I’m workin’ on a 7 1/2 HP “Merc 75” for a neighbor. SN 4746815, which I think puts it in the late 1970s. I’ve got it running great at all speeds above idle, but as it idles down, one cylinder cuts out. I can vary the RPM and throttle position it cuts out at some with the low speed mixture screw, but one cylinder still cuts out if I set the throttle low enough for a nice trolling speeds. Increasing the throttle cuts that cylinder back in like turning on a switch.

I have new spark plugs (NGK surface gap). I’ve disassembled and cleaned the carb and its attached fuel pump. Everything in the carb and pump looked clean when I opened it up, but I sprayed carb cleaner through everything and made sure all passages were open anyway. I removed the brass screw over the low speed needle valve tip and cleaned well in there, and all passages appear to be open. When I get the smoothest, slowest, low speed with both cylinders running (but still not idling as low as it should go), then if I shut it off, it won’t start again until I back out the low speed mixture screw about 1/2 turn, and that makes the cylinder cut out at a slightly higher speed and idle is not as smooth. I’ve used my own gas tank that I know works with other Mercurys, as well as the tank that came wit it – no difference. No ethanol.

I have checked all the ignition wiring and replaced a couple wires that had insulation flaking off. I made sure no wire is shorting or going open as the throttle is moved through its entire range. I disconnected the wire from the stop switch to the ignition, as the switch was broken anyway,m so that’s out of the picture. I have a dark side factory shop manual covering 1975-1978 models, and found the only timing adjustment is done at WOT, and that adjustment is just an advance limit at WOT and does not affect timing at idle. The ground wire screw for the ignition module was slightly loose, but tightening it didn’t help.

What are your thoughts on this??
Thx,
Dave