MERCURY 40 runs great out of the water but in the water its a turtle!

[252SCH]

I have a mercury 40 horse 1991, 4 cyl, 2 carbs, oil injected. i bought this with a bass tracker 3 weeks ago, its been sitting for 3 years and he never drained or winterized anything. it started right up and runs great with the water mufflers on it in my driveway, took it out on the water and it just puttered like a dead dog, with full throttle it was even worse off. gas was shooting out of the top carb ,i checked the gas lines they were all fine but one had black tar substance so i replaced it. sprayed some decarb cleanerfrom the boat store it caused one hell of a smoke screen by my house, but help out a little, i took it out yesterday, ran a little better but only like a 10hp. i have no clue what else to check, motor sounds great out of the water but once under a load it does not want to get up and go, please help if you can.

 

[studlymandingo]

Re: MERCURY 40 runs great out of the water but in the water its a turtle!

I would recommend an impeller change. The water pump sitting that long causes the blades to form a memory. Before you have worse troubles be sure to replace that.<br /><br />As to the other problems, it could very well be tied to dirty carbs. Could have some varnish in the carb. Do a search on Sea Foam and Decarb, probably will take a more intense treatment than spraying some decarb in the carburetor once. With gas shooting out of the top carb, you probably have a needle valve stuck on the float. Try the Sea Foam first, if this doesn't help, you may be in for a carb removal, which is not the end of the world.<br /><br />Good luck and welcome to iboats!

 

[252SCH]

Re: MERCURY 40 runs great out of the water but in the water its a turtle!

thanks for the reply, i forgot to mention once i replaced the fuel line that was clogged i also took the fuel bowls and the needles had the varnish in it too. after i cleaned those off it ran a lot stronger and no more gas shooting out. is the impeller change a difficult task?

 

[252SCH]

Re: MERCURY 40 runs great out of the water but in the water its a turtle!

I HAVE A 1991 MERCURY 40 HP CLASSIC, I JUST DID A COMPRESSION TEST ON IT, CYLINDER 1 READ 130, 2- READ 130, 3- READ 125, AND 4- READ 120. CAN ANYONE TELL ME WHAT THIS MEANS, AND IF THIS HAS A LOT TO DO WITH WHY MY MOTOR RUNS FINE OUT OF THE WATER, BUT ONCE IN THE WATER IT DOES RUN NO BETTER THAN A 10 HP MOTOR.

 

[studlymandingo]

Re: MERCURY 40 runs great out of the water but in the water its a turtle!

The impeller change is relatively simple, the most difficult part is the lower unit removal and replacement. Still not a difficult procedure, I replace my impeller every season. There are generally 5 bolts holding the lower unit on; when reinstalling the L/U make sure the shift shaft and water tube are aligned, everything should go smoothly.

 

[wildbill59]

Re: MERCURY 40 runs great out of the water but in the water its a turtle!

Your jets are clogged also. Run a can of sea foam thru it and see if it makes a difference. If you have to rebuild make sure to spray all the passages out with carb cleaner. running your boat lean like you are will burn da motor up.

 

[roger3645]

Re: MERCURY 40 runs great out of the water but in the water its a turtle!

Your compression is good, the psi is within 15% of each other. I agree with the others. You have dirty carbs, and really need to clean them and change the impeller. Also buy a manual that'll help you alot. Read Awakening a Sleeping Engine in the FAQ section. It has info that will help you. Your problem is in the carbs.

 

[dajohnson53]

Re: MERCURY 40 runs great out of the water but in the water its a turtle!

Does Seafoam actually clean varnish, etc. from carburetors? It hought it was to loosen and remove carbon build up from the combustion chamber.