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1975 merc 50hp 500 runs great then dies at wot

Just one: Do you have an in-line filter BEFORE the fuel pump? If so, eliminate it and put one after the pump.

Jeff
 
I do have an inline filter before the pump to protect the pump. Ditch it? Ok will try tonight. What are your thoughts on that diaphragm pulling fuel up out if the bottom of the boat?
 
Is this a Mercury tank or another brand--------Picture of the tank fitting ??-------------Some aftermarket tanks are known to have problems with cheap hose INSIDE the tank.
 
Atwood tank 6 gallon from wal mart. With the two prong fittings ( I think omc or Johnson.) At the tank. And an inline glass filter. And a quarter turn mercury fitting at the motor. I'm at work so I can't take a picture.
 
Well I took the fuel filter out and ran a straight line to fuel pump did not work. Ran good for halfhour then did the same thing. Only thing left is switch 5/16 line to 3/8 line. Or carbs.
 
It's the coil!!!! Had exact same problem w/ my 1973 20HP Mercury, replaced entire fuel system to no avail. Then started checking ignition. Turns out that when the coil got warm after 5 or 10 minutes, it would short out leaving me running on one cylinder. Then when the motor got cooled down, the coil would work fine again-----until it warmed up after 5 or 10 minutes, then back to one cylinder.
 
You might be experiencing an air leak into the line which kills the fuel pumping action. If so, you have two choices here: Hard pipe (using tubing) the entire fuel system to eliminate any leaks, or install an electric pump at the tank to push the fuel to the motor.

Jeff

PS: Sorry for the delay in answering. That damn snow storm knocked off the power to my home for eight days.
 
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Did you check for good spark? The 36 year old stator or trigger may be failing from self generated heat.
 
duckhunterwa said:
Only other question I had was does anyone know what pitch prop I should be running on this motor? At low speeds it feels kinda like I'm "spinning my wheels" till i get on plane then it feels like it bites.
Click to expand...

I also have a 1975 Mercury 500 50hp. It had a 15" pitch stainless prop when I bought it. It is on a 16 ft. Dixie fishing boat(open hull). I plan to run in places where there is a real possibility that I may come in contact with some rocks so I bought an aluminum prop with a 17" pitch and put it on. It runs out very well with the boat I have with the top end being around 5000 rpm and cruising best at around 3900 to 4400 rpm. Depends on the weight of the boat and wheather you want to get out of the "hole" really quick.
I do however have a problem getting the motor to idle and not die when I put it in gear unless I catch it just as it is starting to react to my tapping the choke, shove it into forward and it will catch up and go great from there. Stll a work in progress.
 
You know, after having problems like that for years--off an on--I took some damn good advise and installed an electric fuel pump, along with a fuel pressure gage. Works slick!

The problem with the stock FP (as you have seen) is that they are WEAK. Any kind of air leak or obstruction and the motor leans out.

Jeff
 
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