Originally Posted by fast68plymouth
If it noses over at 4K in every gear, it’s not fuel related.

Which is why I asked if the recovery was instant after a gear change.

If the recovery after the shift is instant...... it’s ignition or mechanical.

I had a very similar situation many years ago after a cam swap, but at about 5k instead of 4K.

Old cam revved over 6k easily....... new cam was totally flat at 5k.
Not a bunch of drama....... no popping, didn’t really surge or carry on....... just ran into a wall.
You could pretty much just hold your foot on it in 2nd...... it would just maintain 5k.
The instant you pulled 3rd....... it just took off again...... til 5k.

This was in like 1986, and I was pretty puzzled by it. Thought it was fuel..... hooked up a gauge I could see while driving...... never dropped below 4psi........ and the gauge never changed when you went from flat at 5k in 2nd to pulling hard in 3rd.

I was buying speed parts from one of the local engine shops...... took the owner for a ride.
He agreed it didn’t really feel or act like float........ but since I found nothing else wrong he suggested adding a shim under the springs and see if it changed.
So, I added a .060 under each spring.

Totally cured it........ it would rev as high as I wanted after that(63-6500).


At one point I also had an issue with the rev limiter on a MSD digital 6 box coming in at 5k.
It tested perfect with an msd ign tester, but with the motor running, any setting over 5k...... still engaged the limiter at 5k....... even in neutral.
Rolled the knobs back to zero....... motor revved fine.



LOL. I had a cheap customer who just didn't want to change anything, or even check most any thing. He brought in a 6 cylinder ford head and said, kiss the seats with a stone and slam it back together. Don't check it for cracks, don't check the surface, just do it.


A week later he drives the ruck over and it's barely running. The springs were so weak anything more than idle speed or slightly higher RPM and the lifter would pump up and open some valves! He wanted me to "shim" it. I said get bent. It either comes off and gets done right or take your garbage somewhere else.


After that, I never let him dictate what i did whether he was paying or not.

Just because you think it won't make it true. Horsepower is KING. To dispute this is stupid. C. Alston