Johnson 18 hp 1964

[wildwood11]

I have a Johnson 18 hp that does not run well. It struggles to start, and then when running seems to struggle at about half power. It has spark on both cyls and the carb has been cleaned. What could be wrong. Last year it did run at full power once then dropped back to half power and has been there ever since. Ideas?

Thanks

 

[F_R]

Re: Johnson 18 hp 1964

One thing for sure, it isn't the carburetor.

Your motor is running on one cylinder. Check compression. It's probably ok, and if so, try the simple check first---replace spark plugs. (Check for water on the plugs when you remove them and hope you don't find any). Also make sure it isn't arcing out at the boots.

Spark plugs didn't fix it??? The flywheel has to come off to repair the ignition. Yeah, I know you said it has spark. So will a motor with old cracked coils.

 

[wildwood11]

Re: Johnson 18 hp 1964

So let me update this a bit. I did as FR said. Replaced the plugs, found no water on the plugs. Pulled the flywheel and replaced points and coils. But still having the same problem. I dont have my compression tester up this weekend and will try that next but it seems good. I think FR is right about it only running on one cyl. I went out and got 2 new condensers as well just in case and will put them in as well. Any other suggestions.

Thanks

 

[F_R]

Re: Johnson 18 hp 1964

Sometimes new points have an invisible coating on them that prevents good electrical contact. Try cleaning them. If you have a good digital multimeter, check the resistance between the wire terminal connection and ground. Should be around an ohm when points are open and zero when closed. That is zero---not just close to zero. Can we assume you have them both gapped at .020" at their widest opening?

Another suspect is the spark plug wires. Sometimes they will arc through where they are clamped to the armature plate. Makes an almost invisible pin hole where it arcs through. While you are at it, check the resistance from end to end. Should be near zero. If open or high resistance, not good.

If no success yet, at least figure out which cylinder is missing. Remove and ground one plug wire and see how it runs, then repeat with the other wire. Chances are it won't make any difference with one disconnected and won't run at all with the other one disconnected. That is the good one. Now you can zero in on the bad one.