bad vibration

[philo_beddoe]

Vibration getting a little worse, notice at above 40mph and only when i accelerate, even a little. But when i let up on the gas it stops.  Might have to go to my mechanic.  Dont have my shop yet.

Oh, and actually there are quite a bit of posts on this topic.

[philo_beddoe]

The vibrating kind of went away, huh,,must have been the flux capacitor. I'm sure it'll be back.

[Greybeard]

Did the driveshaft get bent? or did the shaft balance weights fall off? Probably not likely because the vibration would simply get worse as the speed of the shaft increased. Don't forget the shaft turns at the same RPM's as the engine in direct drive (meaning not overdrive or one of the lower gears). Also, when you checked the u-joints, did you check them with the wheels chocked and in neutral? If not check it again, any strain on the shaft can lock up the u-joints enough to give a false negative. And don't yank it, just try to twist it while watching each bearing on both ends. Any play at all (rotationaly) and the bearing is on its way out.

[Greybeard]

New tires and factory rims, but i'll check the weights anyway.  It is not "shuddering" like a bad unbalanced wheel, more of a steady vibrate, hardly pulsating i guess, seems to smooth out near 60 mph.  I tried to move driveshaft, tight as a drum.

If this started soon after you got those new tires then it's likely them causing the issue. New tires are poorly built these days, out of round will cause pulsing even if perfectly balanced. Tread squirm will cause it too somewhat. Even tread pattern (most likely) will cause it. Try overinflating them a LITTLE and see if changes the pulsating feel. Most folks make the mistake of running full sidewall pressure in their tires. But do the math, a tire that is intended to carry 1750lbs can carry 1750 by itself at full sidewall pressure, that would be for a truck weight of 1750lbs x 4. Does your truck weigh 7000lbs? Then why do you need 7000lbs of tire capacity? If you go by the pressure on the door jam you are most likely still wrong. That pressure is for the exact same tire that came on the truck. If the brand/diameter/width/ply rating is different, there is a good chance the needed pressure will be different too.   

[philo_beddoe]

I will certainly check thag, thanks.

[Greybeard]

Out of left field...are all your spark plug wires in good shape? No leakage or cross-firing? Wouldn't necessarily cause a pulsating though.

Years ago when I had my first GMC (a '78 I bought after it rolled in 1980 and I got for a song) it was used for river running a lot so it did not have any sound deadening, carpet, or floor covering of any type. I also removed the headliner for more head room during bouncing around in the cab (I didn't have seat belts in it and didn't even know what they were for) LOL.

After I got the 39/15-15's Tall Baja's by Micky-T's put on, the truck droned and pulsed intensely at highway speeds. It was like a surround sound dull bass throbbing feel. Really gave me a headache on long trips (like when I moved from St.Louis MO to Vail Colorado pulling a trailer), I was zombiefied when I stopped for gas every two-three hours. Unfortunately, it was always part of the truck. All that bare sheetmetal got to vibrating in unison. Not to mention I only had a eight track tape player in the truck with one tape...Bad Company-Bad Company....Over and over and over and OVER! UGH! The pain of that trip will never end!   

[philo_beddoe]

Wires and plugs are very good,, replaced both when we did the valve seals.

[philo_beddoe]

no more pulsating since i adjusted the timing, may have been a little retarded if that makes sense, is that possible?

Oh well, see post "another ques. on timing" all is good.

[philo_beddoe]

Ok, spoke too soon, need a little help here. I narrowed it down a little. With engine fully warm and in park, there is a shudder feeling when i rev the engine. If i'm doing 50 down the road with foot off the gas, all is smooth.  I'm saying my shuddering is in the engine, what could it be? Balancer?? My balancer is not "pin" smooth. I can see the pulley on the end with a slight wobble.

[philo_beddoe]

If it works, you can see it in the first few seconds of this video.

https://vimeo.com/150836368

I'm updating this post, because on second thought I don't think it's the balancer at all. Under load, going up a hill or just plain accelerating, I feel a vibrate/shudder coming from engine, because in park I can hit the gas and feel it also. It is not tranny/dr shaft or rear end. Almost like a mis-fire, could this be a fouled plug(s)?

What confuses me is, at idle it is very smooth.

[Greybeard]

Is it tuned correctly? Meaning, the carb is not leaning out during acceleration? The flyweights in the distributor are free to move? No broken advance springs connected to them? No pinhole vacuum leaks?

[philo_beddoe]

Ok, i checked plugs and all were clean, wire brushed anyway. Then i ran engine without main fanbelt, it was hard to tell that way, but this may sound crazy, when i put belt back on, and took for a test spin, it seemed pretty smooth and had its old power back. Now i'm more confused!

[philo_beddoe]

Should the harmonic balancer be absolutely "pin" straight? I have an engine vibrate and my balancer is not perfect. In other words, if it was perfect it would look like the engine wasnt even running. You can see it in the pulley too.

Is this my problem? It certainly feels like it.

The engine is tuned right as far as i can tell, if not the balancer, then back to the drawing board.

[Irish_Alley]

i merged it all together. this was we all know where you were at from point A to point D

[philo_beddoe]

Ok, thanks Irish!