1969 Evinrude 25HP Power Loss

Capt. D

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Thanks for viewing my post. I have a 1969 Evinrude Model 25902B that is significantly losing lower if pushed above half throttle. If the throttle is open quickly, the engine will have full power for about a second, but then rpms will drop sharply. The motor starts on the first crank and idles well, and will run normally until the half throttle position.
I'm not sure if this is a related issue, but I have also notice blueish oil coming from what looks like the bottom of the powerhead. This oil will pool in the bottom of the power head cover. I have also seen what appears to be a small amount exhaust coming from below the carburetor. However, that is only visible when the air is cool.
The motor has been running great until recently.

Here is a link to a short video I made on the water. In the video, I attempt to bring the motor to full throttle twice.
https://www.youtube.com/watch?v=3uxNrBwMi8U&list=UU36w488NxM2j5lUAE8gHDEg

This is what I have done/checked so far:

Thoroughly cleaned carburetor (several times)
Double check carb float levels
Changes fuel line connectors
Checked gas tank vent (no clogs)
Replaced primer bulb
Check for spark (both cylinders showed good spark)
Ran each cylinder individually (both ran)
Looked at the coils (seemed to be in great condition)
Squeezing primer bulb while opening throttle did NOT help
Throttle cam roller contacts plate in correct position ( between the 2 lines)

I would greatly appreciate any thoughts on this matter. Thanks!
 

buddhapi

Petty Officer 2nd Class
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Jul 14, 2009
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check for badly cracked hose going into the intake manifold. there's more than likely one way down at the bottom, underneath the carburetor. seems to me you're experiencing a vacuum leak or fuel loss at higher RPM. I'm sure you've checked the fuel filter and screen already? worse case, it may be a bad lower crank seal.
 

Capt. D

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Jul 17, 2014
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Thanks for you response. I have check the fuel filter, it seems to be clear.I will check out the hose!
 

Capt. D

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There does not seem to be any hoses running into the intake manifold on this motor. Is pulling the power head the only way to diagnose a bad crankcase seal?
 

AlTn

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there should be an oil line running from the lower starboard side of the crankcase up to the upper crank bearing...if it's cracked or leaking it can cause what you are describing... I believe it's the one described in post #2 by buddhapl....leaking crankcase seals can cause similar symptoms ,but in my experience, they affect the idle setting more, as in, hard to ever achieve a smooth consistent idle. Have you checked for gasket leaks from the carb to intake or intake to crankcase?
 

Capt. D

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There is an oil line connecting the upper and lower portions of the crankcase. The hose itself does seem to be covered in a lot of oil, I will will replace this and see if that helps. Thanks!
 

Capt. D

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Okay replaced the hose. Took the boat out, started okay, revved the throttle, motor bogged down and died. Now it will not start at all. I now have black gooey oil coming from the weep holes in the lower unit. I have attached pictures. I'm thinking this is bad:(
 

AlTn

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I wouldn't be too concerned with the gunk leaking from the weep holes as it's probably just unburnt fuel mixed with years of exhaust crud. You may have fouled the spark plugs with fuel if you kept trying to start it for several pulls with the choke on. Are you using Champion J6C plugs gapped at .030? Have you checked the fuel pump to make sure it isn't leaking from the pulse hole in the back of the pump body where it attaches to the block? Post back and we'll keep throwing darts.
 

Capt. D

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Sorry for the delayed response (works, kids, ect)!

I found the cause of the motor not starting. The crankshaft key that spins the magneto cam had broken. Replaced that and put new properly gaped spark plugs in. Now the motor starts and the first crank and idles great again. Took the boat out yesterday and it is still doing the same thing. Power increases until about half throttle, then will not increase any more.
I took the fuel pump off and looked at the back side, and there was a little bit of gas on the gasket, but I don't know qualifies as a leak or not. Is there a good way to test the fuel pump? I squeezed the bulb while underway to see if that would help, it did nothing at all.
Another interesting note- I have an 18 hp johnson that I used to run on this boat. It always idled a little rough and but ran great a high speed. I put it back on my boat for one outing just to get by until I got the 25 hp fixed. When I tried to go full throttle, it acted very similar to what my 25 hp is doing, only about half power. Does this mean I have a fuel line issue? I have thoroughly inspected the fuel line and blub and cannot find any leaks. Bulb stays primed, connections have just replaced. What am I missing here?
 

AlTn

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couple of things to check...the fuel mix itself, any water in it?...the tank vent, is it venting properly? try running with the cap off the fuel tank as a test...in tank filter clear?...in tank fuel line ok? no cracks or plugs in the line....Seems you're on the right track as this is the only common item between the 2 motors....happen to have another tank and fuel line you can use or borrow to test with?
 

Capt. D

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I have gone through the fuel lines and tank already. All filters seem clear. Tried running with the cap off, that made no difference. I have refreshed the gas several times, that did not help. If it is something wrong in the fuel tank or lines it is hiding very well!!!

I have a neighbor that has a 15 hp Johnson that I'm sure would lone me his fuel tank. I will try this on my next outing. Wish me luck!
 

Capt. D

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Well took the boat out with a different fuel tank and hose, did not solve the problem.

When I was out, I applied the choke while running with half+ throttle just to see what happened. The sound and performance of the motor did not change at all when I did this. This is not right is it?
 

AlTn

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took a second look at your video and it appears to me that the carb/timing linkage is slipping back towards slow/shift on its own...one cause for this is the control shaft lever pin has sheared one or both tabs off and will no longer stay in position in the slot on the vertical control shaft gear....easy to get to, just unclip the stainless steel clip at the top of the vertical control arm, unscrew the 2 screws that secure the upper collar around the control shaft, then pull the control shaft up and out of the slot in the gear, then pull it through the rubber boot. You can remove the boot first if you want to but this isn't really necessary. There should be a plastic < nylon> pin at the bottom of the shaft that has 2 tabs which slide into the slot on the gear. If either tab is torn or missing it needs replacing. If this pin is in good shape, post back for other "ideas".
 

Capt. D

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Are you referring to inside the tiller handle? If so this is not it. I have run the motor with the cover off and have had a visual of the throttle cam roller and throttle linkages going to where they are supposed to be.
 

AlTn

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at the end of the tiller handle, the vertical shaft that goes up and is connected to the armature advance arm by that stainless clip..the plastic pin is pushed into the base of that vertical shaft...looking at seconds 9 - 11 on the vid. it appears the linkage is backing down, unless you are doing this with the throttle handle?...do you have a timing light?..if so...the next time out connect it to each plug wire alternately and check the flash when you trying going to wot...are both flashing strong and consistent?..if so...it's back to the fuel system/carbuerator only..post back what you find
 

Capt. D

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Yeah that was me bringing the throttle back down. I will try to see if anyone I know has a timing light. Thanks.
 

Capt. D

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Finally figured it out. I took the carb off again and inspected it very thoroughly. I found a very small crack in the high speed nozzle gasket. I guess it was not allowing enough fuel to get sucked up into the engine. Replaced the gasket and the motor is running good at high speed again.
 
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