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Old 10-06-2013, 07:37 PM   #1
Dan/Kzo
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Default Reasons for deceleration vibration

When the cold weather settles in I would like to investigate and correct the vibration my truck has when decelerating. It is a pretty good vibration/rumble when the foot comes off the gas pedal. I am looking for some pointers and advice on what to check or recheck. Some history...Last year when I installed a rebuilt engine I dial indicated the flywheel housing with horseshoe shims in place and found .005 variation (which I felt was acceptable and better than what I removed), removed the transmission and installed new bearings, and verified the condition of the ujoint, which was tight as new. Transmission gears are not noisy and it does not slip out of any gear. This same vibration was present with the old engine and flywheel housing. It runs and drives smooth on acceleration and all driving speeds, just more rumble vibration than it should have when letting off the gas. I have other A's which are not like this. Any tips are appreciated. Dan Cargo
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Old 10-06-2013, 07:46 PM   #2
Tom Wesenberg
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Default Re: Reasons for deceleration vibration

I would look for a loose pinion bearing in the rear end.
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Old 10-06-2013, 11:12 PM   #3
SeaSlugs
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Default Re: Reasons for deceleration vibration

yea its gotta be slop in something that is being driven by the rear wheels.

is the vibration instantly there when you let off or does it kinda slowly appear?

whats it do when you brake soft? hard?
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Old 10-07-2013, 01:31 AM   #4
Tom Endy
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Default Re: Reasons for deceleration vibration

If you have repo engine splash pans installed, check the four locations where they bolt to the engine pans. There is a tendency for the tabs for break off and cause a horrendous vibration on spool down. I had this happen on two cars. I fixed them by making stepped brackets that fit over where the tabs broke off at all four locations. It required longer pan bolts at these positions.

Tom Endy.
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Old 10-07-2013, 10:14 AM   #5
Bob Johnson
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Default Re: Reasons for deceleration vibration

It is very useful to isolate where the vibration is located. Does the vibration occur only when the car is in motion. If you rev the engine with the car stationary does the vibration occur when you quickly let off the gas? If it does not occur with the car stationary then it is likely a problem in the drive train. If it does occur with the car stationary then it is likely in the engine. Have you disconnected the fan belt? If so did the vibration go away?

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Old 10-07-2013, 10:40 AM   #6
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Default Re: Reasons for deceleration vibration

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Does pressing the clutch to disenage the engine when decelerating reduce the vibration? If so think main bearings/ balance, if not think drive train esp UJ

Last edited by johnbuckley; 10-08-2013 at 03:25 AM.
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Old 10-07-2013, 01:43 PM   #7
john charlton
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Default Re: Reasons for deceleration vibration

It could be the Hyatt roller bearing in the torque tube neck is worn . You can check this by removing the speedo drive cap to reveal the speedo drive gear. with a screwdriver try to push the gear up and see if any slack when it drops down .Should be no or very little vertical movement . Bit of a fiddle but can give an indication of undue wear . A worn bearing/sleeve will cause vibration on the overrun .

John in sunny lovely weather Suffolk County England.
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Old 10-07-2013, 03:35 PM   #8
steve s
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Default Re: Reasons for deceleration vibration

Dan,

You may think I'm nuts, but what heads are on your various A's? Did you try a different carb, or increasing the throttle/idle speed, or opening the GAV?

Steve
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Old 10-07-2013, 06:28 PM   #9
Dan/Kzo
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Default Re: Reasons for deceleration vibration

Thanks for the responses and here are answers to posted questions. post #3) vibration appears instantly when I let off, and it does not appear to change whether I brake hard or soft. #4) original engine pans that have had new ears welded on and are securely anchored to engine and frame. #5) occurs only when in motion going down road. no vibration when A is stationary reving engine. #6) no vibration when depressing the clutch to disengage engine when going down road. (this is what I do going down long steep hills to quiet things down for a minute). #8)standard head currently using Zenith, but have also had Marvel. have not really changed throttle or gav.
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Old 10-07-2013, 06:37 PM   #10
Russell in Tulsa
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Default Re: Reasons for deceleration vibration

I have noticed vibrations on my rebuilt engine. I transferred all components from my other engine to this one. I kinda believe it is the flywheel and clutch not getting balanced as a unit before installation. Happier with the removed engine as I think all it needed was shims tightened up. Report back if you figure out your problem and good luck with your search.
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Old 10-07-2013, 11:40 PM   #11
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Default Re: Reasons for deceleration vibration

I've had deceleration vibrations to some extent in all my A's. float-a-motors took care of it for me. It is somewhat the nature of this (and other) 4 cylinder engines, especially long-strokers.
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Old 10-08-2013, 09:47 AM   #12
George Miller
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Default Re: Reasons for deceleration vibration

I think you will find all Model A's have a little deceleration vibration, all through not that bad. My take is on acceleration the forces push down on the front engine mount. On deceleration it lifts on the front of the engine because the engine is acting as a brake. plus the forces inside the engine reveres.

I have found if the front engine mount is to tight, it will make it worse. Now if you have a bad U joint or some other problem then that is a different deal.

I know a lot of you will not buy this but that is ok. Study how the engine drive train work together.
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Old 10-08-2013, 10:11 AM   #13
steve s
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Default Re: Reasons for deceleration vibration

Quote:
Originally Posted by steve s View Post
Dan,

You may think I'm nuts, but what heads are on your various A's? Did you try a different carb, or increasing the throttle/idle speed, or opening the GAV?

Steve
Quote:
Originally Posted by Dan/Kzo View Post
... #8)standard head currently using Zenith, but have also had Marvel. have not really changed throttle or gav.
The ultimate source of that deceleration vibration--whether it's excessive or not--is the pistons' sudden direction reversal at the top of the compression strokes BUT without the normal cushioning air/fuel charge in the combustion chambers. Thus, it should be worse with the low-compression stock heads and carbs that might be a bit stingy with idle fuel charge--hence the suggestions to increase idle throttle setting or to open the GAV. I'm not sure but I doubt that the Marvel would have the fuel supply problem. So, other than the possibility of high compression heads on your comparison vehicles throwing off your perspective, I'm now at a bit of a loss. Maybe the guy who implicated the rear end knows something I don't (quite possible!), but it seems to me that if that's the problem some noise should be evident from back there, and similarly for the U-joint. I don't understand how any sort of rotational imbalance could give different results depending on deceleration--that effect should just depend on rpm, I think.

Another possibility that occurs to me is that there might be something else on the truck that is of fortuitous size, shape, and looseness such that it resonantly couples with a normal amount of engine vibration to give an abnormally larger total perceived vibration. As I sit here visualizing the truck, that doesn't sound too far fetched--no offense intended.

Steve

Last edited by steve s; 10-08-2013 at 11:23 AM.
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Old 10-08-2013, 07:09 PM   #14
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Default Re: Reasons for deceleration vibration

Much good wisdom and feedback here for me to work with, thanks to all who posted. Steve, it would have never occurred to me that a finely tuned "how low will it go" Zenith could fit into this equation.
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