Need help with Johnson 25 hp

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yotehunter66

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I have a 25 hp Johnson that I want to get running. It has been sitting for several year. Motor has equal compression on both cylinders, 110 psi. The lower cylinder seems to be getting fuel but the upper one does not. I have cleaned the carburetor but have not pulled the flywheel and done anything there yet. Both plugs appear to be sparking. What might cause the lower cylinder to get fuel but not the upper? I included a couple of photos if anyone can guess at what year and model this could be. The one photo I think is the fuel diaphragm. Could that have a hole in it?
 

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Monte1961

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Re: Need help with Johnson 25 hp

yotehunter66 said:
I have a 25 hp Johnson that I want to get running. It has been sitting for several year. Motor has equal compression on both cylinders, 110 psi. The lower cylinder seems to be getting fuel but the upper one does not. I have cleaned the carburetor but have not pulled the flywheel and done anything there yet. Both plugs appear to be sparking. What might cause the lower cylinder to get fuel but not the upper? I included a couple of photos if anyone can guess at what year and model this could be. The one photo I think is the fuel diaphragm. Could that have a hole in it?
Click to expand...

On your first picture there is an aluminum welch plug with numbers on it. You can go to the john/rude site and get year and hp rating. As for the fuel only getting to the bottom cylinder and not the top. It would lead me to think a reed valve is not working properly. But I would try and start it and see what happens and then go from there.
 
fireman57

fireman57

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Re: Need help with Johnson 25 hp

The cover appears to be mid 60's or so. Take the carb off and shine a flashlight into the block. You will see the reed valves, they look like fingers sticking up. See if there is a screw/bolt that may have come loose sitting in the bottom of the cavity. You can also pull your plugs and turn it over and have your/buddy's fingers over the holes and see if they get wet with fuel.
 
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yotehunter66

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Re: Need help with Johnson 25 hp

fireman57 said:
The cover appears to be mid 60's or so. Take the carb off and shine a flashlight into the block. You will see the reed valves, they look like fingers sticking up. See if there is a screw/bolt that may have come loose sitting in the bottom of the cavity. You can also pull your plugs and turn it over and have your/buddy's fingers over the holes and see if they get wet with fuel.
Click to expand...

I pulled the spark plug on the bottom cylnder and it is wet. The top one is always dry. The bottom one will hit but I don't think it will even run on one cylinder and seems to be fouling the plug. I will pull the carb again and look at the reeds. Are they hard to get to and change if needed? Thanks for your help.
 
fireman57

fireman57

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Re: Need help with Johnson 25 hp

Chances are slight that it is a reed valve. Just take the carb off and shine a flashlight in there and you will see them. Look for a loose bolt or something floating around in the bottom. Sometimes you will lose one from the airbox and it will find it's way in there.
 
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larryg

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Re: Need help with Johnson 25 hp

after pulling plugu have to take the manifold plate off,believe there are 6 bolts.they are in there.had same problem one was broke off.when u find model number on the plate.2nd n 3rd letter from last tell you the year. such as J20crcos states its an 85...use (introduces).i=1 n=2 thru e=9 s=0.hope this helps
 
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yotehunter66

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Re: Need help with Johnson 25 hp

larryg said:
after pulling plugu have to take the manifold plate off,believe there are 6 bolts.they are in there.had same problem one was broke off.when u find model number on the plate.2nd n 3rd letter from last tell you the year. such as J20crcos states its an 85...use (introduces).i=1 n=2 thru e=9 s=0.hope this helps
Click to expand...

Is the manifold plate on the side of the cylinders? Or behind the the carb?
 

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yotehunter66

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Re: Need help with Johnson 25 hp

Pulled the manifold off and looked at the reeds. They look like they are in good shape. No cracks or debris in them. They where not stuck. I did manage to get it to start but it sounds like it is still running off one cylinder. I am going to pull the flywheel off next to make sure I'm not getting a weak spark or something. What are the two wires that run from underneath the flywheel? What should they be hooked to?
 
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Daviet

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Re: Need help with Johnson 25 hp

Normally on an engine with one carb, the carb feeds both cylinders.
Without a model number we cannot tell what engine you have and could be chasing our tails.
Does your engine have a pulse type fuel pump, is it driven off of the lower cylinder?
It might have a cracked diaphram in the pump and filling the lower cylinder with fuel.
 
fireman57

fireman57

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Re: Need help with Johnson 25 hp

Check and make sure that your flywheel key isn't sheared. That is what is sounds like.
 
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yotehunter66

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Re: Need help with Johnson 25 hp

Daviet said:
Normally on an engine with one carb, the carb feeds both cylinders.
Without a model number we cannot tell what engine you have and could be chasing our tails.
Does your engine have a pulse type fuel pump, is it driven off of the lower cylinder?
It might have a cracked diaphram in the pump and filling the lower cylinder with fuel.
Click to expand...

I have no idea what year or model it is. I wrote down the number that was on the little plug on the side but couldn't find it on the web site. The carb feeds both cylinders and I believe it is a pulse diaphragm driven off the lower cylinder. Ill check that next and the flywheel key. What should the low idle screw be set at approximately?
 

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Daviet

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Re: Need help with Johnson 25 hp

You can remove the fuel pump and see if fuel comes out of the passage that goes to the block, if so you have a bad pump diaphram.
 
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AlTn

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Re: Need help with Johnson 25 hp

might help getting correct champion plugs and ditching the motorcrafts
 
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yotehunter66

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Re: Need help with Johnson 25 hp

I tried to order the plug for it but this is what they gave me. The correct plug is a J4J. I did find a small pin hole in the diaphragm. Do they sell the whole pump assembly?
 
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Daviet

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Re: Need help with Johnson 25 hp

You can buy a complete pumps and some pumps can be rebuilt with a kit. You are going to have to know the year of the engine to order parts, or take the old pump to your local dealer and match it up.
J4 plugs are correct, if you do a lot of low speed operating (trolling) you can use J6 plugs.
 
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yotehunter66

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Re: Need help with Johnson 25 hp

Daviet said:
You can buy a complete pumps and some pumps can be rebuilt with a kit. You are going to have to know the year of the engine to order parts, or take the old pump to your local dealer and match it up.
J4 plugs are correct, if you do a lot of low speed operating (trolling) you can use J6 plugs.
Click to expand...

There is number on the back on the back of the pump. Could this be a Johnson part number they can cross reference?
 
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