40 hp 3 cylinder bucking and sputter
I have an older 40hp that I purchased with a boat. the motor starts and idles fine. When I apply throttle the motor runs very rough and sputters. It also bucks very hard. I have removed and cleaned the carbs, bypassed the fuel filter completely (O-ring can be an issue). All coils are developing at least 30 kv as per my spark tester at idle. The timing at idle is ~12 deg btdc. It jumps like crazy when the motor is under load and bucking and is not readable! Here is the weird part last year it did the same thing except when you put it into a starboard turn, it would then pick up and run fine until you turned to port, and then it would run really rough again. I have gone over every wire, connection and fuel line looking for a culprit, no luck. Now this year it just runs rough and no power at all, yet it idles fine and you can run for hours at very low rpm. I have sprayed carb cleaner, run fuel conditioner, and done about as much as I know how to do. I will check compression next, should have done it long ago.
I have read fuel pumps are a problem. How do you check them.
Thanks in advance for any assistance you can bring to this vexing problem