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L20B Low end juddering


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Hello all,

My '80 720 get around vehicle vibrates at low rpm when I accelerate--floor it. Evens out after about 3k rpm.. Has anyone had similar symptoms? Does this sound like U-joints? Main bearings? BTW, how hard will it be for an arthritic old bastard to replace rear leaf spring shackle bushings, idler arm and strut rod bushings? PS, I love this truck. Thanks, in advance for any replies!


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Most likely the rubber donut on the carrier bearing is rotted away. The bearing almost never go bad but the rubber does. The rubber supports the driveshaft and when torn or rotted the driveshaft is de stabilized and vibrated when accelerating. It's located at the rear of the front driveshaft. (there are two)




Lug nuts?

Drive shaft to differential mounting bolts

U joints (three of them)

Loose transmission mounting bolts to crossmember

Loose or broken engine mounting brackets or rubber isolators.

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I rolled around under the truck trying to scope it out. I may take it to the guy I had do my ball joints and have him put it up on the rack to check it out. I figure he owes me, because when I was under the truck I noticed that he hadn't tightened the keeper bolt on the torison bar arm/ride height. It makes me wonder if such a blunder was intentional.

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I looked at the driveshaft we pulled from the donor 720 '80 kingcab and that carrier donut thing is way chewed. Looks like most probable cause of typical vibration/juddering. I will investigate when I get some crawling around time. Looking Minnesota and feeling California . . . well, on second thought feeling pretty Minnesota. Burnt out from Roland Rock voodoo space surf gig with son at Redwood Curtain Brewing Company last night and possible impending seasonal lung grunge. Thanks again for info.

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"how hard will it be for an arthritic old bastard to replace rear leaf spring shackle bushings, idler arm and strut rod bushings?"

It depends on the type of bushing. If it's a vulcanized rubber bushing with a sleeve, the leaf spring bushings can be quite a pain to replace. I usually burn them out and the chisel out the remaining sleeve.


It also depends quite heavily on how rusted the fasteners are. Sometimes the rubber itself adheres to the bolt and those can be quite difficult.


Not trying to scare you, but sometimes that job even catches me off guard.

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You can also un-do the U joint in the middle and un-clamp the isolator and remove what's left. Then wrap a strip (inch wide?) of conveyor belt (it's what I used) around the driveshaft to build it up, then clamp the strap back on. I did mine for a temp. fix and it worked so perfectly I never did replace it.

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Thanks for relating your valuable experience! It will help me address these problems.


Update.  Oops--short bed--one piece driveshaft.  Back to the drawing board. Maybe I'm barking up the wrong tree with the driveshaft. Could the bad rear leaf spring bushings be causing the Jah Wobble?

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After reading again, I would check the u-joints. If a vehicle sits for extended periods, the u-joint cap on the bottom of the yoke (the one closest to the ground) can get moisture buildup which rusts causing the needle bearing to become pitted. 


U-joints are an easy fix.

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