'68 Chevy pulsation while driving.

[luke1968c10]

I have a 1968 Chevy C10 250 six cylinder 3 speed on the column. It has been in my family from 1973 and the odometer has returned to 00000.0 four times so now it has 591000 miles on it(engine was rebuilt at 567000 but everything else is original.) It is mostly stock but I recently replaced the points with an HEI distributor. It has a pulsation that coincides with the speed of the wheels turning. It occurs in all gears and goes away when I press the clutch or go to neutral. There is no noise associated with it. I have replaced all four brakes also replaced front wheel bearings. I topped off the oil in the transmission and the rear end. I can usually figure out any problems with this truck but this one has got me. Thank you in advance.

[snipescastle2]

Drive shaft throw a balance weight maybe? Or does the carrier bearing appear to be bad?
That's my guess....not sure.
Ben

[luke1968c10]

Its a short bed so no carrier bearing, but the weight on the driveshaft is something I'll have to check. I suppose there should be some remains of whatever held the weight on. Thanks for the reply.

[Grumpy old man]

it's a drive line / clutch issue if it goes away when the clutch is pressed or placed in neutral . Have you checked the universal joints lately ? Also with that many miles it could be front yoke slop ...

[sloth454]

sounds like it ether threw the balance weight off the drive shaft or you could have u-joints going bad. Do you feel it in the seat or through the steering wheel?

[luke1968c10]

I feel it in the seat. The u joints are fairly new I replaced the rear about three months ago and the front last spring. I didn't notice if it went back in in the same position(could have been turned 180 degrees.) Could that make a difference?

[sloth454]

ok. And if you got the yoke miss aligned it will cause a vibration because the joints wont be "flexing" in sync. Did it start right after you did the joints or has this been an ongoing problem for a while? Worse case its possible something in the transmission could be coming apart.

[luke1968c10]

It started out as barely noticable so I'm not absolutely sure when it started. t has been getting gradually worse over time. I suppose I could try taking the drifeshaft off and turning it 180 and trying it out. If it's a transmission problem wouldn't it make noise? Really hope it's not the tranny, I'm sure those transmissions aren't just laying around everywhere anymore!

[Grumpy old man]

You won't have too much of a problem finding a three speed cheap with so many guys going for 4-5 speeds and automatics . Just a few short years ago you couldn't give away a 250 6 with a 3 speed ,Now guys are buying them to put the trucks BACK to stock.

[snipescastle2]

You can try placing a worm screw hose clamp around the drive shaft and try driving a short distance, see what it does. Then re-clock the clamp at a different position, 6,9,12, 1 o'clock,etc. If it gets better, great, if worse, keep adjusting the clamp to see if its any better.
The weights they put on the shaft were spot welded, and as time goes by, they rust off.....
Hth,
Ben

[67 chevelle]

couple of guesses , clutch / presureplate , and pinion bearing .

[luke1968c10]

Originally Posted by snipescastle2

You can try placing a worm screw hose clamp around the drive shaft and try driving a short distance, see what it does. Then re-clock the clamp at a different position, 6,9,12, 1 o'clock,etc. If it gets better, great, if worse, keep adjusting the clamp to see if its any better.
The weights they put on the shaft were spot welded, and as time goes by, they rust off.....
Hth,
Ben

Thanks, I'll try that. I wouldn't have thought of the worm clamp.

[hamjet]

It would most likely be the rear end pinion bearing, parts in the rear end, tranny output shaft or ujoints if it coincides with truck speed.