Mechanical Noise around 3000 RPM

[m_gdavis]

A couple of weeks ago my boat (1999 5.7l carberated bravo 3 mercruiser)started to make a mechanical noise after 3000 RPM. The boat would not go any faster. If I backed it down a little it sounded great. Last weekend I replaced the distributor, rotor and fuel filter.

The boat ran worse and still made the noise. There was no power at the lower end. The boat sounds great out of the water. If I blip the throttle I can hear that noise at high RPM.

The boat is 200 miles away so I am looking for ideas to try next weekend. If I can't fix it I will bring it home.

Bad fuel? I filled up about 1 hour before the noise started.

Knock sensor bad or picking something up?

Timing not advancing?

I am bringing a timing light, plug wires (plugs), wire jumpers, fuel pressure gauge, compression gauge, shop manual and my standard tools.

Anything else?


thanks in advance for any help.

 

[180shabah]

Re: Mechanical Noise around 3000 RPM

First thought is that you are running lean, could be debris in the carb, water in the fuel, etc. Could also be overadvanced timing.

Most of the time, if there is a mechanical problem, it doesn't go away at different RPM's.

 

[futz]

Re: Mechanical Noise around 3000 RPM

1- compression test, on a warm motor.

2- vacuum test.

Post the results.

99 % of the time these 2 will find the problem.

 

[m_gdavis]

Re: Mechanical Noise around 3000 RPM

The noise is hard to describe. It is a clunking sound. It could be pinging. The best I could tell is it was coming from port side back of the engine. I remove that valve cover and did not see anything unusual. I didn't have time to adjust the valves.


A couple of the plugs looked oilly black. I replaced them with some older ones I had handy and nothing changed.


Hi futz, what would I do a vacuum test on? Is this a leak down test?

Thanks for the response

 

[futz]

Re: Mechanical Noise around 3000 RPM

http://www.secondchancegarage.com/public/186.cfm

Above is the link to the vacuum.