Author Topic: Tach accuracy  (Read 798 times)

CantRepeat

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Tach accuracy
« on: September 03, 2021, 04:52:46 PM »
I noticed that the factory tach in my 67 spins crazy numbers that I know aren't accurate. IE it will spin up to and past the 7k mark.

Is there anything to check or something to look at the fix the accuracy of the tach?
-Tim
67 RS/SS Vert C-1 797-Z LOS 11B
EXDL 2EMGR 3SL
4P

69Z28-RS

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Re: Tach accuracy
« Reply #1 on: September 03, 2021, 08:45:26 PM »
GM installed factory tachs have NEVER been accurate...  which is why so many S-W tachs get installed when the driver cares...
09C 69Z28-RS, 72 B 720 cowl console rosewood tint
69 Corvette, '60 Corvette, '72 Corvette
90 ZR1 red/red #246, 90 ZR1 white/gray #2466
72 El Camino, '55-'56-'57 Nomads, '55-'57 B/A Sedan

CantRepeat

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Re: Tach accuracy
« Reply #2 on: September 04, 2021, 10:07:59 AM »
I can understand a couple hundred RPMs but but spinning up to 7k when it's probably closer to 4500 seems a bit much.
-Tim
67 RS/SS Vert C-1 797-Z LOS 11B
EXDL 2EMGR 3SL
4P

69Z28-RS

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Re: Tach accuracy
« Reply #3 on: September 04, 2021, 11:37:30 AM »
they are bad about 'overshoot' in acceleration (revving), but are probably pretty accurate unless the rpms are changing rapidly... (ie. when driving at a constant speed in a gear)
09C 69Z28-RS, 72 B 720 cowl console rosewood tint
69 Corvette, '60 Corvette, '72 Corvette
90 ZR1 red/red #246, 90 ZR1 white/gray #2466
72 El Camino, '55-'56-'57 Nomads, '55-'57 B/A Sedan

Mike S

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Re: Tach accuracy
« Reply #4 on: September 04, 2021, 02:40:55 PM »
 Are you still using points or an aftermarket electronic ignition module? 
My 67 tach displays approximately 150 RPM's higher than what the engine is running at but its movement is smooth at all RPM's and using points.
Recently I had intermittent stalling and when it was running between stalls the tach would be bouncing and way off in RPM's vs. motor speed. I eventually found the cause to be a loose post nut on the coil that went to the points.

Mike

67 04B LOS SS/RS L35 Hardtop - Original w/UOIT
67 05B NOR SS/RS L35 Convertible - Restored

Stingr69

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Re: Tach accuracy
« Reply #5 on: September 05, 2021, 02:33:21 PM »
You can have the tach repaired.  I sent mine to "Gary the Tachman" and it has worked fine for decades.   www.tachman.com

CantRepeat

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Re: Tach accuracy
« Reply #6 on: September 06, 2021, 08:08:31 PM »
Are you still using points or an aftermarket electronic ignition module? 
My 67 tach displays approximately 150 RPM's higher than what the engine is running at but its movement is smooth at all RPM's and using points.
Recently I had intermittent stalling and when it was running between stalls the tach would be bouncing and way off in RPM's vs. motor speed. I eventually found the cause to be a loose post nut on the coil that went to the points.

Mike

On the advise of a post here I switch to this breakerless ignition.

https://www.lectriclimited.com/breakerless-se-electronic-ignition-conversion

Seems to work rather well.
-Tim
67 RS/SS Vert C-1 797-Z LOS 11B
EXDL 2EMGR 3SL
4P

CantRepeat

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Re: Tach accuracy
« Reply #7 on: September 06, 2021, 08:08:59 PM »
You can have the tach repaired.  I sent mine to "Gary the Tachman" and it has worked fine for decades.   www.tachman.com

Thank you for the link. I might just send the tach off over the winter and get it checked out.
-Tim
67 RS/SS Vert C-1 797-Z LOS 11B
EXDL 2EMGR 3SL
4P