Jumpy timing marks
#1
Pro
Thread Starter
Jumpy timing marks
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
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
#5
Pro
Thread Starter
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
#6
Burning Brakes
Are you running a newer distributor? I have heard that too much end play and/or worn bushings in the distributor can cause bouncing of timing mark.
#8
Pro
Thread Starter
MSD tach Drive distributor 10k miles. I pulled the distributor to change vac advance. Installed new MSD cap and rotor. End play maybe .010
#9
Team Owner
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.
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.
Last edited by Frankie the Fink; 05-26-2018 at 06:07 AM.
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66RBS (05-26-2018)
#10
Safety Car
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
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
Highly unlikely at 10K, just my first and most frequent solution. I’d work on re routing wires.
Last edited by pop23235; 05-26-2018 at 08:42 PM.
#11
Melting Slicks
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.
#12
Pro
Thread Starter
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
#13
Safety Car
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.
Don't think silicon wires will prevent induction/crossfire. Wires would need shielding ie metal grounded.
#14
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
Steve
Last edited by seb67; 05-26-2018 at 09:48 PM.