390 Help
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Freshman User
Joined: Apr 2002
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390 Help
Hey Guys. It's been a while since i've posted anything here. All has been well. Now, I need a little advice. I have a 75 150 with a basically stock 390. The only not stock items are a edelbrock performer intake and a 600 cfm edelbrock carb and a set of headers. I have set the timing at 6 degrees below top dead center. The idle is about 800 rpm. I am just starting to learn how to set the timing and adjust the carb. I think things are pretty good, but, I am getting a sound that sounds like valves rattling. They rattle when the rpm's are getting over 2300 or so. They are not real loud but I can hear them. I do not believe that it is the grade of gas I am using. Can anyone please give me any ideas why the valves may be rattling. The engine was built about 10 months ago and I have put about 3K miles on it. Thanks a lot!
Post Fiend
Joined: Feb 2001
Posts: 6,624
Likes: 8
From: Missouri
390 Help
You are probably hearing the sounds of rod bearings that have seen better days. This could be casue by lack of oil pressure for an extended period of time or just plain old age. It should still last a while if you go easy on it. My 360 was knocking like crazy and it never let me down up until the point I rebuilt it. I would recommend advancing the timing to about 10BTDC but its not going to fix the problem.
Logistics Pro
Joined: Feb 2000
Posts: 3,565
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From: Sun River St. George
390 Help
Rods will knock at low RPM and so will Crank bearings. If the oil pressure is good and it does not make knocking noises on startup I'd look elsewhere for a source of this noise. At 2300RPM are you lugging the engine? Have you tried a full tank of 93 octane? What compression ratio did you build for? Is it possible that something else is rattling, like headers, tail pipe, loose brackets, drive shaft U joints? I also agree with Rat and would move timing to 10 initial advance. Be careful if this noise is detonation it will break pistons.
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Freshman User
Joined: Apr 2002
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390 Help
A buddy of mine said to him it sounds like spark preignition. If that is the case, would that be timing related? I have just started to tinker with the engine because it was starting to lug. This is a rebuilt engine with less that 4K on it. Thanks for any ideas.
Senior User
Joined: Dec 2002
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From: Omaha USA
390 Help
My 390 made that similar sound at about 2500 rpm. When I tore it down, the cam bearings were gone all the way to the metal sleeve. The mains were worn but not gone. I still had decent oil pressure because of the mains not being all the way gone. You could tear down to the cam relatively fast if you have done it before. If nothing else, take the oil pan off and look for shavings. I found all my cam bearings there.
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