1975 Johnson 25 hp surging

[jonmac3569]

Have a 1975 Johnson 25HP model 25R75B, pull start, short shaft. I was having an issue where the motor would run fine at idle and whenever I would open up the throttle it would surge. If I pumped the primer bulb it would pick back up and run fine. It would not run at WOT. I could not pump the primer bulb fast enough to keep up with it.

I changed all the fuel lines, I took apart the fuel line from the tank to the motor and checked all the connections and put back together. I took off the "pulse line" (compression feed)? and checked the connections and line. Next I rebuilt the fuel pump with a rebuild kit. Still same thing. Lastly I just replaced the fuel pump with a brand new OMC fuel pump. Same thing.... No difference whatsoever. Any ideas?

One other thing, I did rebuild the carb recently and the gasket between the bowl and upper half of the carb is moist with fuel. Could air be seeping through there?

Thanks,
Jon

 

[cgd27]

Re: 1975 Johnson 25 hp surging

make sure that the jets on the carb are clean,the ethanol in the gas plays havoc with small engines,I had the same problem with my riding mower,(spray some carb cleaner on a pipe cleaner and wiggle it in the jets)

 

[JDusza]

Re: 1975 Johnson 25 hp surging

Running OK at low speed and not OK at high speed (?)

Could be an air leak, yes. Gaskets should not be noticeably wet...they are there to prevent the wet...if gasket is wet, the seal is leaking, yes.

Also, recheck the carb work with respect to the high speed (main) jet and inlet float and needle valve.
Check the fuel filter screen on the fuel pump.

You are on the right path in that something is minimizing fuel flow to the intake manifold.

Is the engine new to you? A little history on it may help diagnostics.
J

 

[jonmac3569]

Re: 1975 Johnson 25 hp surging

The engine was my grandfathers, he died about 10 years ago. I ran the motor a lot as a teenager. Before he died he had it worked on and in good running condition and then it just set up for about 10 years. I rebuilt the carb, changed the fuel lines, plugs, lower unit oil, water pump impeller, and rebuilt the rewind starter. I replaced the fuel pump today and the filter came attached to the new pump. So it should not need to be cleaned. I didn't even unscrew it.

I guess I need to pull the carb back off and open it up. I used a non-omc rebuild kit for the carb. Since I rebuilt it i have heard they are not that good as far as quality goes. Any truth to that?

 

[AlTn]

Re: 1975 Johnson 25 hp surging

if you'll go to marineengine.com and find the diagram for your carb...make sure part #40 <nozzle gasket > is in place and part #45 < orfice plug (main jet) > is removed and cleaned thoroughly, caution..a special jet removal tool 317002 is recommended to remove this plug and if you use a straight slot screwdriver make sure it fits the slot tightly as it's easy to damage the ears on the plug if the screwdriver is a loose fit...float parallel to base of carb with carb inverted....kit question...some swear by'em , some swear at'em....the last couple of brp kits I've used have gone back to the solid metal tipped needle valves in lieu of the rubber tipped one < for whatever that's worth >

 

[jonmac3569]

Re: 1975 Johnson 25 hp surging

Thanks for the info. I will check those parts. I ordered an OMC rebuild kit last night, so I will just redo it and pay particular attention to that when I do.

I do remember that I did not take out #45. Not having that "special tool" worried me. I just soaked the whole float chamber in carb cleaner for several hours. (The type where you put the part in the basket and dip it in the bucket of cleaner). I was hoping that would clean it from both ends of the plug.

 

[JDusza]

Re: 1975 Johnson 25 hp surging

Thank you for sharing the detail. We better understand the scenario.

I was poking at the impeller change, gear oil change, etc., for some people new to engines do not realize the trouble old, "unknown state" parts can cause. You sound quite mechanically capable and have a great engine here. For the stuff you are queasy about, this is a great forum. The guys are very knowledgeable and friendly. We all enjoy a great rebuild!

Concur with AlTn. A flat blade screwdriver can be used, but be careful not to scar the parts.

I am sorry for the loss of your grandfather. Using and caring for this engine is a great way for him to live on with you! Enjoy his engine. Perhaps you can someday pass it to your son or daughter and that would be a real hoot!

J

 

[jonmac3569]

Re: 1975 Johnson 25 hp surging

JDusza, thanks a lot for the comment and the advice. I will let you guys know what happens after I get back into the carb. I would love to get this thing running like a top again. I have good memories running this motor and would like to get it back in action.

 

[jonmac3569]

Re: 1975 Johnson 25 hp surging

View attachment 122329View attachment 122330

I rebuilt the carb with an OMC kit and checked all the areas mentioned above. When I was putting it back together I noticed that the "cam follower roller" in the pic was acting funny. The service manual says to put the cam follower roller between the two vertical lines on the cam and turn the throttle handle. throttle should start to open as it passes the line.

When i first tried this the cam follwer roller was not touching the area where the marks are. I put more tension on the spring on the throttle linkage below, and adjusted the throttle shaft linkage screw and that will hold it against the cam now.

So now, when i start to turn the throttle handle, it will start to open the throttle at the correct point. Problem is, when I turn the throttel handle back, the cam roller will stay in place about 1 millimeter or 2 from the cam. (see yellow arrow in pic) It will not spring back against the cam. There is a lot of tension on the cam follower roller now and if i pull it away from the cam and let it go it will snap back against it. Only if it is moved a small amount with the throttle control will it "stick" about a millimeter away.

I tried oiling the rod that holds the cam follwer assembly on. (the one that it rotates on) Could that be the issue? Or is the cam roller warn down maybe?

 

[AlTn]

Re: 1975 Johnson 25 hp surging

referring back to the diagrams look under magneto part no. 36 < throttle control cam >...it has two screws you can loosen which allows it to move in and out...try adjusting it so the roller stays against it as you go from start to wot...the shaft the roller rides on could be worn as well and is allowing it to bind

 

[jonmac3569]

Re: 1975 Johnson 25 hp surging

Thanks, I will try that.

When i was working on the carb, I noticed the float was not level when i opened it up. Book says it should be 1/16 of an inch above the body when holding it upside down. As you can see in the pic it wasn't close. I know it was right when I put it together last time but it didn't stay that way. Could this have caused any of my issues? see picView attachment 122385

 

[jonmac3569]

Re: 1975 Johnson 25 hp surging

Ok, played around with it some and now the cam roller seems to seat up against the cam correctly. Just need to take it out now and see if it runs any better.

 

[jonmac3569]

Re: 1975 Johnson 25 hp surging

Great news, I took apart the carb, cleaned all the jets, etc and rebuilt with an OMC kit and now the motor runs great! Not sure if it was the cleaning or the new kit but I am done with the non-omc kits. It took less than 5 mins to get the carb adjusted and then it ran great even at WOT. Thanks for the help!

One question: If you know that you are not going to use a motor for 3 or 4 months is it better (or ok) to unplug the fuel hose while the motor is running and burn all the fuel out of the carb and just let the motor die?

 

[kfa4303]

Re: 1975 Johnson 25 hp surging

Hi jon. Glad you got her running again. yeah, a lot of folks run the gas out of there motors, if they know they're gonna sit for a while, but as mentioned before, the ethanol in modern gas is a pain, and may still leave some gunky deposits behind, but draining the carb should help.

 

[JDusza]

Re: 1975 Johnson 25 hp surging

Score!

 

[jonmac3569]

Re: 1975 Johnson 25 hp surging

Thanks. Yeah I don't really want to leave that gas in the carb after seeing what it can do to one.
View attachment 123880

 

[jonmac3569]

Re: 1975 Johnson 25 hp surging

The motor was running good for a while. I have a couple new issues now. First it has been fouling plugs, I took the thermostat out today and it was stuck in the open positon so I ordered a new one along with the gasket and rubber seal. I also had a recommendation to start running J6C plugs instead of the J4C that I have been running. I was told they would burn hotter and this could also help. Hopefully these two things will solve that issue but any input is appreciated.

The second issue is the one I am more worried about. I made an adjustment to the cam follower because it was not lining up with the marks on the cam. the next time I took it out, I could turn the throttle handle to what should have been WOT and it would only get a little above idle. I knew this was due to the adjustment I made so I loosened the screw on the throttle shaft likage and tried to adjust it back on the fly. (because it was running fine prior to me messing with something that wasn't broke)! Anyway, after the re-adjustment I could open the throttle back up, problem then was that when I would go close to WOT it felt like it would slip out of gear. (felt like transmission slipping) The motor would rev up but the prop would stop spinning until I backed off on the throttle. then it would re-engage. I have been told it may be a clutch dog or forward gear problem. (expensive) Does this sound right? Or maybe something else to check before opening up the lower unit? Also I have been told that if it is the clutch dog that it is possible to just flip it around since a lot of times the reverse gear side is fine. (?)

Thanks in advance

 

[mrjb76]

Re: 1975 Johnson 25 hp surging

As far as the plugs, I am lucky enough to work with a former OMC mechanic. He is religious about replacing the J4's with J6's. That is what I do also. If you are fouling plugs though, I would check to see if you are getting a good spark from your ignition. If not, you are going to have plug fouling and similar problems that you are having. Infact, I would go ahead and order a tune up kit (contains 2 sets of points and condensers) and 6 feet of plug wire and replace them. You will be out about $30, and ~2 hours of time, but it will be worth it.

The motor was running good for a while. I have a couple new issues now. First it has been fouling plugs, I took the thermostat out today and it was stuck in the open positon so I ordered a new one along with the gasket and rubber seal. I also had a recommendation to start running J6C plugs instead of the J4C that I have been running. I was told they would burn hotter and this could also help. Hopefully these two things will solve that issue but any input is appreciated.

The second issue is the one I am more worried about. I made an adjustment to the cam follower because it was not lining up with the marks on the cam. the next time I took it out, I could turn the throttle handle to what should have been WOT and it would only get a little above idle. I knew this was due to the adjustment I made so I loosened the screw on the throttle shaft likage and tried to adjust it back on the fly. (because it was running fine prior to me messing with something that wasn't broke)! Anyway, after the re-adjustment I could open the throttle back up, problem then was that when I would go close to WOT it felt like it would slip out of gear. (felt like transmission slipping) The motor would rev up but the prop would stop spinning until I backed off on the throttle. then it would re-engage. I have been told it may be a clutch dog or forward gear problem. (expensive) Does this sound right? Or maybe something else to check before opening up the lower unit? Also I have been told that if it is the clutch dog that it is possible to just flip it around since a lot of times the reverse gear side is fine. (?)

Thanks in advance

 

[AlTn]

Re: 1975 Johnson 25 hp surging

when it comes out of gear at wot is it with a violent jerk as if you hit something submerged, or more like the engine just revs up and the boat slows down?...if violent, not so good....if it just revs it's most likely a spun prop and can be rehubbed or replace the prop...post back

 

[jonmac3569]

Re: 1975 Johnson 25 hp surging

when it comes out of gear at wot is it with a violent jerk as if you hit something submerged, or more like the engine just revs up and the boat slows down?...if violent, not so good....if it just revs it's most likely a spun prop and can be rehubbed or replace the prop...post back

Thanks for the responses, no it is not a violent jerk. More like you said, it revs up and the boat slows down. But it will not re-engage until I let off the throttle. I saw a post that said you can draw marks across the prop and the end of the shaft with a waterproof marker and then see if they still line up later, if not then it is "spun". I can try that next time I go out and see what happens.

By the way, how would I go about having it re-hubbed? Would I just take it in to a outboard shop?