I have a 1969 Mercury Model 40, 4 hp that I got running tickety boo last summer. I rebuilt the carb, rebuilt the fuel pump, replaced the fuel hoses, replaced the impeller and a couple of lower unit oil seals and it ran like a new motor. Last week I took it out for the first ride of the year for that particular motor and it was running fine at WOT for 10 minutes or so. Then I throttled down to idle for 30 seconds or so to adjust some gear in the boat. When I cranked the throttle again it wouldn’t run at full RPMs. I limped back to my dock and I have now tried the motor in a barrel.
What I found running it in the barrel is:
The plug was fouled. Cleaned it and reinstalled to start the motor.
The motor starts and runs relatively normally up to 1/2 or 2/3 throttle, then sort of bogs. I say relatively because there does seem to be a slight surging to the way it runs (the gas line ball is hard at all times and squeezing it does not impact the way the motor runs, by the way). My hunch is that there is an ignition problem of some sort, as it smokes when it bogs, too. It seems like the ignition is not correctly advancing with the carb throttle after 1/2 to 2/3 throttle. The smoking, rich, and unburned gas is the cause of the plug fouling I assume. I haven’t yet had the flywheel off, but that is my next step once my puller is returned from being lent out.
Does anyone have any insight for me? Any tips on what direction I show proceed to diagnose this?
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