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Tan, MerCruiser Certified Technician
Category: Boat
Satisfied Customers: 11077
Experience:  40 yrs. experience with complete engine repairs both inboard/outboard and MerCruiser Certified Technician.
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I have a 1977 Evinrude 85 HP. It opens to full throttle and

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I have a 1977 Evinrude 85 HP. It opens to full throttle and runs for a little while then bogs down like it is starving for fuel. I already changed the primer bulb which did help a lot.
JA: Sometimes things that you think will be really complicated end up being easy to fix. The Marine Mechanic I'm going to connect you with knows all the tricks and shortcuts. Tell me a bit more about what's going on so the Marine Mechanic can help you best.
Customer: When we prime the bulb and force fuel it wants to run fine. But when you stop forcing the fuel it bogs down again. My fuel filter glass does not remain full of fuel and the bulb does not stay hard and primed.
JA: Is there anything else important you think the Marine Mechanic should know?
Customer: The trip before this happened I ran it very low on fuel. Could it have picked up debris from the tank?

Hello, I'm Tan. Welcome to JustAnswer. I'm reviewing your question and will reply momentarily.

Your symptoms are fuel starvation due to failing fuel pump or a restriction in the fuel tank. Run the engine on a different fuel source. If engine runs better then you have a restriction in the boat side fuel system. If no change you have a failing fuel pump.

If the boat side fuel system is the problem inspect all fuel lines for kinks or deterioration, check the fuel vent line for kinks or blockage then finally remove the pick up tube from the tank and remove screen if you have one and check for debris blocking the tube or fuel supply line.

Post back with findings and questions.

Customer: replied 2 months ago.
That makes sense. Isn't the primer bulb suppose to get hard after priming? And isn't the fuel filter glass suppose to be full?

Once engine is running the fuel only flows through the primer bulb and filter. If they were on the pressure side they would stay hard and full. Squeezing the primer bulb can over come a restriction or act like the fuel pump if it is failing.

Customer: replied 2 months ago.
ok. It will run fine with less throttle and will also idle fine. If I run with less throttle for a while and when I push it full throttle it runs great. But just for a short while. It seems to run through a reserve of fuel and once it is depleted it starves again. What would be the best indication for that symptom?

Clogged or restricted fuel system. Most likely in the fuel pick up tube. Running on another fuel source will verify this as the problem. It could still be a weak or failing fuel pump. It takes a larger volume of fuel to run under a load at higher rpms.

Tan and other Boat Specialists are ready to help you
Customer: replied 2 months ago.
ok. Thank you for your help.

Your welcome. Post back if you need further assistance.

Thank you.....Tan