1987 48 spl has a knock or rattle?

[mdcwells]

I have a 48 spl that is either knocking or rattling. You can hear it well at idle and a low speeds. I pulled the plugs and moved the cylinders to the top. When I push on them and they start to go down the flywheel spins with them right away. After seeing that I don't feel like it's piston slap. If I run in the tank in the garage you can hear it a lot better from the front of the motor than the back. I havn't pulled the flywheel yet (because that is no fun) but it almost sounds like something could be rattling. The motor runs tip top. Pushes a 16 ft alumacraft classic almost 40 mph. Starts good, idles decent. What do you guys think?

 

[tashasdaddy]

Re: 1987 48 spl has a knock or rattle?

if flywheel was rattleing, loose it would not be running.

 

[kenmyfam]

Re: 1987 48 spl has a knock or rattle?

Any rpm range where it is worse or better ??

 

[mdcwells]

Re: 1987 48 spl has a knock or rattle?

Seems worse at idle or lower rpms. It could be I just can't hear it when I'm running hard. Thought it may have been a timing issue, but timing is right on.

 

[mdcwells]

asdf

 

[kenmyfam]

Re: 1987 48 spl has a knock or rattle?

Low pitch rattle or high pitch ???

 

[Benny1963]

Re: 1987 48 spl has a knock or rattle?

have you run a comp check? may have collapsed piston skirt or cyl that has bad rod bearing or scuffed cyl .mine had knock i pulled heads and found one piston had colapsed a skirt from overheat but check under fly wheel first and check to bearing see if your flywheel has any play back and forth .

 

[mdcwells]

Re: 1987 48 spl has a knock or rattle?

Seems like a low pitch sound. very steady. I'm taking this boat to lake truman to crappie fish in a few weeks. You guys think that is a bad idea?

 

[mdcwells]

Re: 1987 48 spl has a knock or rattle?

The flywheel doesn't seem to have any play in it, I still have yet to pull it.

 

[mdcwells]

Re: 1987 48 spl has a knock or rattle?

compression top cylinder 110
bottom cylinder 150

 

[kenmyfam]

Re: 1987 48 spl has a knock or rattle?

compression top cylinder 110
bottom cylinder 150

That is wildly different. You should be within 10% on the compression reading. A lot of compression testers will give a lot of different readings but will usually give a good similarity. Both are good readings but I wonder how accurate your tester is. Can you try it again with another tester ???

 

[jonesg]

Re: 1987 48 spl has a knock or rattle?

compression top cylinder 110
bottom cylinder 150

hello!

 

[mdcwells]

Re: 1987 48 spl has a knock or rattle?

I need to break down and buy a tester. I sold a different motor to a guy today and he brought his tester with him. We tested the compression on the motor I sold (1975 omc 25 hp) and it came up with 125 and 130.
We pushed the throttle wide open to get air through. Bottom cylinder came up right away.

 

[mdcwells]

Re: 1987 48 spl has a knock or rattle?

Bought a new compression tester. Top 110lbs, bottom 145

 

[kenmyfam]

Re: 1987 48 spl has a knock or rattle?

Bought a new compression tester. Top 110lbs, bottom 145

Still wildly different eh !!! maybe something wrong within !!!!

 

[mdcwells]

Re: 1987 48 spl has a knock or rattle?

Another thought. I traded a ruger 10/22 for a 1983 sea nymph with a Johnson 50 hp. The decks are badly rotted was going to do as a project. I have it tarped and put away. I checked it and it was 150 on both holes. Would it make sense to just put the 50 hp on the v bottom and keep the 48 spl for a parts motor. They seem to be very similiar. Or would it be a waste to use the 48 for parts. Only have about $150 in the 50hp and boat.

 

[jonesg]

Re: 1987 48 spl has a knock or rattle?

Runs tip top you say?

Have you tried seafoam, could just be a stuck ring due to carbon or...
it might just be a bad head gasket, only cost you $10 to find out and you can inspect the bores.

Seafoam worked on mine, it started out with 60 psi, went up to 72psi with a 10 minute soak of seafoam spray, then adding startron to the fuel it crept up to 83psi after a few runs. Give it a shot.

 

[mdcwells]

Re: 1987 48 spl has a knock or rattle?

I did just run seafoam in the last tank of gas