Engine vibration at high rpm

[71Nissan240Z]

Just wondering if anyone experiences the same thing I do. A little background to my car is.

1971 240z. Stock 4 speed, stock rear end. Engine is a P54 Block, P79 head 2.8L from a 81zx.

Stock SU 4screw carbs

Mild cam (dont know the specs)

Pertronix ignition

3ohm blaster coil with BR bypassed.

New spark plugs gapped at .44

Carbs are tuned ever so slightly rich, on the border of being perfect. Zhome says its best for power. Carbs are synced. Running premium gas...timing is conservative at around 10degrees BTDC at idle.

The issue is the car starts to vibrate at around 6,000rpms. Not extreamly bad but enough to notice. I understand the redline on my engine to be at 6,500rpms. So im wondering if its normal to have a little vibration at high rpms.

That is all.

[71Nissan240Z]

I forgot to add that after my mods the power doesnt peak like it did before at around 5k. It has power when I shift at 6k rpm and acts like it may have a little more, otherwise I wouldnt even worry about this.

[71Nissan240Z]

No one! come on guys, at least share a theory or two.

[John Coffey]

How are we supposed to diagnose a slight vibration issue across the Internet?

[240260280z]

Bad damper pulley?

Carbon on some pistons throwing off rotating mass balance?

Coupled oscillator?

[Zforce]

Coupled oscillator ? What that ?

[71Nissan240Z]

How are we supposed to diagnose a slight vibration issue across the Internet?

Thanks for your help

Bad damper pulley?

Carbon on some pistons throwing off rotating mass balance?

Coupled oscillator?

I have an accessory damper pully, it appears to be fine

cylinder 6 probably has a little bit of carbon build up, cylinder psi is slightly up on #6 and the car will turn over a couple times once and a while after shutting it off.

Coupled oscillator ? What that ?

I believe its frequencey resonance or something along those lines ....

[Zforce]

Back to the vibration thing, I noticed a vibration at speed due to over advanced timing. When I backed off the timing a few degrees it went away. You might have too much total advance, and steaking of ignition you never did say how you resovled your Pertronix issue.

[71Nissan240Z]

Oh the pertronix issue was not bypassing the ballast resistor with a 3ohm coil. I installed a 1978 tachometer to solve my tach problems. The car totally runs great now and has way more power than before. It doesnt vibrate at speed, there is no pining issues because I can gointo 4th gear at 20mph and peg the throttle with no pinging. Its just kinda hard to explain but at 6k the car sort of feels like its vibrating or it may be just be engine, I dont know.

It may be that I just have too much rotating mass. I.E., AC compressor, fan clutch and mechanical fuel pump.

I can tell tho the car wants to go beyond a little more than 6k so Ill try and rip out the AC and also remove the fan and replace them with electronic ones to see if its better.

I just want to know if anyone else experiences this, is it normal? or should it be smooth all the way up to redline. Which in my case is 6,500k l

Edited June 18, 2011 by 71Nissan240Z

[H4LIGHTS]

Were all rotating and reciprocating parts balanced?

Who did the balancing, what did they balance and what were the before and after results.

Just a few grams difference between rods/pistons and an out of balance flywheel/clutch/pressure plate can make vibrations.

Or, it just may be a weak misfire due to a too large air gap at the spark plugs for the revs.

If you think it's accessories hanging on the engine disconnect and remove them and see if the 'vibrations' move or disappear.

There are harmonic devices that professional NVH (noise, vibration, harshness) folks use to determine what is causing the issue.

So many possibilities.

[d240zx2]

Vibration dampers can fail internally and not be visible externally. Ask how I know....

[Zforce]

Vibration dampers can fail internally and not be visible externally. Ask how I know....

After 40 years it's gotta be a granade, whats better rebuild the old damper or buy a new one. I've heard horror stories about getting new ones to fit properly.