Low RPM Knock

[77sprite]

Well I finally got my '77 850 to start. At idle speed I'm hearing some knocking. It doesn't sound like a metal-to-metal "tink" but more of a "donk" or "clank". However, when I increase throttle from high idle, the sound seems to go away. What's the easiest way to diagnose this problem? If I remove the manifold will I be able to check the rods and crank or do I have to get in deeper? This was out of the water, so could I just be hearing exhaust cobwebs from this old motor? Compression test gave me 110, 115, 90, and 90.

 

[CharlieB]

Re: Low RPM Knock

Check the rubber mounts between the motor and the transom mount, both upper and the lower ones, if that old they could be worn enough that you have metal to metal contact at low speed vibrations.

Use a long screwdriver, rod, or mechanics stethoscope to listen and help narrow down the location of the noise.

 

[77sprite]

Re: Low RPM Knock

Thanks for the quick reply, Charlie. I'm right over here on the Il. side. I'll be taking another look at the motor Sunday so I'll check the mounts first thing.