Mercury 25XD Missing and Rough idle

[realjoe2000]

I have a 1985 Merc 25hp XD with a rough idle and seems to be missing. I rebuilt the carb and fuel pump and replaced plugs and checked coils with an ohm meter. Compression seems even at 120 top and bottom. Is the switchbox the next item? Can I check it somehow? Cant afford to start replacing things. Ive worked on OMC but this Merc is something new to me. Any help and direction is greatly appreciated. Thank you.

 

[will941s]

Re: Mercury 25XD Missing and Rough idle

I have a 89' 25XD and I had the same problem. I cleaned out my carb, checked the built-in fuel pump, and checked all my fuel connections for leaks or clogs. I still had the problem.....when the engine was running I heard a little "ticking" noise and noticed I had a arc jumping from plug wire to plug wire and plug to plug. I changed out my plugs and wires and now it runs great. The engine never was the smoothest thing in the world at idle but it was missing and running really rough. I think the ignition system on the motor is really powerful and it eats plug wires within a couple of years. Instead of spending $20 on new wires I went to the local autoparts store and got a universal high perfomance 8mm plug wire with boots and connectors and cut and made my new wires for $9.99. Hope this can help you out.

 

[jbjennings]

Re: Mercury 25XD Missing and Rough idle

I just wanted to mention that my '84 25xd is the roughest idling 25hp I own, even after a carb cleaning, etc. It cranks easy and accelerates through all ranges fairly smooth, hauls tail WOT. BUT, it idles rough. It's a low hours motor and everything has checked out good as far as vacuum leaks, etc. Has excellent compression, as well. My conclusion is that they're just not smooth idling motors. However, the power and speed makes up for the crappy idle. It has a lot of speed in my opinion for 25hp.
Good luck with yours,
JBJ

 

[will941s]

Re: Mercury 25XD Missing and Rough idle

I think it's a common thing, the motor used to jump left and right and shake the boat because it was idling so hard but the plugs and new wires helped alot. It still idles a little rough but a huge difference. I think I get 35mph with a 14.5 foot aluminum jon and a 4 blade 13 pitch prop. Your right, idles rough but you sure can't beat the speed.

 

[Skinin'Bones]

Re: Mercury 25XD Missing and Rough idle

My 25XD was doing the same thing your motor is.
While I was troubleshooting, I was adjusting the carb and was getting some completely wrong fuel/air mixtures, which was making the motor run even worse. But while the motor was running so bad, it was spitting ang popping down underneith the engine where the crankshaft comes out. Well turns out that there is/was an o-ring that goes on the drive shaft to the lower unit. Soemone forgot to put a new o-ring in it when the replaced the water pump and I was running way to lean.
Sorry for the ramble, but get some carn cleaner adn while the engine is running, just spray it around the engine, especially underneith, and see if it smooths out/changes tone and there would be a big part of the problem.
Now my engine runs great all through the RPM range and my boat gained 2 MPH.
Hopefully this helps!

Matt

 

[realjoe2000]

Re: Mercury 25XD Missing and Rough idle

I think you all may be on to something. The feedback Ive gotten seems to lead to a case leak somewhere. since the carb and fuel pump rebuild didnt make any difference I will try and find a vaccum leak somewhere with carb cleaner or Ive been told to use a propane torch turned off using just the gas to also find leaks. It did run pretty smoothly before so I know something, somewhere is amiss. I will also replace the two plug wires and try that.Thanks

 

[realjoe2000]

Re: Mercury 25XD Missing and Rough idle

It also seems to break down at high speeds. From mid to wide open, she seems to bog out and miss, so I was leaning more toward a ignition problem. But I will play with it some more and keep checking.