1988 Mercury 25 hp power loss

[dont sink]

Hi, can anyone please help me solve this problem. I had a 1988 Mercury 25 hp running hard all weekend with no issues. I stopped the motor and restarted it and could only get about 25% output at wot. the motor sounds fine and runs smooth just with a major power loss. I pulled the plugs and tested them and got spark on both. I then thought it might be a fuel issue so I checked from the tank to the carb and everything looked good and the tank vent was open. I next tested the compression and got 120 on both cylinders. Thought it might be an issue with the carb. Pulled the carb and took it completely apart. Everything looked good and clean with no blockages. Inspected the linkages and they all look good. The valve in the carb opens to a normal position, from vertical to horizontal. Inspected all wires and they all look good with not corrosion or lose or broken wire. Im stumped because i know its not a fuel issue or a spark issue. Im starting to think it might be an electrical issue, ie. switch box, stator, coils, which my knowledge is very limited on. Please help. Thank you in advance.

 

[Dave1027]

Re: 1988 Mercury 25 hp power loss

Does the primer bulb stay full of gas? Try squeezing the primer bulb when it bogs. Does it pick up RPMs?

 

[dont sink]

Re: 1988 Mercury 25 hp power loss

Thanks. It doesnt bog down at all and the bulb stays full and hard. The RPMs dont fluctuate at all and it runs smoothly just at around 25% output wot. the rpm is at the correct level for the output.

 

[dont sink]

Re: 1988 Mercury 25 hp power loss

Wow im surprised on the lack of feedback on this thread. Ok, with the motor out of the water I get good spark from both plugs grounded to the block. While running the motor on the lake at low speed I pulled the spark plug wire from the top cylinder and there was no change. Put wire back on top cylinder and pulled wire from bottom cylinder and motor stopped. I then swapped the coils from bottom to top and vise versa, leaving wiring as it was. Repeated test on the lake by pulling top spark plug wire and no change. Pulled bottom plug wire and motor stopped.

This leads me to believe that the coils are fine. This narrows the problem down to the stator, trigger, or switch box. Correct me if im wrong, if the stator was bad I wouldnt have fire on either cylinder? Triggers rarely go bad? I think it could be the switch box. Is there a way to test the switch box? Thanks

 

[stevarino]

Re: 1988 Mercury 25 hp power loss

You are going to have to check ignition system for #1 cylinder. Spark may not be issue. Pull-off load test does prove motor is not running on #1 cylinder. Could be internal mechanical issue with number 1 cylinder. Do your Peak Reading Voltage tests outlined in manual to rule out spark issue. Remember all spark is not good spark. Spark should be able to jump 1/4"-5/16" gap. Nice blue spark not weak orange or yellow spark. These motor can be very hard to troubleshoot internal engine problems. Especially if reeds are internal. Compression tests are good but do not give full picture of cylinder condition. Could be scuffed on lower part of cylinder covered by piston. I have had a couple of these motors act like your describing and could not fix them. The cost to tear down exceeded motor value. Good luck and I hope you get her running, they are great little motors when running right.

 

[drod07]

Re: 1988 Mercury 25 hp power loss

Wondering if you found the problem with the outboard?

I am having the same problem with my Mercury 25hp. I pull the lower plug and the motor stays running. Pull the top and it dies. Trying to figure it out.

 

[redjmp]

Re: 1988 Mercury 25 hp power loss

I would first try swapping the coils and if the problem remains on that cylinder you know its not the coil. Next I would try to swap the input and outputs from the pack.
If the problem migrates to the other cylinder, you have a bad pack.
If it stays the same, you have a bad tirgger.

 

[will941s]

Re: 1988 Mercury 25 hp power loss

The switch box likes to act up, probably one of the most famous problems on the merc 25. It can cause all sorts of little problems that are almost impossible to diagnose. Swap coils, if no change move on to the switch box...if it's good move on to the trigger. Highly doubt it's a internal engine problem.