Hello all. Hope everyone in the southeastern US is staying safe. I have a sound coming from my port engine and I haven’t a clue. It is a 454 in a 1975, 41’ Flush-deck. Year and hours  of the motor unknown. The sound is at 2000RPM’s and weakens in volume and frequency until it disappears below 1800 rpm’s. It gets worse as I increase rpms so I have not increased beyond 2000.  Any thoughts, troubleshooting tips, solutions, would be appreciated. 

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Engine cold or hot ? under load or not ? engine running rough/lacking power ?

Timing comes to mind first, condition of plugs second.  At 2K the engines on this boat are beginning to work and advanced timing could result in pinging as could plug problems.  It would not be a bad idea to listen to the engine, out of gear as you increase rpm using a long screwdriver or rod to make suere it isn't bearing noise.  If you are not familiar with this, you just press your ear to the screwdriver or rod and place the other end against various points on the block.  Mostly you want to hear smooth regular whirring sounds, on the valve covers you will hear regular tapping sounds.  Anything beyond a smooth regular sound is time to listen carefully so you can describe it to others.

Just realized you had a video link in the post.  That sound is not synchronous with engine, transmision or shaft.  What is loose or hanging in the engine bilge that can bang around when the rpm's get to that level.  It seems to be about 2 per second.  Go down in the engine room and watch, it's amazing what you find when you look.

Engine temps at 150, new Pertronix electronic distributors both engines last season. Happens under load, same noise with a Mallory flat cap distributor that was in it when I bought boat, same noise with mercruiser thunderbolt distributor that was on before new Pertronix. Definitely not coming from something loose in the engine room. With engine hatches off it sounds to me like a backfire with the noise coming up through the carb. Cannot pull plugs or put on new heads I bought until the potential storm passes this weekend in case they call all boats off the dock at our yacht club. If I rev the engine quickly in neutral I can get a pop or two but not like when the engine is under load. I’m looking for my compression numbers that I took end of last season. I’ll do those again after this weekend.

I too just saw the video sounds like it might be a cracked flex plate or loose bolts in the plate - sound normally changes when you put it into gear - sometimes a broken piece gets to rattling around in the bell housing .

It sounds like it is missing as well, true? A possibility is a rocker arm bolt that loosened up. A mechanics stethoscope might allow you to narrow it down. Just a guess, it sounds too slow and irregular, but it may be rattling around. I would be curious enough to pull the valve cover on the noisy side, if it has a noisy side. Other thought is a broken flex plate, at least that is what they are called on automobile. In lieu of a mechanics stethoscope you can use something like a large screwdriver or some other solid object, the sound will travel up that object if you hold it tight against the source of the noise. 

I had that problem and it was a burnt valve. While the engine is running at the RPM's that it normally pops at, pull, one at a time the plug wires. If when a plug wire is pulled the backfire stops then that is the cylinder with the burnt valve. Most of these engines were designed to run on leaded gas and the valves were lubricated by the lead. Once removed, and the valve seats not case hardened, the valves will suck in. On my starboard engine, all but 2 were sucked in. I replaced them with new heads and stainless steel valves.

  Another thing that can cause a burnt valve is water getting to it when its hot. The cause is usually a bad riser and will be the cylinders to the rear of the engine.  Dave

Dave my thoughts were valve related too and I have a new set of heads ready to go but always worth throwing it up on here to see what others with more experience think. Love the simplicity of pulling a plug wire underway , that I can do this weekend. I found compression numbers from the end of last season.

The manifolds and risers/dump elbows are new and the engines are fresh water cooled

left side 1,3,5,7 are 115,110,105,120

Right side 2,4,6,8 are 115,115,100,120  I know these are low and probably require new heads but wanted to get some feedback as to what else I might look at before pulling this apart so i'm not doing it twice.

I didn't mention the carbs have not been touched in many years. There is definitly a misfire as I get the lash thump out of the transmission when at idle. Hoping to get to the boat tomorrow and try a few things. I'll keep everyone posted.

Thanks all.

Dave Bernard said:

I had that problem and it was a burnt valve. While the engine is running at the RPM's that it normally pops at, pull, one at a time the plug wires. If when a plug wire is pulled the backfire stops then that is the cylinder with the burnt valve. Most of these engines were designed to run on leaded gas and the valves were lubricated by the lead. Once removed, and the valve seats not case hardened, the valves will suck in. On my starboard engine, all but 2 were sucked in. I replaced them with new heads and stainless steel valves.

  Another thing that can cause a burnt valve is water getting to it when its hot. The cause is usually a bad riser and will be the cylinders to the rear of the engine.  Dave

Broken/bent rocker arm causing valve noise?

-Darin

HOPING TO SPEND THIS EVENING ON THE BOAT MONITORING THE WEATHER AS WELL AS CHANGING THE HEADS, GETTING COMPRESSION READINGS, CHECKING THE INTERNALS SUCH AS ROCKER ARMS, PUSH RODS, LIFTERS ETC. I'LL KEEP YOU ALL POSTED ON PROGRESS AND WHAT I DISCOVER AS I PROCEED. THANK YOU ALL FOR YOUR SUGGESTIONS, AND RECOMMENDATIONS, THEY ARE VERY MUCH APPRECIATED. I'LL CHECK BACK IN SOON.

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