1976 50HP Mercury 500 4 Cylinder ?

I have just finished refurbishing a 1976 Magnum. And got the outboard running good. It felt good and strong compression was 131 -127 when I started. She was hitting 31 mph (GPS) at 5200 rpm. (ok, I need prop work).
When on the second trip out after warm up and a 1/4 mile no wake zone I slowly increased speed to just below wide open around 4800 -4900 rpm and had ran about 3/4 of a mile 4 minutes or so when she cut out. I backed off the throttle and she pick right back up and ran about another 100 yards then she died. I couldn't get her stared at all. Fearing I would burn up the starter. I gave up and had her towed back in. I tried again the next day and could only get a single fire every few turns.
Both trips were on the same tank of gas and Quicksilver 50:1 mix. I mention this because when tickering with her I pulled each new plug and checked for fouling and compression again, top three were AT 120 psi the forth was 90 psi what happened. any help guys!!!!

Re: 1976 50HP Mercury 500 4 Cylinder ?

Unfortunately,it sounds to me like you rebuilt the motor but didnt address the problem that caused the failure in the first place.Hope Im wrong but it doesnt sound good.Do you have a temp. guage/alarm on your boat?Did you thoroughly rebuild carbs w/new gaskets,needles,seats?Replace all fuel lines?Redo waterpump?Possible causes of failure include overheat,lean running condition,water intrusion into cylinder(s).

Re: 1976 50HP Mercury 500 4 Cylinder ?

starcraft1982 said:

Unfortunately,it sounds to me like you rebuilt the motor but didnt address the problem that caused the failure in the first place.Hope Im wrong but it doesnt sound good.Do you have a temp. guage/alarm on your boat?Did you thoroughly rebuild carbs w/new gaskets,needles,seats?Replace all fuel lines?Redo waterpump?Possible causes of failure include overheat,lean running condition,water intrusion into cylinder(s).

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I hope i'm doing this right! I'm new to forums.
No water temp. gauge but it was the first thing i check on the lake atfer removing the crowling. The head was just warm not hot.

Carbs were cleaned and inspected through out as well as both fuel pumps. I only replaced the carb gaskets, needles inspected they looked, all setting were reset to the exact position when the carbs were removed it sealed with no leaks.
Fuel pump new strainers and check valve diaphragms were replaced. the rest of the part were cleaned and inspected and a main line filter added. lines check and cleaned
Completely new water pump. always checking the witness stream never stopped or slowed.

Water intrusion worst nightmare my question is how? (crached head!).

Burned piston I could handle much better. Which bring up the next question. Do marine mechanic use what I would call a (bore scope?) to look inside the spark plug port to see if the damage to the piston or if the cylinder been scored?
I have been lucky up to this point and got a little discuraged and lost on which direct to go! I guess I just needed someone elses input to get the old thinker working again. Thxs for your suggestion starcraft. I need to crack her open and see what needs to be done!
Have a good one starcraft!

Re: 1976 50HP Mercury 500 4 Cylinder ?

Some mechanics do use a boroscope, but not many as they are 250 and up.Most go by the symptoms and have a 6th sense about whats wrong with them.You may want to buy a infrared/laser point pyrometer[heat gun] they go for 50-60 bucks and up. Its a great tool as you can shoot the whole engine and get intake, exhaust,carb, water temps and its nice to have around the garage for the car when that acts up.Could be electrical, coil,points,etc and just acted up before it let go.