1985 75 hp starves at WOT

[Paul t 608]

I've been trying to cure this problem for a year. Took engine to reputable, large Mercury dealer, and I stumped them. They called Mercury Marine experts, and they're stumped too. Engine goes to full throttle, then acts like it's running out of gas. Idles and runs at 1/2 throttle all day. Have replaced fuel tanks, fuel lines, fuel pump, plugs, stator, had carbs rebuilt, squeezing bulb or choking has no effect. I'm ready to give up on her. I have a line on another '85 75 that runs, but doesn't idle well. If it's problem is reed valves, wouldn't it rattle and not run at hi speed, too? Hate to buy another problem child. Any suggestions anyone?

 

[CharlieB]

Re: 1985 75 hp starves at WOT

Restart at square one.

Verify compression, spark test, then DVA test stator and triggers. Disconnect throttle cable and manually move throttle/timing linkage throughout its range while testing triggers to be sure they do not have a broken wire loosing contact when the wires flex.

Double check the carb linkage and timing is set exactly as spelled out in the service manual.

Temporarily install a clear hose and a pressure guage between the pump and carbs. Watch pressure and look for any air bubbles passing thru the hose during operation.

Use an inductive timing light during test run to verify continued ignition spark, if the light fail indicating an ignition failoure.

Repost your findings.

 

[PHE]

Re: 1985 75 hp starves at WOT

The reed valves only work at or near idle. After 1500 RPM or so, they are wide open. It will run fine at high RPM with a chipped reed. If a reed is bad, it will spit gas droplets out of that carb at idle.

Had a switchbox on a 78 merc that would drop a cylinder at 4000 RPM or so. Acted similar to your desciption. Check each plug wire using a timing light as CharlieB described. Do it on the lake in gear.

 

[Paul t 608]

Re: 1985 75 hp starves at WOT

Forgot to mention, marina checked compression, a.o.k. spark is fine too, also timing is correct. They also checked spark and fuel pressure while running, both were o.k. Mercury Marine mechanics said try a new switchbox, wouldn't the marina have detected this problem? I do have a Weatherby .300 Magnum that should pass completely thru the block if I can't get this thing running at WOT.

 

[PHE]

Re: 1985 75 hp starves at WOT

Switchboxes can get kind of flaky. Had one that would drop a cylinder at low RPM until it warmed up, but would run fine on the top end. Had another would drop a cylinder over 4000 RPM sometimes, but not all the time. Not saying that is definitely your problem, but I understand why Mercury would recommend changing it.