View Full Version : Help diagnosing throttle/drive problem?
01-16-2007, 12:44 PM
Got a 14 year old Johnson 40 hp on my 16.5' side console boat. This is an occasional occurance...maybe 3-4 times last season.
Engine starts and runs good. Sometimes when you increase speed at throttle, the engine speed increases but the boat speed remains about the same. By taking the boat out of gear, pushing the neutral or disengage button, and dropping the throttle into reverse and reving the engine slightly, the problem goes away.
Then, when you throttle back to neutral and then to forward, the engine performs normally, and the boat takes off/comes up on plane.
Does this sound like a throttle/linkage problem, a lower unit problem or what?
Boats...... a hole in the water you pour money into! lol
Thanks in advance for your thoughts guys.
01-16-2007, 02:01 PM
could be a prop hub slipping because of extra weght in the boat on the 3/4 times that it happened last year. just a wild guess.
01-18-2007, 09:52 PM
Linkage - the manual should give the adjustment specs, should be able to find it on the net.
01-18-2007, 10:32 PM
just a thought ... did you go through weeds at a slow speed and then experience this? .... Could just be weeds wrapping around the lower unit causing cavitation which would have the very effect you are describing... when you slowed down backed up etc the weeds came off and you were good to go ...
01-19-2007, 12:12 AM
It's sucking air in from above and it loses its bite. There are a few things to check but it's late and I'm tired. I'll check in tomorrow. You should be able to fix the problem for free.
01-19-2007, 06:23 AM
try a dirrerent prop...sounds like you spun the bushing...can be repaired $80 or so...
01-19-2007, 09:03 AM
Take a permanent marker and draw a mark across the hub and the prop. Check the alignment of the mark after use. That will confirm or eliminate the prop as your source of trouble.
01-19-2007, 10:31 PM
If your hub is worn and is slipping it's going to slip all the time, not just 3 or 4 times in a season. That's about like saying you have a hole in your tire but it only leaks every once in a while.
You have the classic symptoms of a cavitation problem. A 40 hp engine all by itself won't have the power to cavitate a prop if it is correctly set-up or is being run in the proper position. However, if even one thing is wrong, you'll get issues.
OK, so you take off it's fine. (That all by itself tells you the hub is fine.) Once you are running and give it throttle, the engine revs but nothing happens. It does that because the anti-ventilation plate is up too high in relation to the keel of the boat and is allowing the prop to suck air. In other words, the engine is mounted too high or you have a short shaft engine and need a long shaft. Most likely it's the former. A 16.5' foot boat with a 40 is a good boat for this. Chances are the stern doesn't sit very low in the water and you have gear in the boat. That levels the hull and actually brings the anti-vent plate up. Does the engine have trim? I would think a 40 does not but it may. If not, what position is your lock pin in? Is the anti-ventilation plate parallel to the keel or is there an angle?
To check all this and see where you stand, do this. Put the anti-vent plate parallel to the keel. Then measure where the plate is compared to the keel. It should be NO MORE than 1" above the bottom of the keel. If it is higher than that, the engine needs to be lowered on the transom. It will sit under water when you are taking off but should ride on top of (touching) the water when you are on plane. This keeps the prop from forming a vortex and sucking air into the blades. While the boat is on the trailer, do the steps and see where you are. Adjust as needed and take her out for a spin. Your problem should be gone.
If you have to move a pin to lower or raise the attitude of your outboard, keep in mind that the angle can change as the load in the boat changes. I would start with the pin in the second lowest hole and go from there. All the way down is too far down and will result in the boat "plowing" through the water. Too high and the boat will suffer from bow rise and the prop can cavitate. This may currently be your problem. If your outboard has trim & tilt, always take off with your engine trimmed fully down (tucked in) and trim up a bit as she comes up on plane.
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