TX63CONV

I have a 63 with a rebuilt tach gauge by Bill Harrison. All of my other gauges on this car and a previous car were rebuilt by him and have worked/looked flawlessly. The needle bounces at idle and low rpms but at about 2500, it is steady all the way up.

I read on another forum that a hard turn in the cable could be the problem. Any truth to that? Running a new tach cable would not be that difficult.

TIA

68hemi

Have you lubed the cable lately and checked it for burrs?

DUB

Yes a hard turn in the cable housing can cause a binding of the cable in the housing...and also I have seen that lubing the cable can cause the same problem also due to the lube...depending on what type is used...can get thick and cause the cable to bind and pop also.

DUB

DansYellow66

Since the tach is rebuilt I would suspect the cable is worn. The tach cable has to take a couple pretty sharp bends from the distributor to the firewall penetration and those sharp bends increase the cable wear on the cable housing protective sheath inside. Some (including myself) have installed a 90 deg adaptor on the distributor to lessen the bends. Probably helps as mine has remained dead steady over 25 years but I don't have that many miles on it. And GM sent these cars out the door without the adaptor and they had smooth tachs for years.

You could try lubricating it. Lately I've been using this Tri Flow dry lube with some success.

https://www.ebay.com/p/Tri-Flow-Supe...a1afdbffe78fce

But cable lube is kind of a crazy subject with people using everything from chassis grease to the old standard of dry graphite, to auto transmission oil.

Donald #31176

One other issue to check is the correct indexing of the distributor..Correct indexing will allow sufficient movement to time the engine as well as aligning the tach coupling with the cable.

C2Dude

I currently own a 66 and had previously owned a 66 a number of years back. The tach's on both bounce quite a bit under hard acceleration. I believed this to be normal in mechanical tach's. Am I understanding that this is not the case and that I can improve this with an instrument rebuild, new tach cable possibly with this adapter?

DansYellow66

Venturing out on a limb at bit but I would suspect your tach drive has lost some of it's needle dampening capability. They do seem to be a bit more jumpy (little more overrun on a throttle blip) than an electrical tach but shouldn't be real bad. Might talk to an instrument rebuilder or send one a video of it for an expert opinion.

TX63CONV

Thanks for the info. Over the weekend, it stopped completely. I checked at the distrib and it is still connected--I havent looked under the dash yet. I will try to trouble shoot and lubricate--it has never been lubed under my ownership.

I will say that Donald's comment about indexing is interesting. My distrib probably needs to move over 1 tooth as the vacuum advance can is touching the intake to get my initial timing. The motor is new and still needs to be tuned but that might be a consideration as well.

I will say going from the tired 350 when I bought the car years ago to this jumpy 327 is NICE!

Donald #31176

As far as the tach quitting the likely cause is the tach drive gear has sheared. To check you ll need to disconnect the tach cable at the distributor and using a wide blade tool unscrew the tach coupling to remove the drive gear. If the gear is stripped you ll need to also check the mainshaft gear as both gears are usually damaged when this happens. The distributor will need to be removed Also you ll need to check the inside housing where the tach drive gear faces. If the tach drive gear has worn a hole in the housing you ll need to install a nylon bushing to correct the alignment of the two tach gears. While its out perhaps a blueprinting of the distributor is in order. ( optimizing the ignition curve , correct VAC, & shimming the distributor drive gear to under 10 thousands inch.).A blueprinted distributor makes a world of difference in the performance of the engine.

As far as indexing the distributor in a Corvette there is only one position that allows proper timing , proper alignment of the cable & coupling. Moving the distributor a tooth or 2 is not the complete answer to get the correct index. You ll need to search the archives .