Topic: '75 tachometer with an MSD ignition
in Forum: C3 Electrical, Body and Interior
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Greetings, I have a question regarding the 1975 tach. Ever since I have had the car the tach will read around 4k when I start the car in the am, even while idling. After running for a little while it seems to settle down and read correctly. I am wondering if it could be the circuit board behind the gauge itself.
I was not too concerned about it until yesterday when I replaced the oem distributor with an MSD and a 6A box. I now have to wire the tach to the 6a using an MSD tach adapter and want to know the tach is reading correctly.
Looking for some opinions/experiences before I pull the dash apart again
Thanks in advance,
Pat
Daytona Yellow '75 350 Muncie 4 spd

'73 454 on the engine stand.

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Vette(s): #1-1974 L-48 4spd Cp Med Red Metallic/Black deluxe int w/AC/tilt/tele./p/w-p/b/
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#2-1985 Bright Red/Carmine Cp.L-98/auto
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What does the tach do when you turn the key off? They normally go up to somewhere around 4000rpm with the key off, then it should come down to 0 with the key on, engine off. If yours is kinda stuck at 4 until it runs for a while, it may well be the tach meter itself...it may have some dirt, or something sticky on the guts. Normally, I would suggest you try bypassing the tach filter first, but since you've already swapped the distributor, that won't help. Assuming your MSD has a tach output that is to be connected to the factory tach wire from the original distributor, I would say bypass the filter anyway, and run the tach output from the MSD directly to the tach input wire by the dist.
If the tach is still funky. then you get to rip into the dash. Check the wire from the dist to the tach board for continuity. Then check for battery voltage, and ground at the board. Flip the board over and look for any burnt, of corroded areas. If all of that checks out, then it's likely your tach itself is bad.


|UPDATED|11/23/2015 11:45:27 AM (AZT)|/UPDATED|
If the tach is still funky. then you get to rip into the dash. Check the wire from the dist to the tach board for continuity. Then check for battery voltage, and ground at the board. Flip the board over and look for any burnt, of corroded areas. If all of that checks out, then it's likely your tach itself is bad.



|UPDATED|11/23/2015 11:45:27 AM (AZT)|/UPDATED|
Joel Adams
C3VR Lifetime Member #56
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"Money can't buy happiness -- but somehow it's more comforting to cry in a CORVETTE than in a Kia"
Thanks for the reply Joel, thats what I ended up doing. I removed the tach filter and went directly from the MSD tach output to the brown wire after the filter. I still have to get into the dash to replace the board, but it now works as it did before. Basically in the morning it will float at somewhere below 0 at idle to around 3.5 - 4k while running.. at any rpm. Then once I have driven about 20 miles it seems to read correctly, so I am assuming the board could use replacing.
On a similar note, I read a post you responded to in the past regarding setting timing using a vacuum gauge while adjusting the dist/carb. When I replaced the distributor I was able to drop it into the same spot as the original and managed to get it positioned so that the timing was the same as the original, around 8 - 9 *. It started right up and runs 200% better than before, but I wanted to get the carb/dist dialed in better. In the past I would have set the timing by back setting to about 36* at 3k rpm and then dial the carb air/fuel and idle mixture separately. I was wondering if you had any other details on using a vacuum gauge and what your changing on your carb.. Idle screws?.. Btw, I have a simple Edlebrock 1406 that is rather new.
Thanks for your help,
Pat
Daytona Yellow '75 350 Muncie 4 spd

'73 454 on the engine stand.


Moderator
Duncanville, TX - USA
Joined: 11/8/2003
Posts: 20224
Vette(s): #1-1974 L-48 4spd Cp Med Red Metallic/Black deluxe int w/AC/tilt/tele./p/w-p/b/
Am-Fm/map light National/Regional/Chapter NCRS "Top Flight"
#2-1985 Bright Red/Carmine Cp.L-98/auto
Member: NCRS, NCRS Texas, Corvette Legends of Texas
apesa63 said:I was wondering if you had any other details on using a vacuum gauge and what your changing on your carb.. Idle screws?.. Btw, I have a simple Edlebrock 1406 that is rather new.
Thanks for your help,
Pat
You want to adjust both the idle speed, AND the idle mixture screws on the carb. Normally, the idle will change as you adjust the mixture screws, depending on how far off they are to begin with. Start with the carb adjustments, and then go to moving the dist around for the highest vac reading. Then...go back to the carb....then back to the dist....rinse and repeat.

hth

Joel Adams
C3VR Lifetime Member #56
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"Money can't buy happiness -- but somehow it's more comforting to cry in a CORVETTE than in a Kia"
As an update to the original post regarding the tachometer and the MSD Ignition. I ended up just running the tach wire (Brown) directly to the MSD tach output and it works fine. No tach adapter is needed. Just wanted to add this in case someone else has the same question.
Daytona Yellow '75 350 Muncie 4 spd

'73 454 on the engine stand.

in Forum: C3 Electrical, Body and Interior
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