1978 115 hp mercury

[frbal1960]

my mercury wont run full throttle it sounds like its missing when i go to full throttle but when i remove the sparkplugs and leave them out for a few minutes it will run full throttle till i back the throttle down to run slower then it wont go back to full throttle does any one know what could be the problem?

 

[Yepblaze]

Re: 1978 115 hp mercury

Are the spark plugs soaking wet with gas?

Is one of the carbs leaking gas?

Do you think it's maybe flooding out on a couple cylinders then running ok after you dry out a couple cylinders?

 

[frbal1960]

Re: 1978 115 hp mercury

there is no leaking of gas on any carb

 

[frbal1960]

Re: 1978 115 hp mercury

i was told by some friends that the motor may be carboned up and that would cause it to buck at full throttle

 

[Yepblaze]

Re: 1978 115 hp mercury

You could work over the carbs. Clean the dist cap, or check the plug wires.

 

[Chris1956]

Re: 1978 115 hp mercury

That is atricky problem. It sounds like one plug is fouling.

What do the plugs look like? Are they wet with gas, it could be a bad fuel pump diaghram letting raw gas into the crankcase. If they are dry, it could be a weak fuel pump. If the plugs are very clean, it could be water infiltration steamcleaning the plugs...

 

[frbal1960]

Re: 1978 115 hp mercury

the plugs are dry and i just rebuilt the fuel pump thinking that was the problem but i was wrong i guess

 

[daveswaves]

Re: 1978 115 hp mercury

my mercury wont run full throttle it sounds like its missing when i go to full throttle but when i remove the sparkplugs and leave them out for a few minutes it will run full throttle till i back the throttle down to run slower then it wont go back to full throttle does any one know what could be the problem?

What do you mean by sounds like it is missing? Is it knocking or pinging on acceleration? What RPM is full throttle? Are you able to reach 5000 rpm or does it die out before you get there? Does it ever run smooth at full throttle?

There are filters in the top of the carbs that can plug and restrict fuel flow at high speed, causing a lean condition. You have to remove the fuel lines to get at them. They should be removed completely. Check your fuel pressure, should be at least 4 psi to provide enough fuel flow at WOT for that engine.

 

[frbal1960]

Re: 1978 115 hp mercury

yes it runs great at full throttle nice and smooth till i back off the throttle the it will start bucking when i try to go to full throttle I will try the filters then let you know what happens thank you and also i can get to about 10 mph on the speedometer i dont have a tach on the boat

 

[daveswaves]

Re: 1978 115 hp mercury

yes it runs great at full throttle nice and smooth till i back off the throttle the it will start bucking when i try to go to full throttle I will try the filters then let you know what happens thank you and also i can get to about 10 mph on the speedometer i dont have a tach on the boat

Unless you have an extremely big boat 10mph is not indicative of full throttle. There are lots of things that could be happening here. Your prop hub could be spun which would give you high rpm and no speed.