Jumps in reverse
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Junior User
Joined: Sep 2003
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Jumps in reverse
I have an 81 F-150 300 six, four wheel drive, four speed o/d trans, with 3.73 gears and the posi in the rear. It jumps like a kangaroo when I start out in reverse, but not when I get going. It hops so bad I think my u-joints are breaking. It works fine foreward. It also doesn't do it in winter. What is wrong? 
Post Fiend
Joined: Jun 2003
Posts: 5,643
Likes: 8
From: NE Texas
Have you checked the engine and transmission mounts? When they are broken or loose the torque of the engine will cause jumping.
Also, see if the ubolts and mounting plates on the springs are tight.
Also, see if the ubolts and mounting plates on the springs are tight.
Post Fiend
Joined: Jun 2003
Posts: 5,643
Likes: 8
From: NE Texas
I don't think it has anything to do with the diff. You say you have a posi diff. Is it a locker or limited slip? You should have friction modifier on a limited slip diff, but on a locker it's not required as it would do nothing.
Back to the problem.
Jumping in reverse can be caused by loose mounts, supports, etc. from the engine to the rear differential. That would include the carrier bearing (if you have one). The '81 has a manual operated clutch where the "Z" bracket is attached at the frame and the bell housing. That bracket could be moving (caused by a loose engine or tranny mount) and "drawing" the clutch pedal away from your foot. Or the rear spring mounting plates could be loose causing the axle to wrap more than normal.
I would have someone look underneath the truck at the axle, driveline and tranny while you back up to see if there is play somewhere.
Back to the problem.
Jumping in reverse can be caused by loose mounts, supports, etc. from the engine to the rear differential. That would include the carrier bearing (if you have one). The '81 has a manual operated clutch where the "Z" bracket is attached at the frame and the bell housing. That bracket could be moving (caused by a loose engine or tranny mount) and "drawing" the clutch pedal away from your foot. Or the rear spring mounting plates could be loose causing the axle to wrap more than normal.
I would have someone look underneath the truck at the axle, driveline and tranny while you back up to see if there is play somewhere.
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