mercury 35

[weiss lake]

I have a 1986 mercury 35 that will crank and run great 50% of the time but sometimes it will crank, idle rough or low and will really bog down as you try to run it. I thought it might be a fuel problem so I replaced the fuel filter and it still does the same thing. Now i am thinking it may be that the choke is closing and making the engine bog. It will run poorly for a while then i can crank it up 5 minutes later and it will run great then the next day it will run poorly again. Any suggestions?

 

[rat boat]

Yes, make sure that that the choke is tight, some times it can fall in to closed. Check the fuel pump, some times the spinning part can loosen up and you won't get good fuel feed. If the choke is good, disconnect the pump and put a bunch of gas in the carb bowl. Try this a few times and the results will help. If it runs well, do the fuel pump and you can clean/replace the fuel lines. If it does not work, clean the carb, especially the jets, or just do this anyway after the other stuff.

 

[roscoe]

Check the basics first.
Spark on both plugs?
fuel to cylinders?
compression check?

When it is running poorly, do a spark drop test by pulling one plug wire at a time to see if it makes a difference.

Is fuel bulb staying firm?

Examine fuel pump diaphram for holes/tears.
Might as well just get a rebuild kit for the pump, they don't last forever and it is 35 years old.

 

[weiss lake]

Thanks for the responses. Sounds like I may have a fuel pump problem. I am pretty sure the bulb does not stay firm but I will check the next time I am with the boat. (at work now). it is just odd that it will run fine for 1/2 a day and then switch over to running rough. The last time I had it out it ran great and when I was pulling in to the dock I felt it lose power and then it would barely idle to get it onto the trailer.

 

[GA_Boater]

Yes, make sure that that the choke is tight, some times it can fall in to closed. Check the fuel pump, some times the spinning part can loosen up and you won't get good fuel feed. If the choke is good, disconnect the pump and put a bunch of gas in the carb bowl. Try this a few times and the results will help. If it runs well, do the fuel pump and you can clean/replace the fuel lines. If it does not work, clean the carb, especially the jets, or just do this anyway after the other stuff.

Do you know what you are talking about?

A spring holds the choke open and the fuel pump has no spinning parts.