86 Merc 75 starves at full throttle

[Paul t 608]

My 1986 Merc 75 (4 cyl. inline) will take off and run for about 20 seconds at full throttle, then starves out like it's running out of fuel. I have choked it and it gives a burst of speed, but I cannot maintain full throttle. If I back down to about 1/2 throttle, it will run all day - no problem. I have checked the fuel tank pickups, vent is open, new primer bulb, tanks and hose are good. I replaced the fuel pump diaphram, new plugs, had both carburetors rebuilt, I run non-ethenol premium fuel, and only Mercury oil. Am out of ideas. I'm stumped. Anyone have any ideas? Sure would appreciate it. Thanks.

 

[daveswaves]

Re: 86 Merc 75 starves at full throttle

My 1986 Merc 75 (4 cyl. inline) will take off and run for about 20 seconds at full throttle, then starves out like it's running out of fuel. I have choked it and it gives a burst of speed, but I cannot maintain full throttle. If I back down to about 1/2 throttle, it will run all day - no problem. I have checked the fuel tank pickups, vent is open, new primer bulb, tanks and hose are good. I replaced the fuel pump diaphram, new plugs, had both carburetors rebuilt, I run non-ethenol premium fuel, and only Mercury oil. Am out of ideas. I'm stumped. Anyone have any ideas? Sure would appreciate it. Thanks.

If it happens around 3200 RPM it could be the high speed stator winding.

 

[Paul t 608]

Re: 86 Merc 75 starves at full throttle

Thanks, daveswaves! If it is the stator, how can it run wide open for a brief time? Or, does it run wide open, then fail, and it bogs. After it sits for a day it runs wot, then fails again? These are the symptoms, does that make sense to you?

 

[daveswaves]

Re: 86 Merc 75 starves at full throttle

Thanks, daveswaves! If it is the stator, how can it run wide open for a brief time? Or, does it run wide open, then fail, and it bogs. After it sits for a day it runs wot, then fails again? These are the symptoms, does that make sense to you?

If it is the stator it could fail like that due to heat. You can check the stator resistance after it dies, if it is open circuit then you have found your culprit.

 

[daveswaves]

Re: 86 Merc 75 starves at full throttle

Here is the specs on CDI,s website so you can check the stator. http://www.cdielectronics.com/downloads/Mercury_DVA.pdf

 

[Paul t 608]

Re: 86 Merc 75 starves at full throttle

Thanks again... will do some testing.

 

[CharlieB]

Re: 86 Merc 75 starves at full throttle

When you replaced the fuel pump diaphragm did you also replace the valves?

You still have a restricted fuel supply. Have you tried pumping the primer bulb to assist the fuel pump when this condition happens, if you find it improves, that will PROVE there is still a problem with supply.

When you hit the Choke it caused the fuel mixture to richen closer to normal, but as the fuel level in the carb bowls gets lower, the motor speed falls to that which the fuel pump is capable of supplying.

Disconnect the fuel line after the fuel pump, crank the motor and watch for a strong pulsed fuel supply, any weakness and you will run lean at or near WOT and burn the pistons.

 

[johnson20]

Re: 86 Merc 75 starves at full throttle

Stators don't act up like that... it would cut spark completely.

the cooper windings have a plastic coating and if cracked or warn , when the cooper expands and touches another wire it shorts system like a off switch.

I seen coils act up like that but not stator.


did you try pumping the primer when this happens?

 

[Donnieha]

Re: 86 Merc 75 starves at full throttle

Did you ever solve your problem?