Ask The Experts

[magic11305]

I have a 91 Mercruser 3.0 that has me stumped. It is missing and stammering. At idle I can rev the engine a couple of times and it purrs like a kitten.....for 5 or 6 minutes, then back to missing. While running on the water at around 3100rpm, it will run smooth for 10 or so minutes and then begen missing. When it is missing, it is not all the time, just very rough like, fuel starvation. I have changed the riser gasket, complete carb rebuild, new points and condensor, coil, and plugs. I have put Sea Foam in the tank thinking I got some bad gas. I checked all of the ignition wires and everything looks good. If anyone has ANY ideas PLEASE HELP. Thank you

 

[Rocky_Road]

Re: Ask The Experts

Your gas tank vent line may not be 100% clear...and the longer that you run, the more vacuum that your tank develops and slows down the fuel delivery.

Easy way to find out is to 'crack' the gas cap to allow air to pass...and take it for a spin!

Happy boating!

 

[Gary H NC]

Re: Ask The Experts

Are you running a fuel/water separator filter? Sounds like a fuel issue.
If it has water in it Seafoam will not remove the water.
Also the fuel pickup needs to be removed and checked.The screen can become clogged and cause problems.

 

[magic11305]

Re: Ask The Experts

I have opened the gas fill cap and ran the engine, same results. I do have a water seperater, changed it a few months ago, I will try a new one though. The boat was running fine until it sat in the lift for 3 weeks, due to high winds. Thank you for the responses, any ideas would be greatly appriciated.

 

[Gary H NC]

Re: Ask The Experts

If it has water in the fuel it may take a couple filter changes and running it for a period of time.
Those filters will only hold so much water then they bypass it to the motor.

Empty the old filter in a jar and let it set a while and see what you got.

 

[wire2]

Re: Ask The Experts

While it's idling rough, pull 1 spark plug wire at a time, each one should noticeably slow the engine. If one doesn't, there's a problem related to that cylinder.

Taking 5 minutes to show up sounds like a temperature sensitive issue. When it cools, it's running ok again?

If you can, run it at night, with no lights on. Look around the ignition parts for sparks or corona discharge (blue glow) when it runs rough.

 

[Don S]

Re: Ask The Experts

Is the choke working and adjusted properly?
You mentioned you rebuilt the carb. Did the idle mixure get readjusted after running the engine. The idle mixture screw adjustment stated in the manual, is only a preliminary adjustement you make to get the engine warmed up so you can adjust it to your engines condition.

 

[rad1026]

Re: Ask The Experts

That happened to me at Lake Powell. Would run fine then start missing and sputtering. I got it back to camp and went through everything for hours. Finally turned out to be a loose ignition wire. Every time I hit a bump it would loosen. Sounds more like a fuel issue to me but I thought I would throw it out there.