Loud knocking on Deceleration 115 Merc

I have a '97 Astro with a 115 Merc. When decelerating it makes a loud horrible knocking and it sounds as if it is going to blow up. It does it only when decelerating, and when my carbs where gummed, it would do it every time I had a lull in speed. Compression is great and it does not do it when decelerating in neutral so i think it may be lower unit related. I've seen some people comment on the use of SS props making a similar knocking sound but I have an aluminum prop, granted it is a little beat up. Could it be another problem? it is a 19 pitch, maybe getting a new prop with higher pitch, and new hub will help?

Re: Loud knocking on Deceleration 115 Merc

Are you chopping the throtle, or coming down slowly, this is a common problem sometimes with some engines if you chop the throtle too fast. Try slowing sown slower and see if that helps.

Re: Loud knocking on Deceleration 115 Merc

I have one of those noisy engines you mention. I have spent a reasonable amount of time analyzing it.<br /><br />Mine is prop related, but since yours is aluminum the Merc mechanic said that they are much lighter and less prone to do it.<br /><br />What is probably happening is that you are shutting the throttle down fast and the clutch dog on forward gear is freewheeling....the prop is turning faster than the engine (being forced to turn by the water running by it).....that's why it's on there and only on forward gear. <br /><br />If you want to know what I'm talking about, with the engine off, put it in forward and turn (attempt to turn) the prop in both directions. Where you get resistance is the direction of drive when the engine is driving. Where you get clicking is the direction where the prop is attempting to do the driving.<br /><br />When you go to neutral the gears are disconnected so there is no dog engagement to freewheel.....click.<br /><br />Check your lower unit lube; maybe change it if you haven't for awhile, and don't cut the throttle so fast.<br /><br />Mark

Re: Loud knocking on Deceleration 115 Merc

Thanks for the reply! although it does it with little throttle release, I feel sure that is the problem. My main concern is that it isnt going to eventually break or mess something up. I may just get another aluminum prop to see if it will atleast lessen the noise.

Re: Loud knocking on Deceleration 115 Merc

I noticed that Turning Point offers props with a new masterguard hub system that supposedly reduces vibration and may elimate clutch rattle. Do you think this may help? Does anyone have any experience with these props and hubs?