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Mercury 25XD Fuel Problem

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skeeter25

New member
Hey all,

I've got a 1984 Mercury 25XD Outboard that I cannot get running right. I've looked all over the internet, and nothing that I have found has worked. The motor will start and run, but it bogs down and sometimes dies at full throttle unless you continuously pump the primer on the fuel line, and then it will run fine. Obviously, it seems like a fuel issue. I have cleaned the carb extensively several times, rebuilt and adjusted the carb, tried two new fuel pumps, tried a new fuel tank with fresh gas, replaced all fuel line all the way from the fuel tank to the carb, and tried bypassing the fuel filter, I have also replaced the reed gaskets, and replaced the spark plugs. Basically, I think I have tried replacing everything in the fuel system. Any help would be appreciated. Thanks
 
What other trouble shooting have you done ?-----Answer these questions.-----Have you done a compression test ?------Does spark jump a gap of 3/8" on both leads , yes or no ?-----Have you replaced the water pump impeller ?-----Have you done a test run with another tank and hose ?----Gasket between the carburetor the correct one and installed properly ?----Seal between the 2 crankcases ( behind the reed valves ) replaced ?
 
racerone,
Yes, I tested the compression and it was good and even, I can't remember the exact numbers but I think it was around 110lbs on each cylinder. I'm not sure how to test the spark jump, would I just hold the end of the plug 3/8" away from a ground on the engine and see if the spark jumps the gap? Yes, I have replaced the impeller. Yes, I have tried running on another tank and hose. Yes, the gasket behind the carb is correct and I have also tried replacing it. I'm not sure what seal you are talking about between the crankcases. Are you talking about tearing the engine down, splitting the crankshaft case, and resealing? If so, no I have not done that. Thanks for the help
 
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When you remove the reed box there is a seal that separates the 2 crankcases.-----No need to tear the motor apart to check this out.
 
Oh okay, yes I know what seal you’re talking about now. No, I didn’t replace it because it was in good shape and didn’t seem to need replacing. Is it possible that I have a vacuum leak in the crankcase? I forgot to mention that I also replaced the bayonet style fuel fittings. Basically everything in the fuel system is new. I don’t know how to test the spark jump but when I tested for spark there was a nice blue spark on both plugs
 
The fuel pump is operated by crankcase pulses. We are looking for any leak that will minimize that pulse. If that pulse is compromised we will fall short on fuel delivery at high speed, even if the fuel pump has been properly rebuilt and the rest of the fuel system has been replaced. It could, however, be a fuel restriction from improper tank venting. You said that you tried another tank but venting is always an important consideration. How far are you trying to lift the fuel? I have seen problems develop from excessive demand for fuel lift. Here on Lake Superior we have lots of sailboats, extended hoses and fuel tanks below deck can create problems. Rubber hoses absorb pulses especially when out in the hot sun.
 
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Both tanks that I have tried have been properly vented. Usually, I will sit the gas tank in the floor of my Jon boat behind the rear bench seat but I have tried sitting the tank on the seat as well, so it was even with the motor, but this did not help. When looking for a crankcase leak, will I just look for oil on the exterior of the case?
 
The gasket between the carb and block has a little hole in it that must be aligned properly for the fuel pump to work.

The reed system on those has a rubber seal/ gasket that can allow a vacuum leak. Easy to remove the reed assemble and check it.

Jeff
 
I replaced the gasket behind the carb and made sure it was lined up. I also checked the seal behind the reeds and it was in good condition. I may recheck them both today in case I missed something. I’m really not sure what else to do.
 
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