Hi everyone, this is my first post.
I'm Bill, from Ohio, and I just bought a 1972 Cadillac Sedan DeVille for $300. It needs some body work on the passenger side - it was side swiped so the two door handles are gone and there is a streak of surface rust and minor dents along the whole side, and a dent to the front passenger side of the hood. I'm glad the car even ran for $300, and the transmission seems to be in good shape, but I'm a little worried about the engine.. It started up and sounded fine when I test drove the car, but today when I went to pick it up, I started it and it sounded like it was maybe making a 'slapping' sound, but more like a knock... maybe 3 slaps every second at idle.. coming from under the hood somewhere. I don't know if there's maybe a stuck valve or a piston problem or what.. I'm used to a 302, not these huge gm engines... I plan on doing a tune up over the next couple weeks as I have the money, but does anyone know what I might look for when bringing back to life this 472 that has basically been sitting for a year? The car seems like it has a fair amount of power, but I don't know if it feels like 220HP... I'm not really confident flooring the gas pedal though, since I was concentrating on getting it the 20 miles home. There's also a moment of hesitation also when pressing the gas pedal suddenly at idle. If anyone has anything to contribute - advice of any kind - please let me know!
Bill, first things first: Check the oil level and condition (simple, yes, but I almost bought a car with a blown head gasket once for lack of performing this simple step). Does it sound more like a tick or a knock? Ticks usually indicate valvetrain noise and would be loudest around the valve cover area. A tick can also come from an exhaust leak (check around the exhaust manifold gaskets and the manifolds themselves) or even from a belt that has an imperfection or wet spot on it (you wouldn't BELIEVE how loud a screwed-up fan belt can sound). The worst knocks come from the bottom end of the engine and tend to indicate excessive clearance in a bottom end component. Try to determine where the knock is coming from.
A few questions: Does the engine run smoothly or does it misfire (especially under a load)? Does the sound get louder or softer with higher RPM? Does it go away as the engine warms up, or does it get louder? How many miles on the engine? If you have a piece of fuel hose, you can use it as a crude stethoscope. Press the tip of the hose against the valve cover, and press the other end against your ear. Do this VERY CAREFULLY-- there's a lot of dangerous stuff moving around under the hood of a running car (if you have long hair, for God's sake tie it back). STAY FAR AWAY FROM THE FAN AND BELTS!!! This may allow you to hear a valvetrain noise (press the tip onto the valve covers on the engine). Bottom end noises will be trickier, and I STRONGLY DISCOURAGE crawling under a car with a running engine to press a chunk of hose up against the oil pan. Let me know if you can narrow it down at all.
The hesitation is likely a carburetor misadjustment or the need for a carb rebuild. This is relatively simple territory, and I would worry about the noise first.