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Hiccups of my tachometer

1201 Views 5 Replies 2 Participants Last post by  [email protected]
Hello Everybody,
My name is Adrian, and I bought a 1976, L82 Vette 6 months ago.
Overall I am really happy with my new toy, love the way it drives and love the way it looks.
The only issue I had with her since the beginning, is when I push the throttle full down, at 4500Rpm and above she got serious hiccups, but I thought I could live with that as I am not planning on racing it :)

I am currently encountering a problem, let me explain you the thing:
Few weeks ago, in a mechanic shop after having upgraded all the braking system, the guy told me that the timing was a bit off. We adjusted it at the time by ear. The car was running fine, was drinking a bit more than usual but was ok. I realized in the following weeks that it was really really hard for me to start the car when she was hot.
So I bought a timing light and this morning went ahead and tried to improve my timing. First of all at the start the timing was off by 35-40 degrees anticlock wise. I did reduce it to the 12 degrees recommended in the specs of the car, and when I reconnected the vacuum hose the timing did move. So far in my head this morning I was pretty happy.
I took her for a test drive and I realized that my idle is not stable: the tach jumps from 1000 to 1400; and below 3000 the tach vibrates ±200 RPM. Above 3000 rpm no more weird movement of the needle.

Anybody got any suggestions ?
Thanks a lot.
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first, never time an engine by ear..theres not 1 in a 1000 mechanics that can and be close to accurate.. always use a light.. second, is it just the tach needle bouncing or is the engine idle floating around?...and your poor running above 4500rpm, I would inspect or replace cap ,rotor,plugs and wires and check for vacuum leaks ... and welcome to SV
Thanks a lot for trying to help me out.
I think the engine is floating around as well as the tachometer jumping. I have double checked my idling this morning and timing. I still have the issue of bad idling, and this morning when I pushed the throttle down she backfired at me once from the left side.
The plugs, wires, and cap are new and are working fine.
Here's a video, hope that may provide further information.
did you also replace the ign module, with the rest ?.....a backfire can be several possibilities... a timing issue, mixed up plug wires, a vacuum leak causing lean condition ,a cracked dist cap, a bent/burnt valve or broken spring, or a worn cam exhaust lobe... leave vac adv disconnected, see if it still does it, then pull valve covers ,inspect springs, pushrods, rockers ...turn engine to verify all rockers are lifting fully .
No the ignition module did not get replaced at the time.
Could that be the problem ?
could be.. modules are can run fine for 20 minutes and then run like crap, turn it off and restart ,run good again...whacky...... if its old ,I would change it anyway , to eliminate it as the problem , and avoid trouble later on.. they are not expensive.
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