engine and drivetrain tachometer reading 500-800 RPM too high, why?

[jerry]

this morning marked my return to actual 02 driving rather than merely wrenching and related non-driving activities for the last 12 months. it is a 75 02 that was returned to me after the PO (my nephew) drove it into the ground. it's back on the road with a new carpet, front end suspension rebuild, radiator and minor patch panels welded in.

it has its 4spd transmission, a 3.91 LSD and 14 inch bottlecaps and what i believe is a 38/38 Weber (previous PO installed).

i can't believe i should be seeing such high revs at 60mph in 4th. can't remember the exact number, but i'm usually able to listen to the engine and now where it's revving at. my tach consistently reads high. no wonder my idle is soooo low right now. i set it according to my tach and it feels a fair bit lumpy (indicates 1000 RPM, but i'm guessing it's more like 700-800RPM).

i've got a Crane Cam XR-700 installed, too

thoughts?

[Mike87]

Maybe too much resistance in the tach? Corrosion on the contacts in the gauge itself?

[TobyB]

What are you seeing at 60 in 4th?

(crud, now I forget what mine does... 3500?)

The tach should be either on, or off by a multiple-

like, it's reading 2 spark events for every spark.

hmmm

t

[jgerry2002]

70mph = ~4100rpm with a 3.90 and a 4 speed on my '76.

[M3M3]

have you actually compared the readings with another tach like the ones on digital timing lights?

[allbim]

There is at least one potentiometer in the tach. I don't know what it does.

It could be for adjusting calibration or sensitivity.

[jerry]

70mph = ~4100rpm with a 3.90 and a 4 speed on my '76.

on my return trip home this afternoon, i noted 4000 RPM @ 60 mph and around 4800 RPM @ 70 mph in 4th gear....

i know now that i'll need to adjust my idle speed up a little.

[Guest Anonymous]

My tachometer has a pair of adjustment screws on the rear cover. One is called 'span' and one is 'adjust'. 'Span' widens or narrows the entire tach scale. 'Adjust' brings the pointer up or down for a given RPM input from the distributor. I think the one on the right side when you are looking at the rear of the 'can' is the SPAN but I forget ... not sure.

Put an external DWELL / TACH on the motor while running, adjust throttle stop adjustment screw for a typical RPM such as 1000. Then turn the adjustment screw on the back of the tach until the pointer reads the same as the DWELL/TACH. If you do it at a coupld of places you can set the SPAN.

At this point you'll be hoping that someone makes an extension cable for the instrument cluster. You could make the calibration with the guage cluster facing reverse!

[TobyB]

on my return trip home this afternoon, i noted 4000 RPM @ 60 mph and around 4800 RPM @ 70 mph in 4th gear

sounds like you've got a 4.44 diff...

heh

t

[jimk]

to be most accurate at mid scale with only a bit high at max scale.

If you have Electromotive Tec III EFI, you can do it with a crank sensor signal generator.

[NickVyse]

Left of field, but don't suppose you have undersize tyres on the car?

[jerry]

Left of field, but don't suppose you have undersize tyres on the car?

it's my understanding that the tachometer runs off the distributor and shouldn't really be influenced by rear gear ratios, or tire size the same way that the speedometer is influenced.

i adjusted my idle speed with a dwell/tach meter and set it to around 900 RPM. idles much better. i had the engine idling at around 500-600 and it barely idled. i was actually quite surprised it would idle that low at all. i could almost hear each cylinder struggling to rotate the crankshaft....

[NickVyse]

Left of field, but don't suppose you have undersize tyres on the car?

it's my understanding that the tachometer runs off the distributor and shouldn't really be influenced by rear gear ratios, or tire size the same way that the speedometer is influenced.

i adjusted my idle speed with a dwell/tach meter and set it to around 900 RPM. idles much better. i had the engine idling at around 500-600 and it barely idled. i was actually quite surprised it would idle that low at all. i could almost hear each cylinder struggling to rotate the crankshaft....

You're right that under sized tyres or different gearing wouldn't effect the accuracy of the tach, but they could make you experience higher revs at a given speed, which is the symptom you described.