Topic: Flex Plate issues - 67 289 C4

[Tiny]

Hi all I have a 1967 289 fitted with a C4 Auto.  I have vibration issues consistent with issues where the imbalance weight of the Flex Plate is incorrect.  For starters can someone tell me which weight is attached to the Flex plate in the attached images.  Is it 28 or 50 ounce? (Note the stamped numbers on the plate might help 3320071)

The information I have researched online is very inconsistent.

thanks

[Bob Gaines]

Hi all I have a 1967 289 fitted with a C4 Auto.  I have vibration issues consistent with issues where the imbalance weight of the Flex Plate is incorrect.  For starters can someone tell me which weight is attached to the Flex plate in the attached images.  Is it 28 or 50 ounce? (Note the stamped numbers on the plate might help 3320071)

The information I have researched online is very inconsistent.

thanks

What is the back ground of these issues? The vibration start recently or has it been a long time problem? . Did you change flexplates for some reason?etc.

[Tiny]

I bought the car without driving it as it needed restoring. I completely overhauled the motor and C4 and replaced the Flex plate.  From day 1 when I got it on the road it has this vibration noise when I accelerate harshly.  Its not there at idle or any other revs but it occurs only under load.  I have exchanged distributors, carby's, starter motors and altered timing many times but it wont go away.  Through my research my last resort it appears that it could be the wrong flex Plate.  The image you saw was the flex plate that came off the car when I bought it.  I have attached another image of the Flex Plate and weight that is currently on the car.  Hope this may help.
Is the flex plate on the car correct.  Its too hard to tell if it is a 28 or 50 ounce.

thanks

[Bob Gaines]

Some more questions.Why did you replace the flex plate?Yes there is a radical difference in the pictures of how the two are weighted.

[Tiny]

I replaced the Flex Plate because some of the teeth were worn so I thought it better to install a new one.  You mentioned some radical differences?  What are your thoughts

[Bob Gaines]

I replaced the Flex Plate because some of the teeth were worn so I thought it better to install a new one.  You mentioned some radical differences?  What are your thoughts

At first I thought you were showing two different flex plates but now realize it is the same one. I don't remember the stock 289 flex plate having such a large balance weight as in your picture. You could be correct in your suspicions. I hope you are not going to tell me that you disposed of the previous flex plate but since you didn't show it in your picture I am assuming the worst.

[Tiny]

The first 3 images in this thread are of the original Flex plate which I still have.  The second two photos is the new flex plate that is installed on the car.  I took off the inspection plate and took the photos

[Bossbill]

Can you measure the width of the first one (original) to see if anywhere near 9.5 cm?

Do you have the receipt of the second one?

[Bossbill]

I bought the car without driving it as it needed restoring. I completely overhauled the motor and C4 and replaced the Flex plate.  From day 1 when I got it on the road it has this vibration noise when I accelerate harshly.  Its not there at idle or any other revs but it occurs only under load.  I have exchanged distributors, carby's, starter motors and altered timing many times but it wont go away.  Through my research my last resort it appears that it could be the wrong flex Plate.  The image you saw was the flex plate that came off the car when I bought it.  I have attached another image of the Flex Plate and weight that is currently on the car.  Hope this may help.
Is the flex plate on the car correct.  Its too hard to tell if it is a 28 or 50 ounce.

thanks

An imbalance is not subject to "load or no load". If you are spinning at 3,000 RPM with an imbalance this would shake the car regardless of load or not.

Usually shaking under load is due to misfire.

[Bob Gaines]

  The large balance weight on the flex plate in the picture looks oddly large for a stock C4 289 but maybe I am mistaken. I posted a picture of a 289 hipo flex plate that was posted on the SAAC forum. The flex plate on the 289 hipo has a larger weight then stock to compensate for heavier rods and crank counter weight on the hipo and looks similar to the one in the  picture. I am not trying to say it is a hipo flex plate but may be a more modern 68 up flex plate that is weighted different.  Sorry that I can't be of any help.

[Tiny]

thanks everyone for responding.   the measurement of the wight on the original one is around 10cm at its longest point and 19mm wide.  the one fitted to the car is around the same length but I didnt measure te width but it does look wider than the original.  I dont have th receipt unfortunately.

the vibration of the car is only under load and not during revs when stationary so maybe the issue does not point to the Flex Plate.  I have tried to eliminate everything as much as possible as I mentioned, timing, carby, starter motor etc but nothing seems to fix the issue and thats why I have turned my attention to the Flex Plate.

[OldGuy]

You did not elaborate as to the condition of the driveshaft/u-joints. Did you replace the u-joints? A badly worn u-joint (especially the front) can cause the type of vibration that you are alluding to.

Do you have a bent drive shaft? Put the rear of the car on jack stands (positioned on the axle housing), have a helper run the car up to speed while you perform a visual check as to what's happening in the drive line. Is the vibration still felt inside the car as when the car was motoring down the road?   

Have you popped the transmission into neutral while at speed where the vibration occurs? If not, try it. Does the vibration change? While in neutral, try running the engine back up to the RPM where you encountered the vibration. Any change?

As was previously stated, a vibration such as that caused by the wrong balance weight on the flex plate is more RPM-specific and not whether the car is under load.

It costs you nothing, at this point, to eliminate other possible sources for your vibration before you home in specifically on the flex plate. Give it a try.

Frank

[Tiny]

Thanks Frank, the uni joints were replaced both front and rear.  The issue is only under load so dropping it into neutral etc wont help.  Putting it on Jacks and running the car and checking if the tail shaft is running true is something I will try. I'm also going back to my C4 specialist and taking them for a drive and see what thier thoughts on re the Flex Plate.

Cheers

[Bob Gaines]

Keep us posted on what you find.

[midlife]

You did not elaborate as to the condition of the driveshaft/u-joints. Did you replace the u-joints? A badly worn u-joint (especially the front) can cause the type of vibration that you are alluding to.

Do you have a bent drive shaft? Put the rear of the car on jack stands (positioned on the axle housing), have a helper run the car up to speed while you perform a visual check as to what's happening in the drive line. Is the vibration still felt inside the car as when the car was motoring down the road?   

Have you popped the transmission into neutral while at speed where the vibration occurs? If not, try it. Does the vibration change? While in neutral, try running the engine back up to the RPM where you encountered the vibration. Any change?

As was previously stated, a vibration such as that caused by the wrong balance weight on the flex plate is more RPM-specific and not whether the car is under load.

It costs you nothing, at this point, to eliminate other possible sources for your vibration before you home in specifically on the flex plate. Give it a try.

Frank

I like this response, but would like to add that the tranny to driveshaft to rear end angles need to be measured and compared to specification.  While your U-joints may be new, they may be bad as well.  Having not enough angle between tranny and rear end can create vibrations only under load as well.