1987 Johnson 88 spl. The brief recent history is overheat episode back in May, head gaskets, thermostats, water deflectors, and water pump replaced. Initally experienced misfiring due to loose powerpack ground bolt, ran much better after tightening bolt.Now the motor is very sluggish when throttle is opened suddenly. It starts fine and runs fine at low or high throttle operation, but the performance is just terrible under load (acceleration). The symptoms are confusing. When the throttle is opened up, it will "chug" irregularly as if there is an ignition miss, but after 20-30 seconds it will clear up and run with no misfiring once it reaches high rpms. Operation is very smooth at WOT after it stabilizes. Which probably means it isn't misfiring at all. Where do I start looking? I think ignition is a dead-end, although I suppose one of the components may be slowly failing. The only check I have done is ignition coil resistance and resistances are all in range and very close to each other. Also I have sprayed carb cleaner at various motor speeds and this has accomplished very little except maybe the idle operation is slightly improved.Can these symptoms be caused by a vent problem with the gas tank? The breather valve is not operating properly, and sometimes when I fill the tank with gas, the fill line overflows because the vent is not opening as pressure builds up in the tank (once the fuel fill line clears I can add more fuel, so it is obvious the tank is not venting properly). What I am thinking is the vent may be partially plugged and this may restrict fuel flow when the throttle is opened quickly.How do I fix the vent problem? The vent comes outside the boat and there is a screen and a splash guard, but it doesn't look accessible to me.Any suggestions are appreciated.
As you accelerate and the motor starts to bog...try using the primer (push in on key). If the motor picks up then you've got a fuel delivery issue. As SoLittle suggests, time to remove and clean/rebuild the carbs.