1975 mercury Merc500 50HP running out of gas at High Speed

[mstone1968]

Hello,
A friend of mine recently bought a boat with the above motor. It had set unused for several years when he purchased it. Would anyone have any suggestions on what would cause the motor to run out of gas at High Speed. If you run at low to med throttle, the motor runs fine and doesn't stop. But when you start running at full throttle, it will run for a while, then it just suddenly runs out of gas. When priming the bulb on the gas tank, gas comes out of the carbs. Is that normal? Any suggestions would be very appreciated. Thanks for your help

 

[Don S]

Re: 1975 mercury Merc500 50HP running out of gas at High Speed

A bad fuel pump could cause it. Along with water in the fuel, air leaks in the fuel llines. Carbs may also be due for rebuild and be sure to check the float levels.

 

[imported_Voyager]

Re: 1975 mercury Merc500 50HP running out of gas at High Speed

If you go to mercurypartsexpress.com and enter the s/n of the motor, you can access exploded views of the various motor components.

One Possibility: If this motor has carburetors with integral fuel pumps, the diaphragm of one or both carbs could be failing. The integral pump uses pulses of vacuum/pressure from the motor to move a diaphragm that moves the fuel. This works to some extent with a small hole in the diaphragm, squeezing the bulb would push fuel thru.

Rebuilding the carbs would be a good idea, running lean (which will happen if you advance the throttle to just before the problem) will cook the motor.

 

[JB]

Re: 1975 mercury Merc500 50HP running out of gas at High Speed

One long shot possibility.

When my son's Merc 90 started doing that I found a pinched fuel line. Just couldn't supply gas as fast as he was using it at WOT.

 

[mstone1968]

Re: 1975 mercury Merc500 50HP running out of gas at High Speed

Hello Don S, Voyager, and JB

Thank you guys so much for the information you all have provided. We will check out all that you have pointed out and update you on the progress. He just bought this boat and it started up fine and sounds like it is running really good until you open it up at High Speed then it just runs out of gas. Thanks again everyone for your help. I will post an update.

 

[sut195]

Re: 1975 mercury Merc500 50HP running out of gas at High Speed

I bought a boat with a 1974 mercury 650 65 horse which had been sitting for a number of years. I tore down the carbs and cleaned them,all new fuel lines and fittings. It runs great at 1/4 throttle but wont pick up fuel to run faster. If you pump the bulb it will run smooth at wot. i put in a new fuel pump diaphram and was at the limit of my knowledge. The local boat shop put in a new diaphram and said it must have excessive blow by and not enough vacuum to pull fuel.i am not convinced and am looking futher. The shop also told me to put a electric fuel pump on it to solve the problem. I want ton fix the problem not get around it.

 

[Chris1956]

Re: 1975 mercury Merc500 50HP running out of gas at High Speed

I think it is a fuel restriction or a bad fuel pump. Maybe you should replace all the guts of the fuel pump (checkvalves, gaskets etc). Also make sure the pulse lines that run the fuel pump are in good shape and well clamped. Try an external 6-gal tank to remove the built-in tank from the problem list.

 

[JB]

Re: 1975 mercury Merc500 50HP running out of gas at High Speed

Hello, sut.

Welcome to iboats.

A note on forum etiquette. If you have a problem or question it is not considered polite to inject it into conversation on someone else's problem. Please start a new thread on your problem.

Thanks, and good luck.

 

[mstone1968]

Re: 1975 mercury Merc500 50HP running out of gas at High Speed

Update:
Took the carbs out and cleaned them and ran Sea Foam through the gas. The engine fires up on first try and runs great at slow speed and mid speed. As you start running at WOT. The ball on the gas line goes completely flat and looks as if someone has taken a balloon and sucked all the air out - it litterally is sucked dry. It is a brand new line and fittings on the gas tank. The gas tank is fairly new. We tried another gas tank and it does the same thing? Would anyone know what would cause this - it's almost like a vaccum is created sucking out all the air. Thanks for your help.

 

[tmcalavy]

Re: 1975 mercury Merc500 50HP running out of gas at High Speed

If the tank is vented properly and the primer bulb goes flat at WOT there must be a restriction...not pulling fuel from the tank.

 

[rustyscrew]

Re: 1975 mercury Merc500 50HP running out of gas at High Speed

I always think about the easy fix first. Whenever I sense fuel shortage, I feel the fuel bulb. If the bulb doesn't have fuel (i.e. sucked flat), then the engine is pulling fuel fine, but the lines or tank aren't giving it up. Possibilities:

1) crimped fuel line (proper diameter?)
2) closed or crimped vent (open tank and listen for vaccum)
3) bad connection of line to tank. External connectors often look connected but aren't.
4) Inline filter clog

To isolate, you could try using a different hose, and different tank.

good luck8)

 

[mstone1968]

Re: 1975 mercury Merc500 50HP running out of gas at High Speed

Hello Tim and Rustyscrew,
Thanks so much for the information. We will check all the hoses again and try another set of hoses and another gas tank. The diameter of the new hose is larger than the one that came with the boat so we will try another one. We will also check the filter for clogs. Thanks again for your help, I will post an update.

Thanks,
Mike