Hi boaters. My previously running 1976 Merc 500 will not stay running more than a few seconds. If I give it some choke as it's dying, I can keep it running for a little while longer but as soon as don't choke it, it will die. I took it out to the river a few days ago and it ran fairly well for around 20 minutes until it just died. According to my tach, it was running around 4800 rpm at full throttle. It did not like to idle very well and died a couple of times earlier but restarted before finally failing to start. Here is everything I have done since this issue has started:
Checked compression, 125, 122, 123,121. Checked spark on all plugs and it will jump 7/16 gap on all of them. Changed out fuel line from tank to engine including primer bulb and filter and bayonette. Disassembled both carbs, cleaned with carb cleaner and blow out with compressed air. Changed the built in fuel filter diaphragm and gaskets. Set the float on both carbs according to the service manual. Replaced spark plugs with Champion l76v. I have disconnected the kill switch wires from the switch box.
The primer bulb does get hard but squeezing it while running does not keep motor from dying. I have checked for bad or missing ground wires but haven't found any. I have bypassed the tank pickup by placing the end of the line directly in the fuel. I have bypassed the motor fuel inlet by connecting the fuel line directly to the tee between the carbs. It sure seems like a fuel delivery issue but I can't think of anything else to try.
Checked compression, 125, 122, 123,121. Checked spark on all plugs and it will jump 7/16 gap on all of them. Changed out fuel line from tank to engine including primer bulb and filter and bayonette. Disassembled both carbs, cleaned with carb cleaner and blow out with compressed air. Changed the built in fuel filter diaphragm and gaskets. Set the float on both carbs according to the service manual. Replaced spark plugs with Champion l76v. I have disconnected the kill switch wires from the switch box.
The primer bulb does get hard but squeezing it while running does not keep motor from dying. I have checked for bad or missing ground wires but haven't found any. I have bypassed the tank pickup by placing the end of the line directly in the fuel. I have bypassed the motor fuel inlet by connecting the fuel line directly to the tee between the carbs. It sure seems like a fuel delivery issue but I can't think of anything else to try.