Vibration at Deceleration

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domdart

FABO Gold Member
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I put a 340, 833, and 8 3/4" from a 72 Duster into my 68 Dart. I took it for it's first ride to get the front end aligned, and noticed a vibration. I got on the highway, went through the gears, after shifting to 4th and letting off the power it started vibrating. If I get back on the power it stops. It only vibrates at higher speeds in 3rd or 4th when I let off the power. It's at it's worst at about 60 mph in 4th gear coasting. The rear end is 3.23. This is my first restoration. I just swapped all the parts thinking it would work. What should I do now? Thanks.
 
This is classic pinion angle grief.
Under deceleration, engine compression is yanking the pinion nose down. So either the springs are soft (unlikely), the tranny mount is torn (possible),or loose, or the pinion angle needs checking and/or readjusting (most likely).There's also a slim chance that; one or more U-joints is loose or that the driveshaft is too short.http://www.drivelineserviceofpittsbu...-do-you-need-1
 
This is classic pinion angle grief.
Under deceleration, engine compression is yanking the pinion nose down. So either the springs are soft (unlikely), the tranny mount is torn (possible),or loose, or the pinion angle needs checking and/or readjusting (most likely).There's also a slim chance that; one or more U-joints is loose or that the driveshaft is too short.http://www.drivelineserviceofpittsbu...-do-you-need-1

Thanks for the reply. I called the local driveshaft shop. They advised me how to check the driveshaft length. I'm going to check the length then take it down to their shop. I'll have them adjust the length if necessary and check out the u-joints. I remember using the old tranny mount and thinking I should replace it. I'll get a new one. I also tried to replace the rear u-joint but couldn't remove the old one.
 
Thanks for the reply. I called the local driveshaft shop. They advised me how to check the driveshaft length. I'm going to check the length then take it down to their shop. I'll have them adjust the length if necessary and check out the u-joints. I remember using the old tranny mount and thinking I should replace it. I'll get a new one. I also tried to replace the rear u-joint but couldn't remove the old one.
The best way to replace a U-joint is with a press. But it can be done with some correctly sized sockets, and a big hammer. we have all done it at the side of the road or at the track. not that hard, just make sure the needle bearings are in the right place or you will be doing it again.
 
I used the transmission mount from my 904 with no bolt through the crossmember. The Duster just had a piece of 2x4 as a mount. I replaced it with the proper tranny mount. I measured my driveshaft, it was 3" too short. I took it to the shop this morning. They will also install new u-joints. The rear one was bad. So, I had the wrong tranny mount, a short driveshaft, and a bad u-joint. I'll put the driveshaft back in and go for a ride, see what happens.
 
Make absolutely sure you have the correct pinion angle and that it's equal and opposite at the tranny end and the gear end. If one end is 3 degrees down, the other should be 3 degrees up. I found out the importance of that on my race truck - first time on track after installing a Ford 9 inch, I broke a driveshaft, tore out the rear of the tranny, and wrecked my truck. It doesn't get talked about much but it is very important. Also, have the driveshaft shop recheck the balance. I've had them say it's balanced and upon rechecking find out it's way off.
 
I used the transmission mount from my 904 with no bolt through the crossmember. The Duster just had a piece of 2x4 as a mount. I replaced it with the proper tranny mount. I measured my driveshaft, it was 3" too short. I took it to the shop this morning. They will also install new u-joints. The rear one was bad. So, I had the wrong tranny mount, a short driveshaft, and a bad u-joint. I'll put the driveshaft back in and go for a ride, see what happens.

Wowsers; 3 out of 5, it couldn't get much worse. Oh wait, you better change the rear extension bushing and seal, cuz that short shaft will very likely have it "wallered" it out.
 
Wowsers; 3 out of 5, it couldn't get much worse. Oh wait, you better change the rear extension bushing and seal, cuz that short shaft will very likely have it "wallered" it out.

I'm with ya on that. :D
Hope it didn't trash on the yoke too bad.
 
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