Backfire at 4000rpm 350 Mercruiser

[dipthenet]

Hello,
I am new to the I/O world. And its been many years since actually turned a wrench on a real motor, not a computer. Heres my issue, i ran my boat Sunday for the first time since purchase. She seemed to run ok up through 3800, when i tried to open it up it started backfiring. The owner was unsure on fuel age so I added some fresh fuel and fuel stabilizer. Could it be the fuel? Thanks in advance for the help.

 

[gadget73]

Re: Backfire at 4000rpm 350 Mercruiser

Sounds like fuel issues. change the fuel filter first, and dump the old filter into a jar to see what you're working with. If a fresh filter doesn't help, you might want to drain whats in there for lawnmower or whatever use, and refill with fresh gas.

Also, depending on what carb you have, there may be another filter at the carb inlet. The Quadrajet uses this. If you have a Qjet, use a line wrench to disconnect the fuel line. That brass fitting will turn to mush with an open end wrench, then you'll be in there with vise-grips cussing at it. Don't ask me how I know this.

 

[Fun Times]

Re: Backfire at 4000rpm 350 Mercruiser

Sounds like fuel issues. change the fuel filter first, and dump the old filter into a jar to see what you're working with. If a fresh filter doesn't help, you might want to drain whats in there for lawnmower or whatever use, and refill with fresh gas.

Also, depending on what carb you have, there may be another filter at the carb inlet. The Quadrajet uses this. If you have a Qjet, use a line wrench to disconnect the fuel line. That brass fitting will turn to mush with an open end wrench, then you'll be in there with vise-grips cussing at it. Don't ask me how I know this.

.[LOL]I know what you mean:redface:. Dipthenet, What style and year engine do you have, Carbed or EFI?

 

[FreeBeeTony]

Re: Backfire at 4000rpm 350 Mercruiser

Sounds like possible water in the fuel.......

 

[mycirus]

Re: Backfire at 4000rpm 350 Mercruiser

I had the same thing happen with mine and I troubleshot fuel and electrical first. Turns out I have a bad valve and dead cylinder. I would check your compression first and make sure you have a healthy motor.

 

[xltier]

Re: Backfire at 4000rpm 350 Mercruiser

could very well be the coil breaking down.seen two this month.one 4.3 and one 5.0.one had green corrosion on the wire connectors.hth

 

[dipthenet]

Re: Backfire at 4000rpm 350 Mercruiser

Thanks everyone. I do not know the year, the boat is a 89. But I know the motor was replaced after a water pump failure. Its a 5.7 chevy, thunderbolt ignition with a carb. I replaced the fuel filter before I took the boat out, I didnt notice any kind of water separation, but did notice some big chunks of crap in the filter. I will remove again and visually inspect. The boat ran ok at 3600 and 3800. I plan on trolling around 1000 rpms alot, would it be a problem if I ran through the fuel that way instead of draining? I have two tanks, one of which has about 75 gallons in it and the other has about 40 so I have no easy way to drain and use. Thanks again