75 merc

I am having the same problem with my motor except it runs good on the top end.  I have a  1990 90 hp force and it will idle fine but when I put it in gear it wants to die out but if I can get in gear and hammer down it will run really good on the top end but then when I slow down to a troll speed with it it runs for about 20 seconds and then dies out again...I had it in the shop for 3 weeks and just got it back last weekend and put it in the water and it ran worse than when I first brought it there...360 bucks for nothing...so it went back!

Just a thought - one experience I had with an old 35 HP Johnson. It did the same thing ran great just couldn't get any speed out of it. ------There was a pin hole in the gas line and it was sucking air. I got real lucky - found it just by picking up the line in just the right spot when it was running. Held my finger over the hole and it ran great.

Not an outboard mechanic by any means - just sounded like the problem I had.

Good Luck

I had the same problem, and it was a pinhole in the the gas line, they should be replaced every couple of years.

I would start with new fuel line and a new ball and don't us a walmart one they are junk buy a good one, then also change both the plugs on each end of the fuel line, they are plastic and crack sometimes.  If it doesn't work after that I would pull the Carbs off the motor and thouroghly clean all of the air ports and jet holes out and clean out the fuel line to the carbs.  This usually fixes everything.  Just pay close attention to how all the linkage and hookups came off when you take carbs off...its not that hard and saves lots of money!

I had the same problem with more then 1 boat and both times it ended up being spark plugs always start with the cheap stuff first then work your way up.

Some of these older motors have a dried up diaphram or worse yet cracked and the air gas mix will then be wrong.   Several guys I work with that have older boat/motors had this problem diagnosed by their boat mechanic. 

thanks for all the tips, this may also sound like a stupid question but this is a 88 merc, would this have a fuel pump on it? and the right of the motor when looking at it, I am really stupid when it comes to boat motors, it looks like it could be a fuel pump, in the process of switching all the fuel lines, and just curious about that box, it would be between the where the gas comes in the motor housing, and the fuel filter.

quite possibly the fuel pump. i had a similar problem with my 94 75 merc i had it at two different mechanics one rebuilt the carbs and the other adjusted the timing. the motor still did not run at high rpm. i personally replaced all gas lines and that did not help. i ended up buying a rebuild kit for the fuel pump for $25 and less than an hours work it ran better than it has since i owned it.