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67 289 engine vibration

6890 Views 16 Replies 6 Participants Last post by  Dboxman7
On the freeway most noticeably above 60 mph, when on even flat road if I let off the gas, no problem, but when I give just enough to maintain speed I get a heavy vibration/shaking feel from the engine. Not violent like something is breaking, just too heavy to feel right. Doesn't do it when accelerating or when I let off the gas. I haven't started to try to diagnose yet... maybe vacuum advance? or ignition/timing?

Any thoughts or help appreciated!
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Vibrations are sometimes hard to find, any in the drivetrain (engine, trans, driveshaft, u-joints, diff) can feel like it is coming from a different spot. So I will give you some things to check starting at the engine:

1: Check the PCV and the hose
2: If you have an autolite carb, check the power valve, you have to remove the carb, do not turn it over, remove the 4 screws on the cover on the bottom between the idle mixture screws, remove the cover, if fuel comes out the power valve is bad, you can get one at your local autoparts store.
3: Check the torque convertor nuts if you have an automatic trans, remove the inspection cover, rotate the engine with a 13/16" socket, 6" extension, and ratchet or breakover bar, until one is visible, check it and then turn the engine 1/4 turn and check the next one, until you have done all 4.
4: Check the front u-joint on the drive shaft, if there is any play in the joint it is bad, then check the rear u-joint, the same applies.
5: It doesn't hurt to check your ignition timing and vacuum advance.

Post your results. Good Luck.
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If you are replacing the u-joints yourself, then you are missing something in the procedure. Here are some tips, when installing them the zerk fitting should be to the inside meaning it is closer to the driveshaft itself, after the u-joints are installed they should move freely, if they are tight you need to take a blunt chisel and a hammer and place the chisel on a V in the u-joint in the direction of the cup you want more clearance to loosen it up (i do all 4 and then check it for freeness) and tap it with the hammer with the u-joint holder resting on something solid (if it sounds confusing, maybe this helps, pressing or pounding in the cups on the u-joints makes them tight against each other, I use the blunt chisel and hammer to tap the u-joint X shape to push out each cup against its clip, so to release the tension on it. Also the u-joints need to be greased, the easiest way is to grease it before you install the driveshaft is to use a c-clamp or vise to hold the 2 cups on the rear joint that would go in the diff yoke, while you grease the rear one. The only way the diff can be eating up the u-joints is maybe if the pinion bearings are bad causing it to wobble, but I have never seen this, it is just a thought. Good Luck.
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On a used differential, if you are referring to the flange as being able to rotate a 1/2" that should be okay. If it is a conventional rear axle the wear and clearance in the spider gears also affects the amount the flange can move, in essence it is the total clearance of everything from one direction to the other, not just the ring and pinion. What is important is that the pinion bearings are not worn so much as to allow the pinion to move like up and down or side to side, as this will cause the alignment to change on the pinion to ring gear, and in severe cases will cause the teeth to get broken off. My 2 cts. Good Luck.
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