'68 Merc 500, bad fuel pump? high speed sputtering.

[rogueflyfisher]

I've got a Merc 500 50hp outboard 1968. Runs fine starting out, at high speed, with a full tank.... then after several hours of trolling at low speeds... when you try to go full throttle, it kind of sputters and won't run as well as it should. New gas tank. Same problem.<br /><br />After running at full throttle (sputtering, and barely getting on plane)... the primer bulb will sometimes get soft. But, after pumping the bulb, the motor doesn't respond. It stays running at a "moderate" speed... not like it should at full throttle.<br /><br />This is about 1/2, or 1/4 tank of gas, now after trolling for awhile.<br /><br />After filling the gas tank again, and running at full throttle, the motor slowly starts getting back to a good speed.<br /><br />Could this be a bad fuel pump (diaphragm)?<br /><br />It starts and runs excellent in idle, slow speeds, and high speeds early on. Just after trolling around for a few hours, when you want to go fast, it hardly has the power to get the boat on plane.<br /><br />Why would it matter how much gas is in the tank?<br /><br />It seems to do better with a full tank, at the beginning of the day.<br /><br /><br />15 foot fiberglass runabout... 50 hp Merc. 500 - 1968.<br /><br />Any suggestions?

 

[Laddies]

Re: '68 Merc 500, bad fuel pump? high speed sputtering.

If it were a bad fuel pump you could over come the problem by punping the bulb, if the bulb goes flat when running at speed the line or tank is restricted beond the bulb, because it does it after trolling for a long period of time it's possible that a plug is fowling for some reason

 

[rogueflyfisher]

Re: '68 Merc 500, bad fuel pump? high speed sputtering.

Thanks Laddies.<br /><br />Actually we ran it today, and before running, checked the spark plugs. They've got oil on them and they should be totally dry right? So, I'm thinking that it needs to be adjusted to run leaner ??? Carb??<br /><br />Would that cause sputtering at the high speed, and why wouldn't it affect anything at idle, or low speeds?? It also doesn't have all the power I think it should to get the boat on plane.... but it's always later in the day, after running for a few hours. Does the oil residue build up on the plugs a little and make them sputter like that???<br /><br />Kind of frustrating, but fun to work on at the same time.