Knocking and losing power

Hey everyone. New user. Great resource site! I have a '98 Johnson 40 that has run great for the last 8 years. But, I'm having my first problems with it.

The initial start in water it fires up but knocks like crazy in idle. When I increase RPM's or put it in gear the knocking goes away. It doesn't troll at lower speeds the way it use to either. It pukes when I bring the troll speed below 2.3 mph. When I open it up, it loses power and gains power on it's own. I'll go from 36 mph down to 23 without touching the throttle.

I replaced plugs and have gone through 2 tanks of gas with same results. I'm a novice but through searching this site figured it's either the carburetor or plug wires. Any suggestions? Thanks for your help!!

Re: Knocking and losing power

Spark test and compression test are your first steps. Post the results and we'll go from there.

Re: Knocking and losing power

I'm a novice. Do I buy a compression tester and put it where the plug goes and turn it over? Is it that simple? How much compression am I looking for? Thanks again!!

Re: Knocking and losing power

It's that simple. Pay no attention to the value, only how close they are. You'll want to get the screw-in type compression gauge.

Be sure to pull ALL the plugs when testing. You can either ground the plug wires with nails or screws placed against the block, or just jump the starter solenoid with the key in the off position.

Post the results.

Re: Knocking and losing power

I checked compression and they were all pretty equal. I checked it cold and dry. I cranked each check at 3-4 turns and they all came out around 90 (88-92). I noticed that the middle plug was a little darker. I haven't been able to do a spark test yet because I haven't had another person there to turn it over for me. I'll let you know what I get.

Re: Knocking and losing power

Those readings arent bad, they say you want to be within 10 percent of each other, which you are. Have you ever rebuilt your carbs or done a decarb??

Re: Knocking and losing power

No I haven't. I was reading up on cleaning the carb on the Internet because I'd like to do it, but have no experience at all. In fact, I don't know if I can even find the carb. Is it hard to get to?

I just ran the spark test and they passed.

Re: Knocking and losing power

Yeah the compression and spark tests passed, I feel your issue is in fuel delivery or carb related. Carbs are a good job for beginner, but I think a manual would more than pay for itself. There are alot of little orifices and nooks and crannies that need special attention, one of the more experienced guys on here may be able to talk you through it, or send you a link that would help.

Re: Knocking and losing power

Thanks for your help. I'll post a topic about it and hope someone will help.