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Discussion Starter #1
55 Chevy, 454, Holley, TH-400, Street Strip stall converter, new strange driveshaft, 3160 u-joints, new Ford 9 inch, new wheels and tires.

Coasting--no vibration
acceleration to 40-50 at 2,000-2,500 rpms--vibration.
Accleration to higher speeds at 2,500 plus--no vibration.
Let off the gas to coast--no vibration.

Vibration is very slight--almost feels gravely or marblely.

Could it be something with the carburator? I'm running slightly lean--spark plugs are clean and lightly tan. I recently jetted down from a very rich condition and installed a larger power valve. My wife notices it from the passenger seat also. It is not a hard vibration, i.e. doesn't shake the car.

John
 

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I'd see if you rich-end the idle screws to the point just before it affects the idle, then test drive it. You may have gone to far with the jetting. It's doesn't sound like a balance problem to me.:anim_25:
 

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Traction bars of any kind?? could be a pinion angle change under power.
 

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have you checked the driveswhaft angle? go to dennys driveline website for a picture and measurements. unless it's off by quite a bit then it usually occurs on accel or decel. without feeling it it's only a guess.
 

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Discussion Starter #7
I have CalTrac traction bars and Monroe air shocks. Does either of these affect the pinion angle?
John
 

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With Caltracs, you shouldn't get much pinion angle change then,,that is if they are preloaded somewhat tight, how much preload would you say John??

Are you hard into the 4 barrel carb when you feel the vibration??

did any of your new wheels throw a wheel weight that you can tell?? spin balancers usually spin up to 55ish MPH when balancing. Back in the day when I worked in a tire shop, customers would complain of a 40-50 mph vibration under accell, the case was sometimes a thrown wheel weight.
 

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Is the vibration speed-dependent, load-dependent, or RPM dependent? I can't tell from your description of the problem.
 

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Discussion Starter #10
Chevynut,
I would say that it is RPM dependent. I'm on level surface and cruising along at between 2,000 to 2,500 when it does it. If I accelerate to 3,000 or more, it smooths right out. If I coast after accelerating, it doesn't show up. After coasting, if I resume speed, it will vibrate.

Keith,
The more I'm into it, the less vibration. It doesn't feel like a wheel issue.

I think I'm going to have to get it up on stands and see if I can duplicate it to try to pinpoint where it is coming from.

A friend of mine even suggested that it could be a bad plug or plug wires.

John
 

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Sounds like your stall converter what is the stall at 1500, I had one that gave the same troble and after changing a lot of stuff went back and change out the converter and that fix it , the transmission guy said it was not locking all the way at 2500 and it made a little vibration and would stop hope this will help you.
 

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Can you get the vibration, when you are parked and rev it between 2000 and 2500?
 

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The reason why I'm thinking it's a miss-fire, is that it don't show up only on a pull, nothing on deceleration. However if you are going to put the car up, have the rear wheels supported at ride height, and run her while in gear, and observe the drive shaft for any run out. We used to just carefully hold a screwdriver close to the "tube," and observe any run-out in the shaft. Watch out for the yokes and balance weights though. Another thing you might want to check, is if there in something touching the frame or cross member, that would transmit a vibration or noise. Maybe exhaust, bad engine trans mount or the drive train hitting somewhere.
 

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If the vibes occurred parked, it would eliminate the drivetrain and isolate the problem to the engine. Since it doesn't do it then, it's an indication that it's not in the engine, unless it only does it under a load.

A little more info would be helpful.
 

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Your TH-400 is new with low miles??

The tailshaft yoke is also new I would assume.

All converter bolts are still in place and tight?? a buddy of mine had a similar vibration and found two loose converter bolts. once tightened it was gone.

Flexplate looks good??

Engine mounts and trans mount??
 

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A friend of mine has this same problem. I don't think it's the u-joints. But so far no luck on finding the cause. It started when the clutch was replaced, but every symptom is speed related, not engine related.

If your driveshaft is out of balance, the driveshaft itself will vibrate more, the faster it turns. Same with a tire out of balance. Vibration from u-joint angles being wrong will be that way too.

I have had a driveshaft that was clearly out of balance and behaved as above. The faster you went, the more it vibrated. At about 100 mph you couldn't see (this was a drag race car).

With the tire out of balance, what you actually feel in the steering wheel is a resonant vibration based on the tire vibration, the tire's weight, and the stiffness of the suspension spring. This vibration peaks at a certain speed and then diminishes when you go faster.

I think this same thing must be happening on Doughboy's car, as well as my friend's. The driveshaft (or whatever is vibratiing), along with its weight, and the stiffness of the transmission mount, gives you a resonance that peaks at a certain speed, and then diminishes.

Something you could do to see if that was the case would be to change the stiffness of the transmission mount. If it's rubber, go to urethane or solid steel. If it's urethane, go to the softest rubber mount you could find. That won't fix it, but it will help pinpoint where the trouble is.
 

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Discussion Starter #18
Well, as it turns out, the transmission case is cracked just above the torque converter. The crack is on the passenger side and is about a quarter inch wide across half the distance of the housing. I'm not looking forward to pulling the transmission down. Any ideas on what to look for as the cause. Any trans suggestions given that I put a shift kit in this one, bought a new stall converter, and have a new B&M rachet shifter. Any TH400 suggestions? Car is not a race car, but occasionally like to get on it while cruising.

John
 

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Well, as it turns out, the transmission case is cracked just above the torque converter. The crack is on the passenger side and is about a quarter inch wide across half the distance of the housing.
John
Thats odd. it would take quite a bit of side load or leverage of some kind to do that, or just a bad/thin case. Hopefully it is far enough away from the hard internals to be welded up. You might have found your problem.

Your vibration problem might have caused that, and it just found the weakest point. those bellhousing areas are usually quite sturdy, because they have to be.

Nontheless, you found a piece to the puzzle, if not the fix. IMO

If you plan on doing some manual ratchet shifting, I would take it to a reputable trans shop and have them go thru it and tell them what you are going to do with it, so they can check the internals and make sure they are all ok. Maybe new pump and plates, shimming, gaskets and seals.

You dont want to add a new torque converter to a old trans,,or vice-versa, because it could end up costing you more money. (been there,done that to the tune of about $900:rolleyes:)
 
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