Merc Troubleshooting
Could be the carb, see below are some charts from my similar vintage Merc 40hp OB motor. Sound like you are STARVING for fuel, but some OBs have fuel restriction or ‘lean fuel’ alarms (OMC does on V4 & V6 models) but it doesn’t sound like that Merc does.
But here are a few other things I’ve experience directly with OB motors:
Cause – If your fuel lines are old and you’re running ethanol-based E10s fuels, I’ve seen the ID of the fuel lines deteriorate to the point where the OB would run at low speeds, but the vacuum at high speeds pulls the ID of the line shut, as the line is delaminating inside.
Solution[i/] – Running off a separate tank with a different fuel line will prove/disprove this.
Cause – If equipped with an anti-siphon valve on a fixed installation below-deck fuel tank, I’ve seen them clogged up with ‘naval lint’ to the point it limited full RPM operation and/or caused surging at high speeds, especially like after crossing a big wake.
Solution[i/] – If equipped, pull the AS valve and check it. The only place where it would be is right there at the fuel tank fitting connection. It looks like a typical barbed fuel fitting, but inside of it is a SS ball and spring, that gets overcome or pushed aside by the fuel flow. Most times you need to cut the fuel hose to get it off that barb, so you’ll lose an inch or so of fuel line.
Cause – If you had an AS valve and it was clogged, there could crap in the tank, you should then also pull your fuel pickup and check it. Some have a screen on the end, which was intended to protect and keep the AS valve clean/clear/debris free ... but these little screens are fragile and it could either be missing or also clogged up with 'naval lint'. (Nice visual, huh :shock: ?). If missing, I wouldn’t worry about it, less keep an eye on the AS valve for clogging during future high speed ops.
Solution[i/] – The pickups do come out ... mix a 50/50 small mix of acetone and Auto Trans Fluid (ATF) … which is THE BEST penetrating fluid you can get, better than Kroil or PB Blaster and the like. Wipe/weep around threads on boss to the aluminum tank where the square boss is that the fuel barb is stuck into. Use TWO box wrenches 180-degrees apart and with a good oompphhhhh with both hands equally ... and she should spin off easily. Use a good Permatex thread sealant, fuel rated, to re-seat.
Best I can do from cyber-space …