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Old 01-21-2012, 12:47 PM
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SBC knocks up to 1300 rpm, then goes away

Hello everyone,
I just bought my forth c10, 1977 with 350, th350. The engine is bone stock 4bbl quadrajet, adjusted the valves, just did plugs, wires, cap, rotor, new vacuum advance(old one leaked), new mechanical advance springs, one heavy one light, adjusted the carve and got timing where I think it needs to be, no timing light yet. It fires up on a dime, pulls pretty good. My problem is when in gear, giving very light throttle I have a "knock". Above 1200-1500 its gone. And at idle its not there. I can't figure out what it is. Any help would be appreciated.
Thanks

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Old 01-21-2012, 01:22 PM
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The knock you hear is likely detonation since you dont know what the timing is set at. Until you know the timing don't run it again as you can do damage.
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Old 01-21-2012, 01:27 PM
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I thought that but with it Going away I kinda decieded against it. I have retarded the timing and it seems to go away but then I have no power, can't even spin the tires in wet dirt. So figured that wasn't it.
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Old 01-21-2012, 01:53 PM
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Might be a rod bangin around. Does it knock for a short time when accelerating lightly right before it upshifts?That would be a classic telltale rod gettin loose kinda knock.
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Old 01-21-2012, 03:24 PM
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No, it only does it up to about 1300 in first gear. I don't hear it after that.
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Old 01-21-2012, 03:48 PM
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Renamed your thread to help get more responses.
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Old 01-21-2012, 04:14 PM
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Quote:
Originally Posted by DoubleVision
The knock you hear is likely detonation since you dont know what the timing is set at. Until you know the timing don't run it again as you can do damage.
^times 2. Use better fuel until you can rebuild the motor with true flat-top pistons and a tight squish.
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Old 01-21-2012, 06:36 PM
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You have mechanical advance distributor, check the springs are fitted correctly
It could be going to full advance too soon?
Get a timing light (don't need an expensive one if you only use it a few times a year)
You will be able to see if ignition is advancing smoothly or even advancing too fast
Harbor Freight cheap 'strobe will work well enough for what you need

Last edited by crazypj; 01-21-2012 at 06:40 PM. Reason: Added extra
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Old 01-21-2012, 07:55 PM
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your carb is very lean at that rpm range, maybe the power valve is stuck or the jets are wrong, even maybe a small vaccum leak can make that problem.

but definitely you must set the timing first.
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Old 01-21-2012, 09:08 PM
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Don't know if this is important, but it also is after it warms up some. And could it be spark knock or timing with it going away after 1400 rpm and only in gear? Sorry its just a new situation that I haven't delt with before. Thanks everyone.
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Old 01-21-2012, 10:07 PM
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only after it warms up you can get the correct air fuel mixture, also you'll never hear an engine knocking in neutral, they knock under load while driving.
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Old 01-23-2012, 09:17 AM
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Update.
Checked mixture and plugs for signs of running lean, plugs are light brown as they should be. Checked initial timing and it at the 2 on the left side of the 0, 20*? Is that too high? The truck fires up like its on good time, and if I back it off like I said I can't even spin the tires n wit dirt, so if it is too high what could be causing the lack of power.

Thanks again everyone
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Old 01-23-2012, 09:41 AM
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Make sure your flexplate isn't cracked. They can knock like that at low rpms, also. Hole to hole at the crank bolts.
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Old 01-23-2012, 09:55 AM
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I thought about that but if it was the flexplate wouldn't it do it all the time at low rpms. Sometimes this knock I'm having doesn't happen, or atleast isn't noticable.
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Old 01-23-2012, 10:29 AM
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Not necessarily. The one I had crack, the noise was intermittent-that's what made it so hard to pinpoint.
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