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Engine vibration at 3100 rpms

corey6969

corey6969

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Gotta bit of a problem, I notice a bit of a vibration at around 3100-3200 rpms. I just rebuilt this engine with all new parts and had it balanced at the local machine shop. I took it to them and they said a missfire could cause this problem to. Me and my buddies think the rotating assembly is out of balance but is it possible a missfire would cause that.

I have a set of comp cams lifters that wont stay in adjustment and one, maby 2 are really noisy. I tried 1/2 and 3/4 past zero lash and eventually they start making noise again. Could them crappy lifters be causing uneven cylinder pressures and the vibration.

The whole rotating assembly was balanced with the dampner and flywheel. Its usually below 0 here and I dont have much motivation to pull the motor out again, but I could handle the top end work.

Its weird you think youde be able to notice it at lower rpms but not till it hits about 3200 then its instantly there, maby im wrong..


What do u guys think???
 
Have you diagnosed to make sure it's the engine that's vibrating? I.E. does it vibrate in neutral while sitting still? Have you tried unbolting the transmission/clutch to see if that changes anything?
 
In neutral u can feel it just a tiny bit when you rev it up to 3200, but when your moving no matter in what gear you can definatly feel it. Seems like the more load you put on it the more it vibrates. This is the same engine that was in it before just rebuilt, it never did this before the rebuild either
 
What do you have for a cam? With small block chevys its not uncommon for rocker studs to pull out of the heads. I have had them pull out with a "stock replacement" cam before.

Oh what about basic ignition parts like cap, rotor, plugs, wires.
 
I have a comp cams extreme energy cam, but I have screw in studs and guide plates so I dont have that problem :D. I think I have a bit of a head gasket problem so im just gunna pull the heads and replace the gaskets and lifters. Hopefully its just a high rpm miss(or 2), but im not gunna hold my breath. Ill pull the motor out in the spring time if it still vibrates.
 
Couple things,

(1) What year engine are we dealing with?

(2) What engine size is it?

(3) Is it stick or auto (assuming auto since you said neutral).

(4) Regardless of stick or auto is it the same flywheel/flexplate as was on this engine before being rebuilt?

(5) Is the harmonic balancer the same one that was on it before being rebuilt?

(6) Did the shop balance the crank, rods (with a set of bearings), pistons (with a set of rings), harmonic balancer, and flywheel/flexplate?
 
corey6969 said:
I have a comp cams extreme energy cam, but I have screw in studs and guide plates so I dont have that problem :D. I think I have a bit of a head gasket problem so im just gunna pull the heads and replace the gaskets and lifters. Hopefully its just a high rpm miss(or 2), but im not gunna hold my breath. Ill pull the motor out in the spring time if it still vibrates.
Click to expand...

(1) Are you using hardened pushrods for use with guide plates?

(2) If this vibration is from an incorrectly balanced engine the engine will not survive long at all, it will beat the bearings clean out of the engine.
 
4X4HIGH said:
Couple things,

(1) What year engine are we dealing with?

(2) What engine size is it?

(3) Is it stick or auto (assuming auto since you said neutral).

(4) Regardless of stick or auto is it the same flywheel/flexplate as was on this engine before being rebuilt?

(5) Is the harmonic balancer the same one that was on it before being rebuilt?

(6) Did the shop balance the crank, rods (with a set of bearings), pistons (with a set of rings), harmonic balancer, and flywheel/flexplate?
Click to expand...


1. 1976
2. Small block 400 Bored +.030 (406)
3. 700r4 Automatic Transmission
4. Not the same flexplate, brand new one
5. Not the same harmonic balancer, brand new one
6. Yep the shop balanced it all together they said, I gave them a rod bearing and a set of rings from the parts I bought for it and used the same ones when i put it together.

The counter weight is in the balancer and all the weights are in the flexplate.
 
4X4HIGH said:
(1) Are you using hardened pushrods for use with guide plates?

(2) If this vibration is from an incorrectly balanced engine the engine will not survive long at all, it will beat the bearings clean out of the engine.
Click to expand...


1. I am using hardened push rods
2. I have about 800 or so miles on this engine and oil pressure is still the same.
 
Ok, what brand of harmonic balancer and flexplate did you buy? Do you have part numbers for them?
 
Those are the correct pieces, now to find out how your machinist balanced the components.

Have you unbolted the torque convertor from the flexplate and ran the engine to make sure you don't have a bad convertor? If not i would highly suggest you do this first before taking anything apart.

Also, make sure your engine and trans mounts are good, i've seen a vibration from bad mounts in the past.
 
4X4HIGH said:
Those are the correct pieces, now to find out how your machinist balanced the components.

Have you unbolted the torque convertor from the flexplate and ran the engine to make sure you don't have a bad convertor? If not i would highly suggest you do this first before taking anything apart.

Also, make sure your engine and trans mounts are good, i've seen a vibration from bad mounts in the past.
Click to expand...


I havent unbolted the converter because it didnt vibrate like that before the rebuild and i used the same converter. I replaced the motor mounts when i put the motor in. The machinst said he checked and weighed everything, and balanced the crank with the flexplate and dampner on it.

Come to think of it I did put another 400 in it while i was rebulding this one and it didnt vibrate either, ran er all the way up to 5000 rpms. I took the machinest for a ride and at first he said it could be caused by a misfire. I drove him around in all gears 1-3 that is at 3100 or so rpms, torque converter locked and unlocked to. He said he would check it when I took it back apart.

Do u think a missfire could cause that vibration with crappy lifters, i know at least 1 if not 2 are really loud. They wont stay in adjustment so im gunan replace them.
 
Vibration and misfire are completely different and wouldn't feel the same.

Even though you have the same convertor i would still unbolt it and run the engine.
 
Henry's in Carrington (H&J Machine) or Quailty in Fargo. This is not to say that hedahls did a bad job, its just the others have lots more talent working there
 
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