1975 25 HP Johnson w/ issues

[schwar74]

Greetings,

This is an amazing website. Incredibly helpful.

I have the above motor. I bought it very recently to duck hunt w/ and have had trouble getting it to function properly. There are two issues:

1: It starts pretty well w/ the electric choke engaged, and will idle with the manual choke engaged. The manual choke only engages the the choke about 3/4. After the Electric choke is engaged, the motor races for a period of time, then slows down. If I disengage the manual choke, it slows way down and then dies very quickly. It also, at times, will seem to have an out of time issue, where theres a big puffing/coughing sound that comes out of the exhaust. Sometimes it coughs and kills then engine. This doesn't happen all the time.
I went ahead and ran it this morning with the manual choke engaged. As I gave it more throttle (in gear), it lost power, then would have surging power. It ran at lower throttle settings but continued to surge. I never could get it to run w/o the manual choke engaged.

When I got it home, I pulled the plugs. Top cylinder plug had carbon buildup visible on the plug and on the top of the piston. The lower cylinder looked good. Can I assume there's poor combustion in the top cylinder, and that it was only running on one cylinder? When I checked the spark on the top plug, it was pretty poor, sparking from around the central electrode rather than directly across the gap. I pulled the other plug, put it on the top cylinder plug wire, and it seemed to fire pretty normally.

Could all of these symptoms be due to a bad plug causing poor or only occasional firing in the top cylinder. Is just changing the plug likely to fix the problem? (Please, please, please) Is the Champ J4C plug the right one, and what's the recommended gap? Any other thoughts/ideas?

Could it just be the idle mixture setting that's keeping it from idling w/o the choke? I read the instructions on setting the needle valve. It says fully engage the valve. Dumb question, does this mean the screw it in until it stops, and then back it out 1 1/2 turns? There's a stopper on the threads. Is this just a "reminder", or does this indicate full engagement?

Wow, I'm getting long winded. Sorry.

Issue 2: Much simpler, when I was finally running it in gear, it wanted to kick out of gear. Is there an easy way to adjust this, or is it a sign of bad gears?

Thanks so much.

Scott

 

[58rude]

Re: 1975 25 HP Johnson w/ issues

You need too verify spark with a tester. You might have a cylinder not firing. The racing sounds like its running lean. It really sounds like you have a carb problem and you need to rebuild them, along with the rest of the fuel system. Ethanol can eat the hoses from the inside out. I normally rebuild the fuel pump when I do the carbs, the diaphram inside gets stiff and does not work well. The experts will help on the lower unit, but it might be a bigger problem. The clutch dog ears or gears might be worn, but the pros will be along to help you there. Good luck

 

[oldcatamount]

Re: 1975 25 HP Johnson w/ issues

I agree with with 58rude, You have a dirty carb or other restriction in the fuel system. Not a big deal at all to fix. A quality carb kit (from Iboats of course) is relatively easy to install and you'll get advice (if you need it) from this site. Your other issue does sound like a worn clutch dog and that to is fixable (by you). It will be a little more expensive for the clutch dog, but, It's do-able and the motor is well worth it.

 

[tx1961whaler]

Re: 1975 25 HP Johnson w/ issues

Agree with dem guys ^^
A motor that runs with the choke on and dies with it off usually has a plugged up carb.

 

[schwar74]

Re: 1975 25 HP Johnson w/ issues

Thanks for the tips. Its amazing what I'll forget to mention sometimes.

I didn't write in my prior diatribe that I pulled the carb off and removed the pan so I could look in the float chamber. It was spotless. The needle valve was moving nice and freely. The jet going up into the throat of the carb looked clean and clear too. Is there somewhere else in the carb for the fuel to hang up? I couldn't really see where else to disassemble it. The fuel line from the pump to the carb is nice and firm (on the outside at least). There was a screw missing on the side of the float well (??) that the electric choke fits on that was missing. Maybe that was acting as a vacuum leak and was keeping a good stream of fuel from being sucked up into the throat? I was able to find another screw that matched to replace it.

What do I make of the difference in the appearance of the two spark plugs, and the top of the two pistons? The top chamber had lots of carbon buildup, the bottom one looked pretty clean.

Thanks again.

Scott

 

[jbjennings]

Re: 1975 25 HP Johnson w/ issues

Just a few of my ideas:

1. You may have a bad spark plug, and likely have some problem with your igniton system, i.e. a weak coil. Ignition problems can mimic a carb problem. Your motor can use j4c or j6c, but the j4c's are the coolest and best to use unless you do a lot of trolling.
2. It is virtually impossible to look at a carb and determine if the tiny passages and tubes inside the carb are clean. Soak it in some carb cleaner and then blow it out with compressed air or carb cleaner in a can. Sometimes you might even have to take out the welch plug to get the idle jets clean, but for me it is rare.
3. Your lower unit could have a bad clutch dog and gears, the shifter could be out of adjustment, or the connector under the powerhead might be worn out. You basically are going to have to make that determination yourself. It may involve taking the lower unit apart. If you don't know what to look for, a totally worn clutch dog and gear ears may LOOK fine to you. If you've got a camera, you might take pics of what you see and let the experts here take a look.
Good luck,
JBJ

 

[lindy46]

Re: 1975 25 HP Johnson w/ issues

"It also, at times, will seem to have an out of time issue, where theres a big puffing/coughing sound that comes out of the exhaust. Sometimes it coughs and kills then engine. This doesn't happen all the time."

This could be a symptom of a sheared/worn flywheel key. Has the flywheel been off recently? If the nut is not torqued properly, the key may shear, causing the backfiring.

 

[tony edwards]

Re: 1975 25 HP Johnson w/ issues

dont think you have a timing problem. spitting back thru carb indicates a lean condition. also, the choke can be adjusted on this model. in order to close all the way electrically and manually. if you are sure that the carb is absolutely clean, then i would do a linkage and synchronization on the engine. most manuals will have the info on how to do this. this is critical to proper engine operation. these engines typically last foreve



r with proper maintenance

 

[hawkerjv]

Re: 1975 25 HP Johnson w/ issues

Hi,
1975 had a low tension ignition (if I remember right), kind of a precursor to the electronic ignition. Exciter coil, two sets of points and condensers under the flywheel. Two external coils. Try swapping the external coils. Maybe one set of points not opening.
Jim