Fuel starvation at WOT??

[Topol_86]

Recently bought a '73 Merc 85hp outboard...went through an ordeal to get it running and in the water, but after a weeks worth of work, she fired and runs great, except at WOT. It'll run wide open for up to 10 minutes, but after that, it wants to die out unless I ease back on the throttle, after doing which, it regains and picks up again. My thoughts are fuel starvation because the primer bulb becomes VERY soft. Brand new tank, brand new lines/primer bulb, could this be a fuel pump problem, and would holding the tank above the motor while running help determine if this is the problem? (My thoughts would be that gravity would help deliver fuel) also, as far as i can tell, there is no fuel filter except the one in the tank, and it's clean. Any help would be greatly appreciated.

 

[CharlieB]

Re: Fuel starvation at WOT??

Have you rebuilt the fuel pump? It may be weak and unable to provide enough fuel for WOT.

A soft primer may be an indication of either a restriction in the fuel line before the primer

OR

Air leaking into the fuel line anywhere before the fuel pump.

Test by temporarily replacing the fuel line from the fuel pump to the carb with clear hose so you can watch for any tattletail air bubbles moving thru the line.

Remember to switch the fuel line back once all testing and repairs are completed.

 

[B4Istart]

Re: Fuel starvation at WOT??

Disconnect the quick connect at the motor (if you have one) pump the bubble and see how quick you loose pressure. This can help find a leak. Also make sure you have a high quality fuel line. Even a new cheep one can cause fuel supply problems.

 

[Topol_86]

Re: Fuel starvation at WOT??

Thanks for the pointers, no, i haven't rebuilt the fuel pump yet, was just trying to get a sure fire way to make sure thats what it was and not something else...i'll try disconnecting the quick connect and see how quickly I lose pressure and also try the clear hose theory. Thanks again, i'll post back with results. Also, found a complete fuel pump rebuild kit from Oldmercs.com, was just wondering how quick they ship, and if there's any problems shipping to Canada. (I'm right on the border of US/Canada, so shouldn't be too long getting here provided no delays in Customs etc) Anyone with experience using this site please share the good and bad.

 

[daveswaves]

Re: Fuel starvation at WOT??

Thanks for the pointers, no, i haven't rebuilt the fuel pump yet, was just trying to get a sure fire way to make sure thats what it was and not something else...i'll try disconnecting the quick connect and see how quickly I lose pressure and also try the clear hose theory. Thanks again, i'll post back with results. Also, found a complete fuel pump rebuild kit from Oldmercs.com, was just wondering how quick they ship, and if there's any problems shipping to Canada. (I'm right on the border of US/Canada, so shouldn't be too long getting here provided no delays in Customs etc) Anyone with experience using this site please share the good and bad.

Which province?

 

[FishinInMizzou]

Re: Fuel starvation at WOT??

CharlieB - what a great idea! I bought a pontoon last month with the classic 50 on it. I have read everything I can find about the problem it has - it will run up to 5000 rpm's and then it bleeds off to 3000 and less. If you pump the bulb, it jumps right back up to 5K. I have read numerous recommendations from fuel pump to sucking air to a bunch of things. My Dad is an old boat guy (built his own at 12 yo and that was 60 years ago). He said it is sucking air somewhere. Your idea about putting on a clear hose sounds like a great idea. I will give that a try first, then I guess I will progress to the fuel pump - would you agree?
Ken

 

[Topol_86]

Re: Fuel starvation at WOT??

Just an update, I took the quick connect off, and the primer bulbs doesn't lose any pressure, taking the boat out shortly with a clear hose from fuel pump to carb to check for air bubbles. Btw, I'm in Sarnia, Ontario...5-10 min drive to Port Huron, Michigan, right at the mouth of the St clair River on beautiful Lake Huron

 

[daveswaves]

Re: Fuel starvation at WOT??

Just an update, I took the quick connect off, and the primer bulbs doesn't lose any pressure, taking the boat out shortly with a clear hose from fuel pump to carb to check for air bubbles. Btw, I'm in Sarnia, Ontario...5-10 min drive to Port Huron, Michigan, right at the mouth of the St clair River on beautiful Lake Huron

I boat in your water after you are done with it then send it to Lake ontario.
Boat on Long Point Bay. Keep chasing that fuel problem. Dont forget to check the screen in the fuel bowl fitting.

 

[sknight6534]

Re: Fuel starvation at WOT??

I have the same problem with my 60 horse merc. I can go WOT for about 30 second before the engine dies from starvation. I noticed the hose from the cylinder block to the fuel pump was not creating adequate pressure because the it was old and cracked. I haven't tested WOT with the new hose yet but I'll let you know the results when I do.
Scott

 

[Topol_86]

Re: Fuel starvation at WOT??

Bought a fuel pump diaphram kit from the local marine store...just a question, I have a larger rubber o-ring....and am unsure as to where it goes...inside the fuel pump there are two oval cavities where fuel sits on one side (with the in/out check valves) the rubber o-ring will fit these cavities perfectly, just wondering if thats where it actually goes, and if so, which side of the diaphram does it go on?

 

[Topol_86]

Re: Fuel starvation at WOT??

Bump...

 

[corm]

Re: Fuel starvation at WOT??

Hi,
That o-ring does not go to your motor.
The dia pump kit is for several different engines.

 

[Topol_86]

Re: Fuel starvation at WOT??

Thanks Corm, Much appreciated