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Old 05-18-2007, 12:58 PM
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Default Yamaha 150 4 stroke "vibration" at 800-900 rpm


Twin 4 stroke 150s, 2006., 97 hrs. When I go into gear and creep forward at 800-900 rpm, port motor has almost a valve knock sound. Slight increase in forward speed, it goes away. If I put it in reverse, then back into forward, it usually goes away too, for a while. Sounds like something is loose, vibrating at a very low rpm. I checked the rudder zinc, seemed tight, wondered if you had any ideas., maybe some simple things I can try to troubleshoot myself.


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Old 05-18-2007, 02:06 PM
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Default Re: Yamaha 150 4 stroke "vibration" at 800-900 rpm

This was Andy's reply from S.I.M. Thanks.....

Sounds like what you are describing is drive shaft bounce. The vertical drive shaft is pre-loaded in the up position, and at idle\low engine speeds with a SS prop and a 4-cylinder 4-strokes delay in between the power strokes, the drive shaft is loaded-unloaded-loaded-unloaded. For some reason the counter gear cases are worse. What you are hearing is the actual knock from the gear back lash and the drive shaft moving up and down a few thousands.

It happens......not much can be done about it except run a aluminum prop or turn up the engine idle speed . It is a fairly common complaint especially on the counters. I just installed a pair of F150's and sea trialed them yesterday. On the way back in and coming into the dock at a low engine speed, but faster then idle hull speed the counter gear case was knocking away. I purposely stopped the boat and ran at a truer forward idle speed and it went away
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Old 05-18-2007, 03:41 PM
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Default Re: Yamaha 150 4 stroke "vibration" at 800-900 rpm


A little more from Andy.....


If you were to take your gear case off and physically grab that vertical drive shaft you would be able to lift it up and down a little. That movement is the gear set's back lash that is needed on all lower units gears. To little backlash, they do not last, to much and they do not last.

The F150 when its running and from the cut of the reverse-forward and pinion gears, wants to push upward on the driveshaft. This is the shaft that connects the gearcase to the powerhead. Since there is backlash(movement) of the shaft and at low engine speeds much less load is on the shaft, it moves up an down that back lashed amount. With a 4-stroke 4 cylinder, the engines powerstrokes are farther apart compared to a V6 4-stroke. This is the reason you never hear of this on a 2-stroke engine since there power strokes are much closer together.

The water pump has a series of three washers and two keepers that are supposed to hold the driveshaft in the upper pre-loaded position. Most likely they are not. You can remove the lower unit and see if they are pushed down, but chances are high that they will just slip and the noise will come back.

Keep an eye on the gearlube. Yamaha is aware of the nuisance but as of now does not have permanent fix for it.
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Old 05-18-2007, 04:36 PM
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Default Re: Yamaha 150 4 stroke "vibration" at 800-900 rpm

What props are you running? The reason for asking is if your running Mercury props you could get that shift clunk idle rattle. The fix is to install Flow Torque 4 hub kits.
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