1977 inline 4 85hp merc fuel problem!!

[bryanwess2000]

To start off I had previously been using an electric fuel pump with this
motor after a few failed attempts to get the mechanical fuel pump to work.
So I decided to try again with the factory pump for safety reasons, my daughter now rides in our new(used boat). Anyway the problem is starting
out to get on plane sometimes it will bog down then cut off. If I prime the bulb on the fuel line after i start the motor then immediately go full throttle
I have no problems once on plane,the motor runsfine until I try to go over
32mph then after a few seconds it will slow down to 29 or so then I throttle
back a few seconds then i can throttle up to 32mph. With the electric fuel
pump it would run 36.5mph. Sorry no tach yet so I don't know rpms. The motor also dies after coming off plane and idleing for a minute.
I rebuilt the fuel pump except for the check valves which were replaced
a year ago. I also replaced the hose clamps on the fuel line with screw type hose clamps. The fuel line is just a cheap one from walmart,about a year old with no use time.

I wanted to know if it sounds like I have a fuel line/pump issue or a carb issue. I really want to sort this out for safety reasons, I don't like running the electric fuel pump with my daughter on board. And also for fuel economy with the mechanical fuel pump I can get 5-6mpg at 30mph, with the electric fuel
pump I can only get 3-3.5mpg no matter what speed. Any Ideas on helping diagnose the issue ? p.s I cleaned the carb about 3yrs ago and the motor has ran fine until I switched over to the mechanical fuel pump .

 

[2 stroke]

Re: 1977 inline 4 85hp merc fuel problem!!

If you opened the fuel lines at the fuel pump some trash easily could have made its way to the carbs. Pull them off clean em out, you don't have to rebuild em. If this does'nt help you might have your power pack tested.

 

[Chris1956]

Re: 1977 inline 4 85hp merc fuel problem!!

I have serious doubts that the electric fuel pump gave you more top speed. I suspect your test results are flawed.

Pump the rimer bulb continuously and see if the motor stays at full speed. if it does, disassemble and inspect the pump and pulse lines. The pulse line sometimes had a spring inside them to keep them from collapsing. Also check fuel line for air leaks.

 

[OkcOffRoader]

Re: 1977 inline 4 85hp merc fuel problem!!

my 85HP merc had those same symptoms, then it finally started spraying fuel out of the top carb whenever it would start...check your carb floats and see if one is full of gas, it only takes an hour and it may or may not be your problem, but its free to check....

 

[bryanwess2000]

Re: 1977 inline 4 85hp merc fuel problem!!

I retightened all the fuel line fittings and cleaned the carbs and reset
the floats to 11/16 off the carb body, the float levels were out of spec
one too high and one to low. This resolved most of my problems.
The boat will now run up to 34mph constant, but will cut out if I run any
faster. I'm gonna try another primer bulb next. I'm thinking it's an air leak
some where in the fuel line or fuel pump. And I'm positive the boat ran faster
with the electric fuel pump because I checked more than a few times. The boat weight and gear didn't change. the electric fuel pump kept the motor from leaning out in the upper rpm range. It will run 36-36.5mph gps with the
electric fuel pump 37.5mph on my other boat. thanks for the suggestions.

 

[CharlieB]

Re: 1977 inline 4 85hp merc fuel problem!!

The electric pumps greater capacity may be masking an air leak into the fuel system.

As a test, install a length of clear hose between the fuel pump and carbs, look for any air bubbles passing thru the hose.

 

[bryanwess2000]

Re: 1977 inline 4 85hp merc fuel problem!!

thanks for the idea charlieb

 

[bryanwess2000]

Re: 1977 inline 4 85hp merc fuel problem!!

update! replaced the primer bulb with a mercury brand and the issue is resolved. I had the boat running full throttle at 37.3mph gps after a decarb
with seafoam.