Vibration upon deceleration

[Jred]

If I get the 1970 Avanti up to about 80mph and decelerate there’s a vibration that seems to be in the general area of the firewall. Original Sbc350 w/65k, 700r4 installed while under previous ownership. Fresh tires. 
Any suggestions on where to start?

[silverstude]

tie rods or general  alignment

[Avanti83]

Does it continue to vibrate as you decelerate or stop vibrating at a lower speed?

[mfg]

Possible defective engine mounts issue...since your Avanti ll has a 700R4 conversion look everywhere for metal-to-metal contacts.....Since driveshaft was modified, check for depth of front yoke in trans extension housing.

[mfg]

I'd also remove the driveshaft and check for any looseness and/or tightness in the universal joints ...especially the front one.

[Jred]

On 3/23/2022 at 10:58 AM, Avanti83 said:

Does it continue to vibrate as you decelerate or stop vibrating at a lower speed?

It will stop at around 65/70mph.

On 3/23/2022 at 8:59 AM, silverstude said:

tie rods or general  alignment

I'll check those out when I install the fresh coils and shocks.

 

On 3/23/2022 at 4:14 PM, mfg said:

Possible defective engine mounts issue...since your Avanti ll has a 700R4 conversion look everywhere for metal-to-metal contacts.....Since driveshaft was modified, check for depth of front yoke in trans extension housing.

Good call.

 

On 3/23/2022 at 4:41 PM, mfg said:

I'd also remove the driveshaft and check for any looseness and/or tightness in the universal joints ...especially the front one.

Will do!

 

thanks all!