1,800 RPM
 

I have a 67 GTO that I recently purchased. Its a great car and it very fast with a butler built stroked 455 with ported eheads hyd roller and Doug's headers. It has an M20 transmission with a Centerforce dual friction clutch. There is a pretty significant vibration at 1,800 RPM. It's not the driveshaft and it's not the transmission because it has the same vibration in neutral without the car moving. I believe it's either the engine or the clutch causing this vibration. I was wondering if anyone has experienced this issue? I only notice it at right around 1,800 RPM. Any ideas? Could this be caused by a malfunctioning hydraulic valve lifter? Could the pressure plate/clutch be out of balance?

If the engine balance is good, likely a flywheel out of balance. You can run the engine without the transmission installed, if you believe it's the clutch, pull the trans and run it with the clutch removed from the flywheel to see if the vibration disappears. If the vibration persists, you can have the flywheel balanced at a machine shop or replace it.

Just a thought, I’m probably the worst mechanic on here but what about the harmonic balancer, especially if it’s 60 years old or a cheap one

Does the vibe peak at 1800 or satr at 1800? If it's a mechanical balance issue like the flywheel. I suspect it will get worse with rpm.

George

It's very specific to 1,800 +/- 1000 RPM not present at 1,500 goes away before 2,000

Flywheel balance mismatch. The motor is probably internal balanced not external.

A stick car will amplify a lean-rich condition in part throttle operation, that rpm range is where different circuits overlap.
Just throwing it out there. Good luck with it.

John

Hmmm, could be mixture transition beating with the Timing Advance dancing (mech and maybe not Vacuum). What type carb, intake, dizzy?

850 Dbl pumper
MSD RTR Distributor with vacuum advance
Intake Performer RPM

The vibration is there in neutral, clutch in, clutch out, during acceleration, during steady, during deceleration. I feel it the strongest when down shifting and hitting 1,800 RPM. In all cases it is at the same RPM.

I would remove clutch and run engine bare. That will tell if its engine side or trans side

Before making any changes, take all the belts off-loose. That will eliminate a cooling fan-clutch and strange harmonics with alternator etc…

My thoughts. I fact the 400 I am freshening was an auto flexplate it was balance to. I took the flexplate to the machine shop to match to the new stick flywheel.

I do know some guys that take the pressure plate to balance as an assembly and mark it and the flywheel.

I removed the belt and eliminated the accessories. Next step pull the trans, flywheel etc.

During decell; not mixture, nor Timing.

Vibe due to rotating metal.

After removing all those parts, maybe install an automatic flexplate, start engine, see what happens...

Could this vibration be caused by a bellhousing misalignment?

Alignment wouldn't do it.

What kind of balancer is on there? If it's an SFI, it's most likely zeroed. If it's a generic factory replacement, I would pitch it in the circular and dig on the build to determine if it's internally or externally balanced.

If it's a stroker assembly, my first guess would be internally balanced, but still need to troubleshoot.

You can in many cases visually inspect the flywheel and determine if it's a zero or externally balanced one.

After above, I would be looking at the PP.

I would also inspect the pilot bushing carefully. If it's damaged etc, would also be looking at the trans front bearing. (if not the entire trans).

If you get it apart, it wouldn't be a bad idea to check the general alignment of the bell. If it's an OE one, behind a stroker, I would replace it regardless.

EDIT: If it's a CF PP, check the weights to ensure they are correctly seated and the retainer isn't damaged or missing.

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Power Bond SFI

So yes, that's an internally balanced unit.

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Pontiac balancers are all neutral balanced, IIRC. The difference with an internally balanced engine would be the flex plate/flywheel would also be neutral balanced vs an own where the flex plate/flywheel have an imbalance.

If you have access to spares, maybe try a different carb and distributor. Those are easier to swap around than flywheels :-)

Update:

Removed the trans, clutch, pressure plate and flywheel. Installed two different OEM flex plates and the vibration is still present at 1800 RPM. I did notice that the clutch springs have been making contact with the bolts attaching the flywheel. The flywheel is .135 under the OEM spec of .960. Those issues are unrelated to the vibration but need to be resolved. I am baffled as the what is causing the vibration. It appears the flywheel is an aftermarket SFI unit. (see pic)

Since the vibration is still present during deceleration with my food off the gas I doubt it is carb or timing.

Maybe try a neutral balanced flywheel/flexplate if you haven’t yet?

Take advantage of doing this experiment if you still have the clutch removed.

Take one of the clutch mounting bolts ( or a long enough different bolt ) and stack two big washers under when you bolt it to the flywheel.

Start the motor and hopefully one of 3 things will then take place .

1) the vibration will be less or gone.

2) there will be no change .

3) the vibration will be worse.
If the vibration is worse the move the bolt washer combo across to the other side of the flywheel as best as possible to be 180 degrees out .

Try this in all six clutch mounting bolts holes or until you hopefully get some kind of result either way.