Mercury 25XD won't go full throttle.

[Big_Boat]

Hi all,

I recently picked up an aluminum boat with a mercury 25XD (Serial #A194312) which would start and run for about 2 seconds then die. Previous owner rebuilt the carb with fresh gaskets, I still decided to take it apart and blow everything out, set idle mixture (2 1/8 turns out) and it ran fine.

Took it out to the river and got it warmed up and punched it to WOT, boat planed no problem with two people in it, after about 2 miles of ~80% throttle it started to bog down. I stopped it restarted and went WOT again same thing again, hits WOT for about 20 seconds then bogs down.

I took the carb apart again check what the float was set to and it was in spec according to the manual.

Back on the water boat won't got WOT and stays at about 60% or about 3600rpm, but it would run mid throttle and idle not issues, then it began running only at idle and any throttle would cause it to bog down.

I checked the plugs afterwards they were wet, I suspect the carb is flooding thus not allowing WOT.

I don't think it's timing and sync since it hit WOT no problem before, but I will get back on the lake with a timing light and try to check it. Any info would be greatly appreciated.

 

[racerone]

Trouble shooting starts with a compression test.----Post your numbers.----Then check for spark on both leads.-----Check flywheel key.----Float in carburetor could have a crack and have fuel in it.----You did put a new water pump impeller in it when you bought it I hope.

 

[Buddy3197]

Check fuel hose connections for air leaks (bad o'rings in fuel line connectors). Also check that fuel tank is venting properly. If all that checks out ok I would then suspect a fuel pump issue.

 

[Big_Boat]

Forgot to mention that there was no spark on the 2nd cylinder, which was solved by a new sierra switchbox.

So I know that I have spark on both cylinders, carb float was replaced when previous owner rebuilt it (I checked the carb by submerging it in fuel for a while and checking that it still floats). Not sure about the flywheel key, but the is no play on the actual engine flywheel.

Now onto the interesting one, the compression, it read 70psi on both cylinders, hot and cold with throttle at idle with same numbers at about 50% throttle, I couldn't turn the engine over at WOT to test. 70psi seems low, but It has no issues idling and starts 1st pull after sitting for a few days.

I just changed the impeller yesterday with a sierra one, old one was in good shape.

Boat has a 6 gallon west marine fuel tank that's EPA compliant so it does not let air out and expands under the sun, I will make sure that it is allowing air in. Might run the boat with the fuel cap unscrewed a bit to make sure it is venting.

 

[racerone]

Test with another gauge.----Or remove covers on side of block to inspect pistons and rings.----And no there is no head gasket to possibly cause low compression.

 

[Big_Boat]

I believe it's the gauge, I don't have another one to test the compression.

When I was removing the spark plugs I noted that both cylinders were very clean and the cylinder walls were clean with no pitting or scratches.

I highly doubt the engine would run as it does right now with bad compression.

What would the next thing to check after compression?

 

[jimmbo]

That the Spark can jump a 3/8" Gap

 

[flyingscott]

Clean and rebuild the carb again. Get a good kit with the fuel pump and primer diaphragms.

 

[Big_Boat]

I added sea-foam to the tank a few days ago and ran on the muffs to get it into the system.

Took the boat out again today, I can get it on plane at about half throttle and get to WOT about 50% of the time. I noticed when throttle is anything over half there is a slight oscillation in the rpm's which leads me to believe that the fuel pump is the main culprit.

Will find a kit and get it rebuilt again some time soon.

Edit: I just took a look at my photos when I took the carb apart and noticed the the fuel pump assembly seemed to be missing. I attached two images 1 & 2, the 1st one shows what my carb was like when disassembled, while the 2nd is what I see in videos of others rebuilding their carb with the same engine??? According to marine engine the one without the plastic fuel pump was used on 18 and 20 hp models, not 25hp.

Would that mean my engine is only producing 20hp instead of 25?

Any info would be greatly appreciated.

 

[flyingscott]

The plastic spacer in the pump is used on later models. Make sure the correct gaskets were used in yours. What is your serial #

 

[Big_Boat]

Here it is (Serial #A194312) it was in the first post, but might be hard to spot since it's a lot of text.

 

[flyingscott]

They show both styles of carb for your motor. So you probably have the right one. After your compression and spark test make sure the right gaskets are used. That carb kit has both sets of gaskets in it. Easy to screw them up.

 

[Big_Boat]

Carb kit is on the way. I also got new spark plugs, old ones look fine but who knows how old they are. I could not find NGK BUHW plugs any where which is what is currently installed so I purchased some Champion L76V plugs which should work fine (please correct me if I am wrong).

Will update once boat is in the water again.

 

[flyingscott]

You want the NGKs in t ha t motor

 

[flyingscott]

You want the NGKs in t ha t motor

 

[Big_Boat]

So here is the conclusion, I rebuilt the carb and noticed the gaskets were not that fresh.

New fuel pump diaphragm and spark plugs and engine runs like a champ, have had it out twice and had zero issues with the motor.

Thanks for all the help!