Old johnson loses power after a while

[raywm3]

I'm a little stumped by this. I have a 1969 25 hp, 2-stroke. When I first get out on the lakes, it runs like a raped ape at about 25-27 mph. But by the end of the day it gets down to about 15 mph at WOT. Just way less power and rpm's.

I've replaced both coils, ran new wires, and of course many new plugs. It's not dropping a cylinder and the compression seems okay. I was told it could be water pump, but I checked it and it looks pretty good.

I was also told it could be reed valves. I have no idea. What confuses me is that it runs great when it's been sitting and the engine is cold.

Any ideas???

Thanks.

 

[jtexas]

Re: Old johnson loses power after a while

"compression seems okay"?

no point going forward till you have the numbers......(IMHO).

water pump "looks pretty good"? Wonder how many outboards been ruined by "pretty good" looking water pumps?

How bout the thermostat?

 

[Rick.]

Re: Old johnson loses power after a while

Sure sounds like your running on one cylinder. I know you changed the coils but have you tried pulling the plug wires when it is heated up and acting sluggish? Coils often work until the get hot and then fail. Best of luck. Rick.

 

[raywm3]

Re: Old johnson loses power after a while

Thanks for the responses. I was told that it could be the water pump and that the motor could overheat and lose power or compression. Before I dropped the lower unit though, I threw a compression gauge in one hole (2 cyl motor) and after a few pulls I had it close to 100 psi so I stopped and figured that was good enough.

The impeller remarkably looked really good. The housing was a little scored, but no more than normal. So I'm ruling out water pump.

It's not dropping a cylinder- I did that test in the water last time. Maybe I should do a compression check in the water when it acts up. The plugs, by the way are almost always wet with fuel/oil, could that be an issue?

Do these old OMC 2 cylinders have thermostats?

 

[eavega]

Re: Old johnson loses power after a while

Do these old OMC 2 cylinders have thermostats?

Yup. That triangular piece at the top of the head is the t-stat housing.

-Eric

 

[jbjennings]

Re: Old johnson loses power after a while

I don't understand your comment about "at the end of the day". Could you clarify that statement? Does it run great for a few minutes and gradually lose power? Does it run fine all the way to your fishing spot and then when you crank it back up it doesn't run well?
If it is overheating and you keep running it the motor will lock up. Have you tried feeling the cylinder head when it's running bad to see how hot it is, or throwing a little water on the head and seeing what happens? If you can hold your fingers on the head for 3 seconds without yelling "yeeeeooowwchhh!" then it's not too hot to run good.
Is your tank vent working properly, letting air inside the tank? Have you tried taking the gas cap off and seeing if that helped?
BTW, checking compression on only 1 cylinder is a waste of time, check them both and report the numbers pretty please.
Have you tried pumping the primer bulb and seeing if that had an effect?
Last of all, your comment about the plugs always being wet has to be due to either weak spark, or your high speed adjustment is loose and vibrating out, causing the motor to run rich and ruining your high speed adjustment which would definitely make it run weak. I just can't understand why it would run good at first, if the high speed adjustment is off.
Just some ideas to try,
JBJ

 

[raywm3]

Re: Old johnson loses power after a while

I only run the boat/motor a couple times a year. I take it to Canada for a 1 week trip. When I first put it in a lake and start it, it runs great. It seems to run great after it has been sitting a while. My fishing style is wot from spot to spot on the lake. After a few half mile jaunts it will just start losing speed(power). I'll be motoring along at about 25-27 mph and you can hear the motor slowing down, like I'm letting off on the throttle or something. It will get down to about 12-15 mph.

Once I get to a spot and let it sit, it'll either start back up and run great or it'll start and struggle to accelerate and find rpms. It's very inconsistent. I've felt the powerhead before while running and I don't remember it being scorching hot, because I was feeling to see if one cylinder was colder or hotter for losing spark. I know I've pulled the plugs before on the water when it's acted up to check for spark.

I still think it's in the fuel system, but I have no idea. The tank is new and I've removed the cap while running and that didn't do anything. BTW- I've just pulled the thermostat and I can't see how that thing EVER worked- its very old, rotted and siezed. ??? Could that be an issue??

Is it possibly over-riching the mixture, and would that give me my problem? Can the rings fail after the engine is warmed up?

 

[freddyray21]

Re: Old johnson loses power after a while

check the coils. one or both are warming up and putting out a weak spark or no spark when warm.

 

[tigershark2]

Re: Old johnson loses power after a while

change t-stats (both). Since you don't use the boat that often and its a 2-stroke you may want to get the Carbs cleaned and rebuilt each year, before your trip to Canada.

Tune and time the engine each year. Take it to a shop if your not comfortable in doing it yourself.

I want to stress PROPER Maintenance throughout the year and especially before fishing trips.

Winterize properly and use FRESH gas.

 

[jbjennings]

Re: Old johnson loses power after a while

check the coils. one or both are warming up and putting out a weak spark or no spark when warm.

I agree with FreddyRay. You either got a bad coil from the factory, or maybe have some dirty points. I have found spark to be inconsistent sometimes due to several problems, but hardly ever have I found fuel delivery inconsistent. It either works or doesn't work. Definitely replace that thermostat, but I doubt that has anything to do with your problem.
JBJ

 

[jtexas]

Re: Old johnson loses power after a while

Electric start? Is this the 1969 battery-driven CD ignition, the one with an amplifier? If so a weak battery means weak spark, and a blown rectifier means weak battery....

 

[Chinewalker]

Re: Old johnson loses power after a while

It's a standard OMC magneto - two sets of points, coils and condensers, not a battery driven ignition.

I highly doubt it is a water pump - they don't partially slow you down. At least not for long. If it's going bad enough to slow you down you're on your way to full stoppage pretty quick.

I'm leaning towards lack of spark during these instances. Also, did you check compression in BOTH cylinders?

 

[tmcalavy]

Re: Old johnson loses power after a while

Sounds like a coil or wire/connection is dropping out when it gets up to operating temp after awhile. Check compression, then work your way up each plug wire to the coils, checking with a meter. If the coils are original, I'd do a tune-up under the flywheel and new wires.