Tach light coming on at idle.

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SGanny

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I have a 1972 Chevy Nova with a 454 BBC. I had problems with my gas and air mixer in the carb figured out that problem. I don't know if it's just a loose wire but when I started it up, at 500 RPMs to 600 RPMs the tack light is not on. (Idle). When I give it some gas only up to 1000 2000 3000 RPMs the tack light comes on.

Any thoughts
 
Is this an in dash factory tachometer?
 
SGanny said:
No, it's a add on. On the column.
Click to expand...
If wired correctly it should only be on when the dash lights are turned on. Either way, it sounds like a bad connection at the power source or inside the tachometer.
Not to clear what you mean by gas and air mixer. What carburetor?
Is the problem at idle or all engine speeds?
 
zora said:
If wired correctly it should only be on when the dash lights are turned on. Either way, it sounds like a bad connection at the power source or inside the tachometer.
Not to clear what you mean by gas and air mixer. What carburetor?
Is the problem at idle or all engine speeds?
Click to expand...
The problem is at all idle speed. I was messing around with the carb and was wondering if it has something to do with the tack light being on. I just bought the car and didn't touch anything but the air/fuel mixture screws and the tack light came on.. could be a loose connection. If it idles.really low at 500-600 rpms it doesn't come on. Only after 600 witch it shouldn't come on till 7000
 
SGanny said:
The problem is at all idle speed. I was messing around with the carb and was wondering if it has something to do with the tack light being on. I just bought the car and didn't touch anything but the air/fuel mixture screws and the tack light came on.. could be a loose connection. If it idles.really low at 500-600 rpms it doesn't come on. Only after 600 witch it shouldn't come on till 7000
Click to expand...
What manufacturer carburetor GM Quadrajet, Holley, Edelbrock?
Not clear what Tach light you mean? For night time illumination or some other purpose since you typed 7,000 rpm.
 
It's a Holley carb.
When your RPMs are way too high and you're going to redline, a light comes on next to the tachometer that light is illuminated. From 500 to 600 RPMs the light is not on after 600 RPMs. The light stays on constantly but I'm not at redline 7000 or 8000 RPMs.
 
Do you have the instructions to see how to set the redline light?
Unless you have a true race engine, not street able, you should limit your redline to 6,000 to 6,500.
Which Holley? The number is stamped on the front of the choke housing.
Do you know what your idle vacuum is?
 
No, I don't have the instructions to set the red line, I just purchased the car last month and I have not done anything to it except for adjusting the idle screw and fuel air mixer on the Holley carb. I can get you the Holly stamp number when I get home. I don't have it off hand and I'm not sure what my idle vacuum is.
 
Does the tachometer have the manufacturers name on it?
 
I'll get that information too when I get home. Won't be for a couple hours. Thanks for all your help
 
Hollie carb is 83870- 1537
Vacuum is at 10 Hg
RPM is auto gage by auto meter
 
Just noticed under the RPM gauge on the back side there's a little knob that I just twisted. It looks to be the set screw which was turned way off for some reason. Light is now off
 
SGanny said:
Just noticed under the RPM gauge on the back side there's a little knob that I just twisted. It looks to be the set screw which was turned way off for some reason. Light is now off
Click to expand...
If there is also a shift pointer on the dial, with the engine off, key on run position, adjust the pointer to 6,000 rpm.
 
SGanny said:
Hollie carb is 83870- 1537
Vacuum is at 10 Hg
RPM is auto gage by auto meter
Click to expand...
That's a healthy size carburetor, 870 CFM with 4 corner idle mixture.
What transmission do you have, automatic or manual?
 
Thank you for all your help Zora, I really appreciate being new to the hot rod world..
Steve
 
With the engine off, turn all 4 mixture screws gently clockwise till they fully seated. Then turn all 4 screws counterclockwise 2 turns. Start the engine and let it warm up. Set the parking brake, I assume it works. Chock the front of all 4 wheels to prevent the car from moving or have someone sit in the car ready to apply the brakes but they should not have their foot on the pedal while adjusting the screws. Place the car in Drive. Set the idle speed screw on driver's side to 750 to 800 rpm depending on how rough the idle is. Start with the drivers side front screw and turn it a 1/8 turn clockwise to lean the mixture. Watch what the tach shows for idle. If it goes up, lower it. Then go to passenger side front screw, then passenger side rear screw, then drivers side rear screw. Each time you turn any screw recheck the speed. Continue this until a screw causes a drop in idle, if it does go back 1/8 and go to next screw to see if it also causes a drop.
 
Thanks again, I'll give this a go tomorrow. Appreciate all the help
 
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