78 mercury outboard power problem

Scott Hebert

Recruit
Joined
Oct 19, 2010
Messages
3
i have 78 mercury outboard 115hp,when i bought the boat it would bogg down at full throttle from 5000 rpm to 2300,had in the shop and was told electric choke was stuck on.i have since got boat back it is still bogging down at full throttle,but it boggs down to 4000 rpm now,mechanic said everything checks out try running fuel cleaner thru it which i have done and still no change,have changed fuel water seperator also.seems like something simple but not sure what to do now.any ideas?thought about cleaning carbs but figured mechanic would have suggested that also.guess he is stumped too!
 

Chris1956

Supreme Mariner
Joined
Mar 25, 2004
Messages
27,036
Re: 78 mercury outboard power problem

Scott, it could be a few things. While running at WOT, have someone pump the primer bulb. If that fixes it, rebuild the fuel pump and clean or replace all the fuel filters. That motor likely has stainless steel screens under the brass fittings that alow fuel into the carb covers. Sometimes they get full of junk.

If the primer bulb is collapsed at WOT, the fuel tank is either not vented properly, the antisiphon valve is gunked up, or the fuel line is too small or partially clogged. A 3/8" fuel line is best.

Post back with the results.
 

Scott Hebert

Recruit
Joined
Oct 19, 2010
Messages
3
Re: 78 mercury outboard power problem

chris,i checked carbs for screens but found none,i replaced all fuel lines going to carbs one of which i think was leaking air,took it out today and at wot it went to 5000 rpm then slowly bogged down to 4300.so we pumped the line at wot and it would finally run at 5000.so i got to looking at fuel line coming out of tank realized it was 1/4 in so i swapped it to 3/8 and when i pump the fuel its seems to be holding but ran out of time to take it back out.when my mechanic had it earlier this year he put a new fuel pump kit in it,if i still have the same problem do you think i need to try another kit?
 

Chris1956

Supreme Mariner
Joined
Mar 25, 2004
Messages
27,036
Re: 78 mercury outboard power problem

Normally a fuel pump kit will fix the fuel pump for years. However, your mechanic should have seen the undersize fuel line and replaced it. 5/16" fuel line is the absolute minimum for that motor. 3/8" is what you want, however.

Check the fuel pump screen under the black cover for dirt, and all connections. The fuel pumps can be marginal, so inspect the pulse lines as well. If you have an external fuel filters, you might check them as well.
 
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