25 hp Merc. problems

Kickin Back · 02-28-2010, 08:50 AM

I have a 1998, 25hp two stroke with about 500 to 600 hours. The last outing the motor started and idled fine for about 1/4 mile then died as soon as I applied power. I purchased the gas the day before.
I just emptied the tank, bought new gas, replaced the water/fuel filter, fuel hoses and checked the primer bulb. The motor started and ran for about five minutes at low and high rpm's then died. I could start it but it wouldn't run for more than 5 seconds. Yes, I had the hose hooked up.
My second concern is that for the last year the cooling water stream runs strong for 10 to 20 seconds tapers off for 10 to 20 seconds then strong for 10 to 20 seconds and does this all day long. It never seemed to be an issue as the boat ran fine under all conditions.
I am religeous about running the fuel out of the carburator after each use and changing the lower end oil each season.
What do you think? Carburator, water pump or other?

Lou · 02-28-2010, 09:47 AM

Carb and plugs (you did not netion plugs). Sounds like the water pump needs looking at also. Your carb(s) are dirty.

Laguna29 · 03-19-2010, 05:47 AM

Not to cause you undue alarm but it sounds like what happened with my two stroke short shaft 25 HP. Bottom line is that the lower cylinder was scored. No real reason just went out on me. There is an inspection plate on the left side of the piston wall. You will know if this is the issue as soon as you pull the two bolts off and the inspection plate. My entire lower piston was scored. I always took excellent care of the motor....I just had a bad one. Symptoms....motor became hard to start....would run but I had to give it bursts of power to keep it running and would not idle.

If you have not found the problem yet you might explore this diagnosis.

1pump · 03-19-2010, 03:43 PM

Is your primer bulb staying firm while it's running? And is the fuel filter staying full? The reason I ask is because I had a bad check valve in the primer on my Merc 25 and it wouldn't keep running. Another time the fuel pickup in the tank went caddywampus and would cut the fuel to the motor. The carb would drain itself, and sometimes it would recover quickly without stalling, and sometimes it wouldn't.

Make sure you have good spark, too. I had an ignition switchbox go south and cause intermittent problems like that. Sometimes it would run for an hour, other times 30 seconds. In salt water it was worse. It about drove me nuckin' futs before I figured it out.