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Mystery Knock Still There200 Hours Now

stumedic

Contributing Member
My 502 RH rotation has 500 hou

My 502 RH rotation has 500 hours on it now. I bought the engine with 300 hours. Had a knock from day 1. Very hard to describe. Lighter than a rod knock-closer to a piston slap but not quite. Using a stethoscope it's impossible to isolate. Ditto to pulling off ignition wires. Loudest from the timing cover but that's also the thinnest part of the engine. Doesn't seem to be the fuel pump. I give up. If it was something serious wouldn't you think 200 hours would tell the tale? It's there in gear or in neutral and it seems to get quieter over 1500 rpm's. The engine has adjustable hydraulics and they adjusted very easily. Stu
 
..."...it seems to get qui

..."...it seems to get quieter over 1500 rpms..." would indicate it might be the damper plate springs. They make a clunking sound at idle--especially if the motor is running rough--that goes away at speed.

Jeff
 
"My stb (RH) has a lit

"My stb (RH) has a little knock at about the same rpm, also independant of load. It was there right after a rebuild some 800 hours ago, it's the same now. It could be related to the gear cam drive, a rod guide, or piston slap, or other. It's not loud and i'm not concerned."
 
"Hot or cold; any difference?

"Hot or cold; any difference?

You adjusted the valves - no change?

Never heard the damper plate rattle in neutral, always in gear - Jeff, will they do that in neutral?"
 
"Hot or cold there isn't a

"Hot or cold there isn't any big change. I adjusted the valves and there was no change. It's one of those sounds you just can't put your finger on. I like diver dave's answer! the "I'm not concerned " part.
happy.gif
I was thinking something to do with the reverse rotation as the sound is loudest from the timing cover. Stu"
 
"If it ain't changed in 20

"If it ain't changed in 200 hours, it's probably not serious.

FWIW, one time we had a high pitched chirp that would come and go, from the front of the engine. after about two months, we found it - that 90 bracket that holds the oil coolers. It had cracked right where the upperr bolt hole is, at the pump."
 
"Stu, my experience with a noi

"Stu, my experience with a noise @ the timing cover on my 270Hp Crusader turned out to be a bad belt on the water pump pulley. The belt was narrow in one spot and was "popping" out of the pulley each time. Was louder at lower speeds and faded with increase in rpm. The water pump pulley is like a "bell" shape and amplified the sound. Good luck.
Chuck"
 
Thanks Chuck. I ran the engine

Thanks Chuck. I ran the engine without any accesories however and the noise was still there. As Mark says a serious problem wouldn't last 200+ hrs but I wish I knew what it was-just for peace of mind. Stu
 
"Sounds like the drive damper

"Sounds like the drive damper from what I have heard and read. My port engine has a loud diesel knock that eventually gets drowned out with increasing RPMS. Its still there over 1500 RPM's, but its much harder to hear over the other engine machinery whirring.

It is present in neutral and in gear and does not change with engine operating temperature, unlike piston knock. Mine has been knocking for 3 years and about 200 hours. Its probably a broken or loose spring or cracked damper plate. I'm going to replace mine over winter layup. I'll post what I find."
 
"The symptoms you describe mim

"The symptoms you describe mimic mine exactly. I will know for sure later this fall when it all comes apart, and I will post my findings on the board. Rick Sweeten was the one who weighed in and provided the original diagnosis, and his opinion is highly valued.

Your motors are much newer than mine, which are 24 years and 1500 hours old. It seems likely that a latent defect in the drive damper or faulty installation was responsible for your present situation, if indeed the drive damper is at fault. Like I said I have run mine now for 3+ years with a very noticeable knock. It is so loud at idle that the other engine can barely be heard above it. The engine itself starts and runs flawlessly and actually appears to have more power than its quieter neighbor, for what that’s worth.

The noise is very difficult to isolate. At first I thought it was coming from the top end through the valve covers, but I removed them and its not there. I took all the accessory belts off—no change. When I get down on my hands and knees and listen by the bell housing, its pretty loud down there, but then again the knock is loud at the front of the engine and up top, too…."
 
"Stu, a while back I also had

"Stu, a while back I also had a mysterious "knock" on my 270HP Crusader that the experts said was a rod knock and the engine needed teardown. I refused to believe the recommendations because all symptons didn't coincide with rod knock. Well after several months of investigation I believe my problem was excessive carbon build-up on #7 cylinder and the knock was actually "severe spark knock" in only that cylinder. The noise was as if someone was tapping the intake with a ball peen hammer. I couldn't determine where the noise was coming from either. The key was a compression check which indicated that #7 was 178 psi(1200 hr engine) and all other cylinders were also high (155-165). I've been running my engines @ 1800 rpm for several hundred hours to save fuel. I overlooked this carbon build-up potential. Anyway, I also had corroded distributor advance weights that were not working properly. So I rebuilt the dist. advance, new wires, cap, rotor (I have a solid state distributor) & new NGK plugs (recommended due to more exposed center electrode-less likely to carbon foul). Afterwards the knock was evident on first fire-up but began to fade and after 2 hrs run was gone. This run also included 4200 rpm for 20min. Compression on #7 then down to 150. The poor girl just needed a comprehensive tune-up and a brisk run on the open water. I've learned now to run these engines on plane regularly to keep them "happy". I hope this story helps you narrow your investigation. Good luck,
Chuck Lawrence"
 
"Chuck, funny you should menti

"Chuck, funny you should mention cylinder #7. Although I can't seem to pinpoint the location, and using a stethoscope it's loudest at the timing cover, just listening to the engine while standing above it the sound "appears" to be coming from the area of #7. The tune up is recent but quite honestly I never push the engine-anything above 2450 and my fuel consumption shoots through the roof. I never thought of carbon. Thanks. Stu"
 
As far as the drive damper the

As far as the drive damper theory-that's something I have to look into also. I've owned quite a few boats over the years and this is the only one-other than my 175 Johnson outboard-that bangs into gear with a vengeance. No matter how low I set the idle it bangs into gear. I thought the prob was something in the velvet drive but maybe not. Stu
 
"Stu:

1 + 1 = 2....what


"Stu:

1 + 1 = 2....what I mean is Tom may have pointed you right to it. That damper works like a shock absorber when you shift (or the prop gets unloaded). If it is going WHAM when it goes into gear, I'd say there isn't any shock absorbing going on. I'd go even further and bet the springs on it are gone and the hub is locked.

You may want to may a quick check inside the flywheel cover (bell housing) and see if there are any 'filings' on the inside."
 
"Unless you have a Vee drive,

"Unless you have a Vee drive, # 7 cylinder on your big block is lower than the rest on that side, especially when on plane. Any flooding of the carb will, therefore, send excessive gas to # 7 (and #8 on the other side). That could account for the carbon build up.

Note: This isalso true with TBI (centrally located fuel injection) if you have a bad injector.

Jeff"
 
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