64 Tach Needle Jitter at idle.

[Jeff B.]

I'm preparing my 64 300 HP for PV and I just can't get the tach perfectly still at an idle. It's a very small bounce of less than 100 RPM (one increment line on the tach). Smooths out completely after 1000 RPM. I did my own instrument cluster restoration but did very little to the tach except clean and lubricate. I also rebuilt the distributor and drive gear, replaced the tach cable twice, and tried different lubricants. I also tested the tach out of the car using a drill motor on the end of the cable at a steady speed and had the same problem when at RPMs less than 1000. I talked with one of the well known instrument rebuiders and he said the 64's have a heavy needle which causes problems and that this might be normal. Is a 50-100 RPM bounce at an idle going to fail the PV. And if so, anyone got any ideas how to fix it. New tach?

[Stuart F.]

Re: 64 Tach Needle Jitter at idle.

Let's see if I have this right; your needle has a steady bounce or is it one that varies as might be expected with say an L-76 or L-84? I would think an L-75 would/should be fairly steady - not influenced by actual idle speed fluctuation.

Hope you get some answers.

Stu Fox

[Ken A.]

Re: 64 Tach Needle Jitter at idle.

Your tach bushing is worn and the frame assembly, including first worm & 2nd worm needs to be replaced along with the jewel in the strap. Find a rebuilder who has these parts and you're good to go.

[Bob R.]

Re: 64 Tach Needle Jitter at idle.

A bad cable can also cause the needle to jump. If that is the case a little lube should help. Mine bounced like crazy just before the cable broke. They usually break near the distributor. The sharp bend at that location causes the cable to brake.

[Jack H.]

Re: 64 Tach Needle Jitter at idle.

Consider hooking up an electronic dwell/tach to see what RPM profile it shows... If it's the real McCoy idle RPM fluctuation you're seeing on the mechanical tach in the instrument panel, you could be chasing your tail!

[Jeff B.]

Re: 64 Tach Needle Jitter at idle.

Stuart: The bounce is constant and present both when the tach cable is connected to the distributor, or removed and conneced directly to a constant speed drill motor. The drill motor test was done with the instrument cluster out and a new cabel pulled straight with no bends.

Ken: Your solution with the worn tach bushing and worm gears sounds correct since the tach is the one component that has not been fully rebuilt or replaced. I'm confident with the cable, distributor and smoothness of the engine idle. Any suggestions on the best rebuilder?

Bob: Great suggestion, I did connect a dwell tach and the problem is not present on that meter.