1994 Johnson 25 runs great then begins to knock

[Rdeas1]

My 94' Johnson electric start will run absolutely perfect sometimes for two hours or more then lose power and begin knocking violently over idle speed. I'll get it back to the launch and to my shop and It'll run fine. I've got new plugs, fuel pump, hose and tank but I don't think it's any of these. I haven't gone to in depth with trying to diagnose the problem....any suggestions before I delve into it??

 

[Walker]

Re: 1994 Johnson 25 runs great then begins to knock

Not sure if your motor has "SLOW" but it sure sounds like that is whats happening. Are you sure your motor isn't getting hot?? Does your alarm system work?/

 

[Rdeas1]

Re: 1994 Johnson 25 runs great then begins to knock

It does not have SLOW or an alarm system. I have thought of overheating but assumed that because it's spitting out water that could not be the problem. That would be a very likely solution considering that it runs fine to a certain point then runs again after it sits a while.

 

[iwombat]

Re: 1994 Johnson 25 runs great then begins to knock

How long since you've changed the impeller and/or thermostat?

 

[Rdeas1]

Re: 1994 Johnson 25 runs great then begins to knock

I have'nt. I bought it used a couple of years ago. The previous owner ran it in a test tank for me and there were no problems. I was redoing a boat at the time so it was several months until it was actually put to use other than cranking it occasionally. I can't really point a finger at when the problem started occuring. Someone suggested that it could be a power pack or ignition coil, but I disagree. I would appreciate any further advice on the problem.

 

[iwombat]

Re: 1994 Johnson 25 runs great then begins to knock

Regardless of whether the problem is an overheat or not, you should replace the impeller every couple of years as standard maintenance.

All that being said, when running it in the test tank can you hold your hand on the powerhead for a few seconds before it gets too hot? If not you've got an overheat problem.

Given that this is a used motor though I'd just replace the impeller and thermostat anyway and have a little piece of mind. They're cheap and easy to replace.

Normally replacing parts w/o diagnosing the problem isn't the way to go. But, this is one of those cases where a possible cause can be eliminated by standard maintenance items. So, why not?

 

[Rdeas1]

Re: 1994 Johnson 25 runs great then begins to knock

What are the chances that its an electrical problem?

 

[iwombat]

Re: 1994 Johnson 25 runs great then begins to knock

Until it's all been diagnosed it could be just about anything. Your first order of business on diagnosing an electrical problem is to do a spark check.

I'd still tackle that impeller before going any further. Who knows how old it really is. If it goes, you can easily toast the whole powerhead.

 

[Scaaty]

Re: 1994 Johnson 25 runs great then begins to knock

I'd still tackle that impeller before going any further. Who knows how old it really is. If it goes, you can easily toast the whole powerhead.

absolutely perfect sometimes for two hours or more then lose power and begin knocking violently over idle speed.

Get off the impeller issue.....its something else

 

[iwombat]

Re: 1994 Johnson 25 runs great then begins to knock

I never said the impeller was _the_ issue.

In the process of asking questions, we came up with a used motor that has an impeller of indeterminate age and a _possible_ overheating problem.

 

[Walker]

Re: 1994 Johnson 25 runs great then begins to knock

The impellor could very well be the issue. Waterpump condition may just be on the edge of being not quite good enough. After running for a while the engine retains enough heat to cause problems.

 

[Scaaty]

Re: 1994 Johnson 25 runs great then begins to knock

The impellor could very well be the issue. Waterpump condition may just be on the edge of being not quite good enough. After running for a while the engine retains enough heat to cause problems.

But TWO HOURS?

 

[Silvertip]

Re: 1994 Johnson 25 runs great then begins to knock

One thing is certain, knocking and losing RPM says we have a heat, lubrication, or ignition problem. Are you sure the fuel/oil mix is correct at 50:1? A sticky thermostat is a possible cause for overheating. As for the impeller, I agree its probably not the issue but since its condition is unknown its worth replacing it. Just so you know, an overheating engine can have a perfectly functioning water pump. Since we can't hear the engine we must assume the description is indeed a "knock" and not a "dropped cylinder" that is being confused as a knock. That could very well be a pack issue. When it acts up a spark check is in order. Tough to do when you are in the middle of the lake. Since it is intermittent, check the pack ground connection.

 

[freddyray21]

Re: 1994 Johnson 25 runs great then begins to knock

I've seen packs get hot and fail to fire. Best way is to run it until it starts acting up then check for hot fire on all plugs.