and put new fuel filter and fuel line on it. still have the same problem. the engine runs at normal operating temp too. has new cap roter wires and coil. The intake seem quiet hot to the touch could it be a vapoer lock? The engine has a hard time turning over after a while of running too. Any Ideas. I like the motor just need to figure this out. How can I fix it??? Thank you. You guys have been so helpful in the past and recomend to anyone with a boat.

 

[guyjg]

Backfire at high RPMs is a lean condition. Worn fuel pump?

 

[firemanlowden]

Good point I will have to look at it. mabay the diaphram is bad. mechanical fuel pump.

 

[looseplay]

Have you found the problem, I am having same issues.

 

[sandkicker]

Backfire at high RPMs is a lean condition. Worn fuel pump?

Backfire thru carb also indicative of leaky or burned intake valve. Check compression on cylinders.

 

[firemanlowden]

Replaced fuel pump and still have the same problem after about a hour of running hard. will start to turn over hard after shut down and try to restart after 20 min or so. temp is still normal too. ???? runs great till it warms up. can runn full trottle till then

 

[guyjg]

"...will start to turn over hard after shut down..."

Pull the spark plugs and see if it cranks better and if water comes out of the plug holes. The engine may be getting a little fluid in the cylinders from a bad manifold or gasket. Read the plugs: tan to medium brown is good, white is too lean, clean is liquid intrusion.