Johnson 40 RPM's rise while at trolling speed

tiger king · Oct 17, 2004

I have a 1990 Johnson 40 (model J40EESR) mounted on a 14' rigid inflatable. The problem I am having is that while at trolling speed (about 7 knots)the engine occassionally increases RPM's and speed to the point where I have to back it off with the throttle. It seems to happen more often when travelling downhill and a swell gives it a push. 7 knots is also the only trolling speed it will hold. If I try to attain 8 or 9 knots the engine cannot sustain this speed and wants to go faster. It seems as though the boat wants to jump up on plane. Somebody suggested a hydrofoil mounted on the cavitation plate. Is this a problem with the engine or will a hydrofoil help? Recent work: new fuel line, fuel filter,carbs cleaned, new plugs. thanx, john

fireman57 · Oct 18, 2004

Re: Johnson 40 RPM's rise while at trolling speed

The push from a swell increases the speed of both my boats. Nothing you can do about it. It's going downhill. I would suspect when you try to go a little faster that the powerpacks are kicking in to give it a boost. This usually happens when you are coming out of the hole and that is their job. They give the power when it feels the boat needs it. I don't know that it is a problem as much as an inconvenience.

Mark42 · Oct 18, 2004

Re: Johnson 40 RPM's rise while at trolling speed

when you try to go a little faster that the powerpacks are kicking in to give it a boost

Fireman, can you expand on that statement? What are the powerpacks doing to give a boost? Thanks, Mark

tiger king · Oct 18, 2004

Re: Johnson 40 RPM's rise while at trolling speed

Originally posted by fireman57: The push from a swell increases the speed of both my boats. Nothing you can do about it. It's going downhill. I would suspect when you try to go a little faster that the powerpacks are kicking in to give it a boost. This usually happens when you are coming out of the hole and that is their job. They give the power when it feels the boat needs it. I don't know that it is a problem as much as an inconvenience.

tiger king · Oct 18, 2004

Re: Johnson 40 RPM's rise while at trolling speed

thanx fireman, I did not know that the power packs give the engine a boost when in a hole. I can understand that my boat would speed up when a swell passes but the problem I am having is that my engine just takes off and never backs down on its own after the swell passes. Is that normal?

fireman57 · Oct 18, 2004

Re: Johnson 40 RPM's rise while at trolling speed

The powerpack or coils supply the spark to the plugs. a lot of times one will be weak and kick in and out slowing down or speeding up the engine as it gets spark to the plugs. I don't know why your engine would take off and not back down once you are off the downside of the wave. It isn't normal for your rpms to go up basically for no reason and not come back down until you turn the throttle back. I don't think a hydrofoil would make any difference. Your engine wants to get up to plane about 8 or 9 knots and I don't know that there is anything wrong because all engines want to kick in and plane at a certain speed or when the engine feels like it needs more power it asks for it and it is getting it. Maybe Joe Reeves or one of the other Life Masters will find this post and can guide you.

tiger king · Oct 19, 2004

Re: Johnson 40 RPM's rise while at trolling speed

I just found a local outboard mechanic. I'll check his opinion and let you know how this ends up. Thanx fireman. john

fireman57 · Oct 24, 2004

Re: Johnson 40 RPM's rise while at trolling speed

Any word yet?

tiger king · Oct 31, 2004

Re: Johnson 40 RPM's rise while at trolling speed

I was told to adjust the throttle linkages- I basically performed a synchronization without adjusting the timing or idle speed. I adjusted the idle speed screw, throttle control rod, spark control rod, throttle cam and replaced a broken follower roller on the spark lever cam. Well, just returned from a test run and no change. The boat still will not hold a constant speed in the 7 knot to 10 knot range. I encountered another problem today. The engine went into a series of surges. When I squeezed the bulb it stopped surging. Could a failing fuel pump be the cause? john

fireman57 · Oct 31, 2004

Re: Johnson 40 RPM's rise while at trolling speed

Jphn, glad you're back but not happy you still have the same problem. If pumping your primer bulb helped then you definitely have a fuel delivery problem. A weak fuel pump could be the culprit.There is a rebuild kit that is very inexpensive. For my 25 it was around 9 bucks. Replace the diaphrams and all other parts that are in your fuel pump. It could be that not all parts you receive in your kit will go to your pump as they make some of these to fit a range of engines. You might also want to take the carb apart and check the jets in there and make sure that they are clean. You might even let them soak overnight and blow them out with compressed air to be sure. If you have a low speed jet that is clogged this could cause the same symptoms.

v12mac · Nov 1, 2004

Re: Johnson 40 RPM's rise while at trolling speed

I am not a mechanic but could this engine be running a bit lean when going "Down Hill"? Could be re-create the condition with trim? Mac

tiger king · Nov 1, 2004

Re: Johnson 40 RPM's rise while at trolling speed

I will trouble shoot the fuel delivery system today. I have a new fuel line, fuel filter and we recently cleaned the carbs (although we didn't soak them or replace any parts). I just read every post on surging and will replace the female fuel line connector O-ring and get a fuel pump kit as you recommended Fireman. If that doesn't work, off come the carbs again. I don't know if I can play around with trim as that is broken- I lift the engine manually. john

fireman57 · Nov 1, 2004

Re: Johnson 40 RPM's rise while at trolling speed

There are posts on fixing the trim too!!

tiger king · Jun 17, 2005

Re: Johnson 40 RPM's rise while at trolling speed

The increase in rpm's was fixed by replacing a second follower roller and having the throttle linkages adjusted by an outboard mechanic. Now the boat will hold a constant trolling speed. Thanx Fireman and others! john

R.Johnson · Jun 17, 2005

Re: Johnson 40 RPM's rise while at trolling speed

fireman57: I have heard some wild explanation's of what a power pak will, and will not do. Your's is the best I have heard. Will it respond to a dirty look?

fireman57 · Jun 17, 2005

Re: Johnson 40 RPM's rise while at trolling speed

What I was trying to say was that a lot of engines when you are trolling or at a slow to medium speed have a range where they run a little rougher like they are wanting to get up on plane, they feel like they are cutting in and out. If the downside of the wave he was on helped it get over that hump then it would increase his speed and the rpms would rise thus seeming like he was throttling up a little. If he had an intermittent pack that might have been the rpms that it would fail and recover at. Not a good explanation back in October. Must have had too many cool tasties that week.

Johnson 40 RPM's rise while at trolling speed

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