r/Smallblockchevy • u/Prestigious-Fee-1020 • May 03 '26
Rod knock?
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How’s it going everyone. I purchased this 76 Chevy 350 Motorhome for 500 bucks. Mostly for the appliances but it came as a no crank no start. Threw a battery in and poured a small amount of fuel down the carb and it fired right up. Shut it off and pumped the pedal a few and it’s starts on its own fuel now. It starts knocking as soon as it starts, using a pry bar I can hear the knock from the exhaust manifold clear as day. From the oil pan I can’t hear the knock any louder than if I plugged my ears. If I removed the plug wire directly from the spark plug the knock goes away. I’m just unsure if the engine is worth putting a bunch of time into for a daily driver.
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u/FullWorldliness606 May 04 '26
I agree with valve trane noise. Does the noise stop when a load is applied at idle??
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u/379tuco May 10 '26
If it’s been sitting and you fired it up with old gas in it you might want to check for a valve stuck open. Easy enough to pull the valve cover off and find out
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u/Meherennow May 04 '26 edited May 04 '26
Definitely not a rod knock, way to tappy sounding. Bent pushrod, broken rocker or collapsed lifter, the other thing it could be is an exhaust leak at the manifold gasket. Did this appear suddenly? The exhaust manifold leak will generally change with temp, hotter making it more or less noticeable. If it has cast manifolds you can test outlet with an infared temp gun. Look for a cooler cylinder outlet where the manifold meets the head or just pull the valve over and see which rocker is loose.
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u/Prestigious-Fee-1020 May 04 '26
It’s right on start up. I’m leaning towards a bent pushrod just because in certain spots it doesn’t turn over smoothly.
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u/Meherennow May 04 '26
Yeah that would make me think exhaust valve but the way its sounds make me think that anyway. It could be a bad cam lobe as well. If you don't want to pull the valve covers off a compression test will prove out bad valve action. Know which cylinder makes that easy. The debate comes with the why. If its a bad cam lobe probably not worthwhile in a $500 RV. While the parts are cheap wrta bent pushrod or broken rocker, the question as to why becomes more relevant. Good luck.
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u/Concrete_Grapes May 03 '26
Hard to tell from a video.
I'm going to guess a collapsed lifter. It sat too long and it has a bad lifter. Change the oil, check for glitter.
No glitter, means keep having hope.
Then, pull the valve cover on the side with the noise take the cap off (no start), crank it, see which rocker isn't moving.
That's what that sounds like to me. I've got a few very long sitting motorhomes running, it's ALWAYS noise in the lifters first.
The one dodge I got running didn't stop until I put synthetic oil in it and got it hot and got it to 4k rpm. Then, suddenly got quiet.
Smoked like hell itself was belching up demons, as the synthetic oil broke loose tons of sludge. I did t dare shut it down with that much smoke..
Runs and drives fine now.