Mercury 25HP Wont Start

[kevGreatBearLake]

I have a 25HP Mercury (1980's motor I believe) on my pontoon boat and I cant get it to start. This happened last year also but then once I got it to start it started the rest of the season. I assume it has spark because it starts when I spray starting fluid in a cylinder or all around the motor (because if I dont have a spark plug out I dont really know where to spray it on the motor but I do know there has to be an air intake somewhere), It will run until it burns off the starting fluid then just die.

I have new gas so its not bad gas. The fuel filter fills when I pump the ball so I know fuel is getting that far. When I pull the choke and pump the ball and crank you can smell gas so it does seem I can flood it if I want (and the tips of spark plus seem wet..though that could be my imagination)..

It seems I can crank it forever and it doesnt even get close to starting

I switched gas tanks last year and it seems with this tank and new line the ball never stays hard. Not sure if it is always supposed to be hard or what. When I pump it hard the crank it seems to get soft. Not sure if this means the gas is going in or what. I also dont remember is the vent on the cap for the gas tanks supposed to be open or closed? I am thinking open but dont really remember

HELP????

 

[Mi duckdown]

Re: Mercury 25HP Wont Start

The vent on the tank should be open. AND NEVER USE STARTING FLUID.
VERY BAD ON 2 STROKES. they need oil in combustion to the pistons.

 

[will941s]

Re: Mercury 25HP Wont Start

Replace the fuel lines, tank to motor line, and rebuild and clean the carb with compressed air. All pretty cheap. THEN, perform a compression test on the motor to make sure that starting fluid did'nt kill it. If there is ANY trash in the carb it will clog up the idle speed jet and cause it not to start or idle. Next time you try to cheat start it use a mixture of gas and OB oil in a spray bottle. Starting fluid/carb cleaner/brake cleaner, well, cleans and removes oil....which is the exact opposite of what you want to do to the inside of your motor. Needle bearings become dry and seize, cylinder walls become dry and the rings just scrape and gouge the walls everytime you spin the motor over. The Gas oil mixture runs through your crank case while running, the gas keeps moving while the oil sticks to the parts in the motor. Good luck. Keep us posted.

 

[kevGreatBearLake]

Re: Mercury 25HP Wont Start

Replace the fuel lines, tank to motor line, and rebuild and clean the carb with compressed air. All pretty cheap. THEN, perform a compression test on the motor to make sure that starting fluid did'nt kill it. If there is ANY trash in the carb it will clog up the idle speed jet and cause it not to start or idle. Next time you try to cheat start it use a mixture of gas and OB oil in a spray bottle. Starting fluid/carb cleaner/brake cleaner, well, cleans and removes oil....which is the exact opposite of what you want to do to the inside of your motor. Needle bearings become dry and seize, cylinder walls become dry and the rings just scrape and gouge the walls everytime you spin the motor over. The Gas oil mixture runs through your crank case while running, the gas keeps moving while the oil sticks to the parts in the motor. Good luck. Keep us posted.

Thanks for the response. Never thought about not using starting fluid because the engine needs the oil..duhhhhhh,,How hard is it to take the carb off. Typically what tools would I need for that. I will probably try that today and see if I can get it off.plus check all the fule lines..THANKS AGAIN

 

[will941s]

Re: Mercury 25HP Wont Start

Take off the starter, the air box on the front of the carb (if it has one), disconnect the fuel line and you will see the two nuts on both sides of the carb holding it to the intake. Where the choke knob goes through the lower coweling, on the inside of it you will see a spring steel retainer holding the choke knob assembly in place, remove that retainer. there will be two screws on the front of the carb on both sides of the bore that also holds the choke assembly in place, remove them, then remove the nuts holding the carb on the intake, then just wiggle the carb off the intake and off the choke assembly and your golden. Takes all of about 10 minutes to get it out of there.

Find a clean spot to work, and begin to take the carb apart. There is a small ball bearing in the bottom of the primer assembly under a small spring. When you take apart the primer assembly DO NOT LOSE THAT BALL.

When you take the float bowl off, and remove the float, turn the carb right side up over your hand or something to catch the needle when it falls out.

When you take apart the fuel pump make sure all the parts are assembled the EXACT same way when you put it back together. Also make sure the carb to intake gasket is installed correctly when you remount the carb. There is a small hole on the intake plate that mates with a small hole on the carb that runs your fuel pump by suction. Make sure you mount that gasket with the hole in the gasket matching the hole on the intake. If it's not then the motor will not run. Clean the carb parts with carb cleaner and compressed air. Clean all ports, holes, passages, everything. Put it back together and go fishing.

 

[kevGreatBearLake]

Re: Mercury 25HP Wont Start

NOW THATS DETAILED INSTRUCTIONS!!! Awesome.. Though I have good news and bad news.. Took it all apart as you instructed and cleaned the heck out of everything as you instructed with a couple cans of carb cleaner and compressed air..Let a few parts sit in a small bowl of carb cleaner overnight. Put it all back together this morning and I can get is started (very rough) after some effort but then its hard to keep runnning and even at full throttle its seems its not throwing a lot of rpm. I can rev the engine up and down in nuetral (though again doesnt seem like a lot of power) but I almost have to slam it into gear to keep the motor still running when I put it in gear (forward or reverse). Also seems like having the choke on doesnt help much.(I did check to make sure the choke knob was pushing the little blue button down so I know that part is working)..When it is running it doesnt really sound like its missing but just seems like there is not a lot of power

Take off the starter, the air box on the front of the carb (if it has one), disconnect the fuel line and you will see the two nuts on both sides of the carb holding it to the intake. Where the choke knob goes through the lower coweling, on the inside of it you will see a spring steel retainer holding the choke knob assembly in place, remove that retainer. there will be two screws on the front of the carb on both sides of the bore that also holds the choke assembly in place, remove them, then remove the nuts holding the carb on the intake, then just wiggle the carb off the intake and off the choke assembly and your golden. Takes all of about 10 minutes to get it out of there.

Find a clean spot to work, and begin to take the carb apart. There is a small ball bearing in the bottom of the primer assembly under a small spring. When you take apart the primer assembly DO NOT LOSE THAT BALL.

When you take the float bowl off, and remove the float, turn the carb right side up over your hand or something to catch the needle when it falls out.

When you take apart the fuel pump make sure all the parts are assembled the EXACT same way when you put it back together. Also make sure the carb to intake gasket is installed correctly when you remount the carb. There is a small hole on the intake plate that mates with a small hole on the carb that runs your fuel pump by suction. Make sure you mount that gasket with the hole in the gasket matching the hole on the intake. If it's not then the motor will not run. Clean the carb parts with carb cleaner and compressed air. Clean all ports, holes, passages, everything. Put it back together and go fishing.

 

[will941s]

Re: Mercury 25HP Wont Start

Ok next thing. Do a compression test to make sure the compression is fine after the cleaner incident. Over 100 psi and no more than 15% variation between cylinders. Check it with the throttle open, engine warm, kill switch killed, plugs out and crank it over til you get the max reading. If that checks out then adjust the carb.

With the motor in the water, and secured to the trailer, turn the idle screw in til lightly seated (top face of carb). Back it off 1 1/4 turns. Start the motor. Put it in forward (just forward....nothing more) and rotate that screw slowly and by very very small incriments clockwise (lean) til it begins to die. Then turn it counterclockwise (rich) slowly and by very very small incrimants til it begins to run like crap and die. Then clockwise about 1/8th to 1/4 of a turn until it runs even and smooth. At that point the carb will be correctly set on the rich side of the idle curve, right where it needs to be. While it's running hit the gas and let off and put it in neutral. Do this a few times to make sure it will run at idle, idle in gear and will be able to make the transition from the idle jet to the mainjet when it hits that 12-1,500 RPM mark. Shut it off a few times also to make sure it starts easily.

Let us know what happens.