Need help debugging 72 Johnson sea horse 25

[EBenz]

Hey folks,
I'm new to the forum so I hope I'm not making a duplicate thread but I need a little direction in working out some bugs on a 25hp 1972 Johnson seahorse that came with a boat I just recently purchased. Took it out for it's maiden voyage under my ownership and everything ran great for about 10-20 minutes. All of a sudden the motor cut down to roughly 30% power even though the throttle was wide open. When I would shift to neutral and back into drive it would go to full power again for about 10 seconds then cut right back down again. The plugs look good and it has a new fuel pump. I don't want to spend all summer replacing parts trying to figure it out so any tips or advice would be greatly appreciated.

 

[racerone]

Did you install a new water pump impeller ?-----Sounds like it might have lost spark on 1 cylinder.----Test run with a timing light hooked to monitor spark.----Or perhaps water in the fuel.----Mixing at 50:1 in your fuel tank ?

 

[EBenz]

I have not replaced any parts so far. It does seem to be pumping water efficiently. I'm positive there's no water in the gas, I did a fresh fill up with premium gas at 50:1 with Evinrude brand 2 stroke oil. I'll double check the plugs to make sure.

 

[tphoyt]

Is the tank vent fully open and working properly?
Any new to you motor should at least have a water pump inspection and may as well replace the impeller for cheap insurance.

 

[EBenz]

Sorry for such a delayed response, I dont have as much time to take it out as I wish I did, but I have a bit of an update. After running in it a garbage can I noticed there seemed to be gas present inside the motor case which lead me to find the carb drain plug screw had fallen out. I purchased the boat from a very good friend of mine whose deceased father-in-law previously owed it and I know for a fact he took good care of it. I'll post an update after we take it out on the water again.

 

[racerone]

If you do not know how old the impeller is , it needs a new one.-----Check thermostat too.-----I often find them missing or stuck open.