66RBS

Playing with my car today adjusting timing

When adjusting timing at idle the mark on the damper occasionally jumps up about an inch then returns. I was thinking my advance weights may be deploying early

However when checking total timing at 2500 or 3000 rpm I get the same twitch or jump. At these rpms I would suspect my timing is fully advanced.

Bad timing light? It is and old dial back

Randy

pop23235

How is your timing chain and gears?

66RBS

ZZ4 with 10k miles.

pop23235

Unless you are getting induced voltage in adjoining spark plug wires, the only time I've seen erratic timing is with bad chain and sprockets.

66RBS

Thanks pop23235

I suppose bad chain and sprocket is possible in a engine with 10k miles
Car drives fine and I don’t hear any changes in rpm when it jumps momentarily

Would the induced voltage have to be in #1 wire since that is where the timing light pickup is? I see #5 and #7 which fire sequentially are neatly parallel in the wiring loom.

I’ll borrow another timing light

Randy

vettepoor

Dave Tracy

With the dwell gauge, does the needle jump? I replaced my distributor bushings and the needle is a stead as a rock whereas before there was a several degree wobble.

66RBS

MSD tach Drive distributor 10k miles. I pulled the distributor to change vac advance. Installed new MSD cap and rotor. End play maybe .010

Frankie the Fink

Try another timing light - most are induction based with circuitry that turns the induced pulse into a strobe and some of the older ones can be a bit twitchy. I use an Actron digital dial back light from AutoZone and its very good... Its not a Sun Machine but I can map out advance curves fairly decently with it.

Try moving the inductive pickup - near the plug tower at the distributor perhaps. Stray inductance getting into the device will cause issues.

You can also rubber band your centrifugal weights fully closed and recheck things and eliminate those... A timing chain issue is highly unlikely IMO.

pop23235

Highly unlikely at 10K, just my first and most frequent solution. I’d work on re routing wires.

Donald #31176

Assuming the above solution does not change try pulling up on the rotor . If you detect play then remove the distributor and measure actual end play and shim between 2 thousands and 10 thousands. Any speed shop or Long Island Corvette carries shims that you will need.

66RBS

Update. I believe it to be the timing light. Thanks to all who responded. I tried an old Penske/Sears non inductive timing light. Remember putting a spring on the #1 spark plug?

Timing marks were steady.

No vertical slop in distributor


Plug wires are 8.5 MM silicone which I hope minimize crossfire

My dialback/inductive timing light has a 4 pin plug in connector at the handle that may be sloppy.

Randy

pop23235

Glad you found it. I also have one of those old Penske TL! When all else fails, I pull it out too.

Don't think silicon wires will prevent induction/crossfire. Wires would need shielding ie metal grounded.

seb67

Crossfires (shorts) are possible in the secondary side of the ignition system, but not inductive firing of a spark plug by merely being adjacent to another plug wire.

Steve