1975 Merc 500 shut-off at WOT

[boatmanwayne]

After decarbonizing powerhead & exhaust on 50 hp, when I run at 3100 or higher rpms engine shuts down like the key is being turned on and off, throttle back a little it runs fine. Checking timing I find timeing goes from 35 BTDC back to around 15-20 BTDC when advancing throttle past 3100 rpms. Could my switchbox be going out?

 

[12Footer]

Re: 1975 Merc 500 shut-off at WOT

Check the tanks ,carb bowls for water . Check tank vent to.Does primer bulb go soft when it does this?

 

[boatmanwayne]

Re: 1975 Merc 500 shut-off at WOT

Rechecked all fuel possibilies everything is good to go, Also all coils have been replaced due to previous problems. distributor & wires clean and good,timing is set. Is there a test for the switchbox other than ohms test I could do?