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Mercury 25hp missing

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realjoe2000

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"I just bought a merc 25hp 198

"I just bought a merc 25hp 1985, with a miss and rough idle. I went thru the fuel systems with a new carb kit and fuel pump. Still the same.New plugs and checked the coils with an ohm meter and they show no problems. Is there a way to check the switchbox or anything else? Seems to be missing on one cylinder but I dont know where to turn next. I just cant afford to replace parts to find the problem. Thank you."
 
"More info,,,,
Seriel number

"More info,,,,
Seriel number 6621542 25XD compression is stable at 120psi top and bottom. It maybe a 1986? Thanks for your help. This is my first try at a Merc. Most of my experience is with OMC."
 
You could put a timing light o

You could put a timing light on each plug wire and watch the strobe for the miss.

And to look for a case leak use a plumbers torch (off) to wave around the case looking for a leak
 
"If it does this only at idle,

"If it does this only at idle, you might try opeing the idle mixture screw a 1/4 turn.

Jeff"
 
"It shakes at idle and seems t

"It shakes at idle and seems to bog at higher rpms. Using a torch, how would this find a leak in the case halves? Does the gas make the motor run differently? I am not sure I understand how this trick works. Thanks again"
 
"Jeff is right, I assumed you

"Jeff is right, I assumed you would have the carb set or tried adjusting it if you went through it. Your engine has all the indications of running lean and fuel starvation. Try pumping the bulb while running and see if it helps. Try opening the idle mixture screw as Jeff said. Be 100% sure the float and main jet in the carb is set and clean.

The propane when waved in front of the carb will cause the engine to change rpm, when waved in front of the case it will do the same if there is a leak. After reading your problem at all rpms I would look at the fuel system closer."
 
"Joe, I am no pro and the guy

"Joe, I am no pro and the guys above are typically the ones helping me but I just went through the exact issue with a 1985 25xd.

Here is what I had and what I found.
Engine ran rough at low rpms and idle. Appeared to be missing on #2 cylinder. I concluded this by pulling a plug wire while the engine was running. When I pulled the bottom wire the engine changed very little if any. Replace the bottom wire pull the top and the engine stops. Swaped the coils around and the problem remained the same with the #2 cylinder unable to run the engine at all. The top cylinder would continue to run with the lower wire unplugged.
1. Compression good.
2. Checked fire and found both cylinders firing properly.
3. I knew my fuel supply was good, clean fuel, new lines and filter, no leaks.
4. Replaced both plugs and ran engine in tank for 5 minutes. Removed plugs and noticed lower plug #2 cylinder showed no sign of combustion? Plug was as clean as new and there was excessive oil/fuel in water of test tank.
5. Appeared as if no fuel was making it to lower cylinder? Search revealed a leak in crank case halves. I could see it coughing out fuel as engine coughed. There are seals at the bottom of the crank that can cause this per the guys here but they were ok in my engine. If can't find a leak in the crank case halves and after everything else still have a no fuel issue with #2 the guys here can help you with replacing the lower seals.
6. After re-sealing the crank case the engine ran better but still not right.
7. Had the carb completely rebuilt by dealer and that completed the repair.

From what I learned (some the hard way) after 25 years the seals and gaskets begin to fail. While I had mine apart I found a leaking water jacket gasket also and the seals from the carb were less than good. My engine was garage kept since new and lightly used. Father time just took it's toll.

If your fire is right, I bet you're not getting combustion from a leaking crank case or bad carb.

Good Luck and keep us posted."
 
A bit more. Mine ran better a

A bit more. Mine ran better after all the fixes but I still had to open the idle mixture screw from 1.5 open to 2. and advanced the timing linkage slightly which smoothed the idle right out.
 
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