'84 Merc will not accelerate!

[westjr8]

Ok gentlemen,
I have an 1984 70hp Mercury. The other day while out on the lake i was carrying a lot of camping gear at full throttle, and all of a sudden the engine bogged down and wouldn't move faster than 11mph (before it was moving at about 33mph). I took all the weight off of the boat, and surprise, it was able to get back up to normal speed, 33mph. This problem had never happened before, as I have always rode with either 4 people in the boat or some camping gear. So the other day, I took 2 buddies out to do some fishing and with them on the boat I couldn't go faster than 11mph. Without them on the boat, im flying like a banshee again. This never was a problem before and my first thought what fuel supply/carb. I rebuilt and cleaned the carbs thinking this was the solution, and nothing... the same problem occurs. Any thoughts?

 

[jerryjerry05]

Re: '84 Merc will not accelerate!

Serial# Model any other info.Do a compression/spark test.That check OK then do the rest.
I'm guessing it's carbs.Start with a complete hose replace.All the hoses on the carbs and pumps squeezie too.Replace any filters.Install a water/fuel seperating filter.Clean the carbs and reset the float level(manual?)
Reset the air screw and rebuild the fuel pump.On the Force section is a sticky at the top read these and look for a linc and sinc.Make sure the idle is set right.Warm,in gear about 750-800rpm.J

 

[westjr8]

Re: '84 Merc will not accelerate!

The SN is 6230127. It is an '84 70hp mercury engine. 3 cylinders, 2 carbs. Other side issue is that if I have it in WOT and and then quickly slow down to idle, the engine bogs out... not sure if this problem is related.

 

[westjr8]

Re: '84 Merc will not accelerate!

bump

 

[CharlieB]

Re: '84 Merc will not accelerate!

That it runs fine lightly load yet bogs and die heavily loaded is a fuel supply problem.

Continuing to operate the motor with a restricted fuel supply WILL MELT PISTONS.

Faulty fuel pump, restricted fuel supply by an old deteriorating hose/primer bulb, restriction in the pickup tube/tank fitting, or air leaking into the system from a loose hose clamp, poor sealing connector, or cracked hose.

Any or all of the above. Check them ALL. Rebuild the fuel pump, replace all fuel hoses.

 

[westjr8]

Re: '84 Merc will not accelerate!

Before the season I got a new tank and a new fuel hose from the tank to the motor, including a new bulb. The first few times i took the boat out, it ran fine. And then the problem started occurring. CharlieB, do you know where I can get a new fuel pump kit from?

 

[westjr8]

Re: '84 Merc will not accelerate!

Ok, so I just rebuilt the fuel pump. I also removed the quick connect from the gas tank hose to the motor, so now the gas tank line goes directly to the fuel filter. No dice, still won't get up to speed. I also noticed the engine makes a very low noise while WOT, it used to make a slightly higher pitched sound at WOT. Any clues?

 

[westjr8]

Re: '84 Merc will not accelerate!

Finally got a chance to check the compression, and of course, cyc. 1 was putting out about 30psi. I was able to scope out the cylinder with a camera and discovered severed dimpling of the piston wall. After removing the exhaust plate, I verified that a ring had busted off the piston and probably dinged up my piston! So right now im in the process of overhauling the powerhead!