115 Merc engine dies at WOT

[woodsyfeller]

I bought a used 115 1979 inline 6 Merc.Took out this weekend and ran it around for a test drive. Idles fine after I adjusted throttle cable(was running at higher rpms at idle till I adjusted it) Runs fine when in gear from idle to 3/4 throttle.Only issue is when i got to WOT it will run for about 20 seconds then just dies.

It wont start up again until I reprime the the fuel line.Once it starts again it will run fine up to 3/4 throttle. I am guessing the issue is either;

1-A vacum leak in the fuel lines that doesnt seem to be a problem until I run engine at higher rpms and the engine cant keep enough fuel in the carb resivior because to much air is getting in.

2-A bad fuel pump that is going out and cant pump fuel fast enough at WOT

3-Carbs may need to be rebuilt

The fuel line is currently hooked up to a portable 6 gas gallon tank that I use for my Jonson 9.9 motor.I bought a new universal fuel assembly and put a Merc engine fuel adapter on motor side and a Johnson tank adapter on the other side.

What I would like some suggestions on is ways to rule out the possible problems.

I really think its a vacum leak somewhere in the fuel line and was wondering what methods I can use to check the new fuel line assembly.

 

[aussieflash]

Re: 115 Merc engine dies at WOT

Check all Fuel hoses for cracks,leaks and loose connections.If good start with the fuel pump and inspect the diaphram.Dont run engine in this state as it will be running lean and could cause damage.

 

[Faztbullet]

Re: 115 Merc engine dies at WOT

Also the supply line for the Johnson may be to samll ,need to be at least 5/16id hose as some of the small Johnsons use a 1/4id hose

 

[CharlieB]

Re: 115 Merc engine dies at WOT

Also the supply line for the Johnson may be to samll ,need to be at least 5/16id hose as some of the small Johnsons use a 1/4id hose

Exactly!

That small fuel hose may flow enough fuel for low to moderate engine speed but totally fail to flow enough for full power, causing the carbs to run lean until they get low enough the motor starves and dies, out of fuel.

The grave danger is pistons melting while the carbs are running lean!

Get REAL fuel fittings and hoses before you melt down your motor!

 

[Chris1956]

Re: 115 Merc engine dies at WOT

Woodsy, I always found that the original fuel pump on that motor was barely adequate. Make sure the pulse lines are in good shape, and clamped securely. If you have a 5/16" or larger fuel line, you can remove all the fittings and stick the bare end into the fuel tank. if the motor still starves for fuel, it is fuel pump or fuel lines. If the motor makes WOT, it is likely the fuel line quick connect fittings.

 

[woodsyfeller]

Re: 115 Merc engine dies at WOT

Replaced all the fuel lines and rebuilt fuel pump today (the diaphram was trashed im actually suprised it ran at all) ran well on the hose, I'm going to take it to lake tomorrow and see how it performs .I should of done the fuel pump first, I guess thats how you learn. The primer bulb actually gets firm now like it should.I will post tomorrow and give a report if it took care of the issue.

 

[woodsyfeller]

Re: 115 Merc engine dies at WOT

Rebuilding the fuel pump did the trick can now run at WOT and reach speeds almost to 38 mph/GPS. I can now change speeds without worring about engine cutting out.Now its time to go kill some halibut.