98' 120 hammering/knocking

stormy weathers

Seaman Apprentice
Joined
Apr 24, 2009
Messages
31
Motor runs stong and idles good. At 4200 to 4800 it makes a hammering sound. Over 4900 to WOT runs strong. Any ideas?
 

foxtrotboss

Seaman Apprentice
Joined
Jul 25, 2009
Messages
46
Re: 98' 120 hammering/knocking

I would check compression. Mine was making noise in the mid range so I checked the motor for basics and found a bad cylinder with a broken ring. it had 75 psi.

That's all I know to do for now just check it out for basics the problem ight show itself.
 

john from md

Commander
Joined
Apr 13, 2008
Messages
2,184
Re: 98' 120 hammering/knocking

Is the engine jerking any when you are hearing these noises?

Give us some history on the engine and what kind of maintenance it has had.

John
 

stormy weathers

Seaman Apprentice
Joined
Apr 24, 2009
Messages
31
Re: 98' 120 hammering/knocking

Both salt water and fresh use. I run for 30 to 40 min out in the Gulf at 5000rpm. When I slow down and run at 4400 this is when I hear the quick hammering noise. If I speed up or slow down it goes away. I have had the motor for 1.5 yrs rebuild carbs, 1 set of reeds and water pump, 2 set of spark plugs.
 

john from md

Commander
Joined
Apr 13, 2008
Messages
2,184
Re: 98' 120 hammering/knocking

When you rebuilt the carbs, did you pull the rubber needle valve gasket and replace it and the little ring that holds it in? The reason I ask is that gasohol is causing rubber materials to swell and stick. If you have the old type seats, they could be expanding which does two things, it causes the needle to stick and leans the engine. The knocking noise can be detonation from a lean burning engine.

This is only a theory that I have worked on with some other guys. We figure that at WOT, the fuel pump has enough pressure to overcome the sticking valve and at slower speeds, the engine doesn't need the same amount and the weight and spring of the float overcomes the sticking.

It is only a theory but it has happened to my 85 and seats fixed it. They only cost about $5 each and the clips are $2.50 and are easy to lose.

Hope this helps,

John
 

stormy weathers

Seaman Apprentice
Joined
Apr 24, 2009
Messages
31
Re: 98' 120 hammering/knocking

I have one more run this year to my secret reef and a great gas well next week. Do you think I can turn my carbs a little richer? I will not have enough time to go through the carbs. I will try to rebuild after this trip. The last time I rebuild the carbs I did not have a diagram, but I did not have any extra part:D:D. I plan to run this motor till Jan, and then repower my boat for the spring.
 
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