Engine vibration at 40 mph and higher

Tiny
BIGBADPIRATETOM
  • MEMBER
  • 1940 CHEVROLET
Hello. I own a 1951 Chevrolet Styleline DeLuxe 4 door passenger sedan with a 216.5 cu. In. (3.6 litre) inline 6 cylinder valve-in-head engine. The car has about 153,000 miles on it. The engine vibrates steadily with a smooth humming vibration at about 40 mph to 50 mph. Over 50 mph the vibration starts to become a deep rumble, definitely at crankshaft speed. The clutch pedal vibrates slightly, and the throwout bearing chatters lightly, too. I am wondering if this is from an out-of-balance flywheel. At about 100,000 I rebuilt necessary parts of the eninge. Original pistons were cleaned and re-used, since they are perfect, and the cylinders had less than.003 inches of taper. New rings and bearings and valves, guides, oil pump, and seals were installed. Appropriate machining was performed. The machine shop I had do the crank machining recommended balancing the crank and pistons. I didn't see how this was necessary. The engine was always a smooth running engine, and it says clearly, several times, in the Chevrolet shop manual that the engine is precision balanced at the factory as a unit (assembled). At the time I did not know that this included the flywheel.
After I got the engine all back together, it was quite smooth, but after a long 200 mile run, the humming and vibration started. I opened the engine to see if I could find trouble. I found that the rear bearing had started to fail. I had the rear bearing journal dressed and checked, and installed a new bearing, which doesn't seem to have troubles. I try not to take the car over 55, though it was originally quite capable of going 99 mph (not that I ever sustained speeds over 65). I am wondering if my not having the flywheel balanced with the crankshaft and pistons is what is causing this vibration, since they were all originally in balance as a unit. With the crankshaft now being balanced as its own unit (including pistons), I feel that it is now out of balance with the flywheel. Would balancing the flywheel and pressure plate as its own unit now correct the trouble? I really don't care to remove the crankshaft again.
I also wonder if this vibration is robbing power and mileage. The car has very good acceleration at lower speeds, but the power tends to be a bit lacking at higher speeds, and gets worse after a long run (everything else is well adjusted--valves, heat riser (which can severely rob power if intake overheats), carburetor, timing, and all the other things that can rob power after a long run). I used to always be able to get 18-20 mpg with the car prior to the vibration, which is typical mileage for this car. Now I usually get 16, and sometimes worse. Only rarely do I get 18 mpg. The carburetor is well adjusted and well jetted, and the engine does not misfire as far as I can tell, though I have noticed that one spark plug is a shade ever so slightly more brown than the rest, which are, only after much driving, a very light shade of tan or gray. I don't feel that the vibration is from misfiring or anything in the fuel/ignition department. Valves seal well, and compression is good. All valve springs seem to function well, and were shimmed after the rebuild. All valves have good clearance--.015 inches for exhaust, and.008 for intake, after engine is thoroughly warm and normalized.

What causes the vibration?

Thank you much!

Thomas
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Monday, September 17th, 2007 AT 11:26 PM

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Tiny
BIGBADPIRATETOM
  • MEMBER
Man. This site is helpful. Noone responds to my questions or anyone else's. I was the only one who responded to a lot of questions, and noone even thanked me.

I can happily say that I also solved my own problem. The problem was in fact due to a lean mixture problem. The vibration sort of started to happen when gas started containing a full 10% ethanol. I noticed that about 5 or 6 years ago that the engine was always running lean, and would run hot, too. No matter what I did to it, I could not get it to run rich without ruining mileage. I would increase the carburetor jet size, but that would ruin mileage. High speed driving still overheated the engine, and had a lot of misfiring (sensed as a vibration). More recently I have read that you also have to increase the float height in the carburetor. I increased the jet size one step from original size (changes diameter by a thousandth of an inch). I also increased the float height 1/32 of an inch at a time. What seems to work flawlessly is to set the float about 5/32 of an inch higher than what the factory recommends. Vibration at freeway speeds is almost non-existant. Idle is perfect. Mileage is between 18 and 20 mpg, which is excellent. If I went for a long freeway drive, I could possibly do even better. This car is amazingly efficient. I love the 216.5 cu. In. Motor. If I put in a mixture regulator (along with oxygen sensor and throttle position sensor, etc.), As well as a device that would close the main metering jet when coasting, my mileage would be even better. Overdrive would be another plus. I know that Nashes and Studebakers, with their automatic electric overdrive, and Nash's twin ignition (two spark plugs per cylinder), could get in the high 20s, if not 30 mpg (this was in the 1940s and 50s).

Thanks for the help! I'm so glad that I could help myself!

Thomas
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Sunday, September 23rd, 2007 AT 12:24 AM

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