Bogging at full throttle intermittently

[mrboatman3354]

Intermittent bog at full throttle

Hello! I have a 1999 40hp 2 cycle Mercury 3cyl. Very periodically I will all the sudden completely loose full throttle. During these times where this occurs the engine will misfire slightly and run not so great at lower speeds too. It's like a light switch that is entirely unpredictable.

I ran the boat for 3 days straight with no problem, and then boated across one of the largest inland lakes in Wisconsin, on my way back about 400 yards from the boat launch it happened just like that. I've replaced the fuel pump, fuel lines, fuel tank, and spark plugs to no avail.

It will always, except when the issue suddenly occurs at full throttle, purr like a kitten and runs great. It has good compression and oil consumption rates. My only two theories at this time is that one or more of the coils are starting to breakdown and it only is an issue when warm, or that gunk is in the float bowl that only periodically causes an issue. Any thoughts? Just need help with direction at this point.

*Note: problem seems very similar to when you forget to open the vent on your portable gas tank. I've checked it when this occurs and it's not the problem*

 

[jimmbo]

Welcome to iboats

Might be a Fuel Starvation issue, That last 20% of the Throttle Lever Arc, really increases the Demand for Fuel. You say you replaced the Fuel Lines. does that include the Lines between the Connector to the Pump, and from the Pump to the Carb(s)?
When you have the issue, have you looked at the Primer Bulb? If it collapsed, it would indicate a plugged Tank Vent
It could also be an Ignition Issue. As the Throttle is moved, the Ignition System is moving in relation to the Block, with wires Flexing. Broken/Corroded wires can create Intermittent Opens, loose or Corroded Connections too, As you mentioned Heat can cause Electrical issues, as can High Humidity

 

[mrboatman3354]

Welcome to iboats

Might be a Fuel Starvation issue, That last 20% of the Throttle Lever Arc, really increases the Demand for Fuel. You say you replaced the Fuel Lines. does that include the Lines between the Connector to the Pump, and from the Pump to the Carb(s)?
When you have the issue, have you looked at the Primer Bulb? If it collapsed, it would indicate a plugged Tank Vent
It could also be an Ignition Issue. As the Throttle is moved, the Ignition System is moving in relation to the Block, with wires Flexing. Broken/Corroded wires can create Intermittent Opens, loose or Corroded Connections too, As you mentioned Heat can cause Electrical issues, as can High Humidity

I did not replace the carb lines. The bulb is not sucking in at all and depressing the bulb when running in this condition changes nothing. Do you think I should start replacing ignition coils or clean the carbs first or at all? I do have some chewed wires standard with all old engines, but nothing that is exposed or seriously damaged or corroded.