99 moomba outback Noise under load

[mr408]

1999 350 indmar assault 310 hp carburated 750 hours

Rough idle all year, except after coming in from skiing, then the idle is fine .
Last night developed strange knocking under load. No noise when idling or revving the engine to 2500. We took off the valve cover and checked rod spin and springs and wiggled the rocker arms. Sound seems to be coming from front piston area. Not sure what to check next. Oil was p[resent in all areas under valve cover. We ran it without cover under load to check the noise. It decreased as I let off the power, increased as I gave it gas. Sound is not constant, like a tap tap tap, it seems a little out of synch with motor rotation. I checked all plugs on that side. One was a bit dry others were okay.

 

[mr408]

Re: 99 moomba outback Noise under load

Wondering if detonation could be occurring?

 

[Bondo]

Re: 99 moomba outback Noise under load

Wondering if detonation could be occurring?

Ayuh,... That'd be My guess...

Was the clean plug White,..??
Was it from nearer the noise,..??

Check the contents of the fuel filters,...
Ya can't burn Water...

 

[krisnowicki]

Re: 99 moomba outback Noise under load

sounds like spark knock? carbed???

 

[mr408]

Re: 99 moomba outback Noise under load

Plug was not white. I had a friend listen and he thinks it's loose valves clattering. I did check fuel water separator and it was good. No water.

Will let you know after I get valves adjusted.

Sound seems to be moving around the top of the engine. Spark knock moving around to different cylinders?

 

[mr408]

Re: 99 moomba outback Noise under load

Had a chevy mechanic listen to the engine with the valve covers off. He said spark knock, and the Holley was skipping a bit. Needs a rebuild and the jets cleaned out.
He recommended higher octane fuel.

I put 10 gallons of 93 octane in and a bottle of 104 octane booster. Boat has a 28 gallon capacity and is now full with 18 gallons 89 octane and 10 gallons 93 with the booster.

Sound of the knock is diminishing, but still there under load. Running rough, maybe due to deposits being cleaned out.

 

[krisnowicki]

Re: 99 moomba outback Noise under load

spark knock is caused by a lean condition and will tear up that engine which has given you 700 hours of service. Do your self and the engine a favor and get that carb done asap before running it with any octane booster in the engine. If the carb is running lean then there is still a shortage of gas.

 

[Bondo]

Re: 99 moomba outback Noise under load

spark knock is caused by a lean condition and will tear up that engine which has given you 700 hours of service. Do your self and the engine a favor and get that carb done asap before running it with any octane booster in the engine. If the carb is running lean then there is still a shortage of gas.

Ayuh,... Kris is Right,... Fix the lack of fuel issue...

Detonation will turn a boat motor, into an anchor Very Quickly....

 

[mr408]

Re: 99 moomba outback Noise under load

I'm ordering the rebuild kit today. The guy that diagnosed the knock will come to the lake and rebuild it on the boat.

Thanks for your advice and help. Will keep the thread updated.

 

[Bondo]

Re: 99 moomba outback Noise under load

The guy that diagnosed the knock will come to the lake and rebuild it on the boat.

Ayuh,... The Carb Kit,..??

If it ain't goin' back to a Shop, with a Carb Dip-tank, 'n abundant compressed air,...
It ain't really a Rebuild....
It'll be 1/2azzed, at Best...

 

[mr408]

Re: 99 moomba outback Noise under load

I'll ask him about that option this afternoon. Thanks for the heads up.

 

[krisnowicki]

Re: 99 moomba outback Noise under load

Take a couple wrenches to the boat and take that bad boy off and run it up to him. Usually fours bolts holding it to the manifold and then, the fuel inlet hose should take 5 mins.

 

[mr408]

Re: 99 moomba outback Noise under load

Having trouble locating the correct rebuild kit.
Holley Model number 75021 600 cfm

 

[krisnowicki]

Re: 99 moomba outback Noise under load

http://www.nationalcarburetors.com/marine/fourbarrelholley.aspx

 

[mr408]

Re: 99 moomba outback Noise under load

Carb is off and heading to the shop for cleaning and rebuilding.

 

[mr408]

Re: 99 moomba outback Noise under load

Carb rebuilt and installed. Knock is still there, runs very rough. Knock is worse than it was before rebuild. Any thoughts? Could the detonation have hurt the motor other ways?

 

[Bondo]

Re: 99 moomba outback Noise under load

Could the detonation have hurt the motor other ways?

Ayuh,... It's 'ell on the rods, 'n their bearings...

 

[mr408]

Re: 99 moomba outback Noise under load

Ayuh,... It's 'ell on the rods, 'n their bearings...

Why is it still detonating? Timing? The rods still spin freely. Valves don't sound loose. I Put higher octane fuel in. What else could cause the spark knock?

 

[Bondo]

Re: 99 moomba outback Noise under load

Why is it still detonating? Timing? The rods still spin freely. Valves don't sound loose. I Put higher octane fuel in. What else could cause the spark knock?

Ayuh,... My guess is, it ain't detonating anymore, the damage is already done...
A hammered bearing will lower the compression on that cylinder...

Do a compression test...

 

[mr408]

Re: 99 moomba outback Noise under load

More bad news. I have milky brown oil now on the dipstick. Did not have it on Sunday or Monday when I first heard the knocking sound. Checked the oil a little while ago and saw the water in it.

I think I'm in for an engine overhaul. The sound is the same as when it first happened. I think the bearing was going all year and then finally gave out. I probably damaged it more by having several people hear the sound it made, and then running it with higher octane fuel.
Possibly damaged the jacket from the rod and piston clattering about.