Tach misreading issue on 67 427/435 car with TI

[John S.]

The tach on my 67 427/435 car reads correctly when one lets rpms build slowly, but if you mash throttle or advance it quickly the tach fluctuates, goes down in rpms rather than up with motor/throttle above about 3000 rpm. All up to about 3000 rpm is fine, then it breaks down when quicker/rapid rpms are called for. Slowly build rpms up and it reads correctly, but faster acceleration and it fails to read correctly above about 3000 rpm, actually goes down and fluctuates. Would the issue be the tach itself? Fittings are tight on both ends at dist and tach cable. Appreciate any tips to correct.

[Rob M.]

Re: Tach misreading issue on 67 427/435 car with TI

The tach on my 67 427/435 car reads correctly when one lets rpms build slowly, but if you mash throttle or advance it quickly the tach fluctuates, goes down in rpms rather than up with motor/throttle above about 3000 rpm. All up to about 3000 rpm is fine, then it breaks down when quicker/rapid rpms are called for. Slowly build rpms up and it reads correctly, but faster acceleration and it fails to read correctly above about 3000 rpm, actually goes down and fluctuates. Would the issue be the tach itself? Fittings are tight on both ends at dist and tach cable. Appreciate any tips to correct.

check the dist cable itself, is the cable still square on both ends and is the receiver of the cable (inside the distributor and tach gauge) still square?

[Joe L.]

Re: Tach misreading issue on 67 427/435 car with TI

The tach on my 67 427/435 car reads correctly when one lets rpms build slowly, but if you mash throttle or advance it quickly the tach fluctuates, goes down in rpms rather than up with motor/throttle above about 3000 rpm. All up to about 3000 rpm is fine, then it breaks down when quicker/rapid rpms are called for. Slowly build rpms up and it reads correctly, but faster acceleration and it fails to read correctly above about 3000 rpm, actually goes down and fluctuates. Would the issue be the tach itself? Fittings are tight on both ends at dist and tach cable. Appreciate any tips to correct.

John------


Most likely it's in the instrument. Corvette tachometers are not direct drive; they use a type of "magnetic drive" with dampening. Yours probably needs service. However, I have found that once these things go bad, getting them "right" is problematic.

[Dick W.]

Re: Tach misreading issue on 67 427/435 car with TI

Tach head needs to be cleaned, re-calibrated. Several companies can do this for you. A frequent poster here, Joe Ray, provides this service. Reasonable, capable, fulla BS, etc. Easy to deal with

[Bob H.]

Re: Tach misreading issue on 67 427/435 car with TI

Can't it also be a worn small gear drive in the dist or a cable that needs cleaned and lubed?

[Dick W.]

Re: Tach misreading issue on 67 427/435 car with TI

Can't it also be a worn small gear drive in the dist or a cable that needs cleaned and lubed?

If the cable was binding enough to cause his problem, either it or the gear would give up. The gear is pretty much an all or nothing deal