Timming jumps around
Thread Starter
Advanced
Joined: Jul 2007
Posts: 63
Likes: 1
From: Newnan Ga.
Timming jumps around
My 75 with HEI ignition has timing issues I think. With timing light hooked up, the mark on the vibration dampener moves around quit a bit. Should it be very steady at what ever my timing is set to? If so, would this be caused by maybe something not firing good or properly? Thanks in advance for any help.
Pro
Joined: May 2016
Posts: 708
Likes: 150
From: Minnesota
If it is jumping around, assume wear in the distributor mechanism, or more likely in the timing chain. How many miles? Usually it's the chain, and running long or hard on a worn chain set will eventually break it, then larger issues.
Pro
Joined: May 2016
Posts: 708
Likes: 150
From: Minnesota
Are you plugging off the vacuum advance to time it, and it is not steady at idle or at a steady rpm? Not uncommon to see the mark move a degree one way and the other, but if it's jumping up and down a lot I might assume timing chain wear.
Le Mans Master
Joined: Aug 2017
Posts: 8,336
Likes: 2,809
From: Cool Northern Michigan
I too have the dreaded light jumping around like a Mexican bean. Rumor has it that its not your IGN but rather the timing light. More specifically, the pickup probe can not receive a good signal from the plugwire. I use a fairly cheap $50 light w/ dial-back and just ignore the occasional false firings.
And timing chain / dizzy are only two yrs old. (<1,500 miles)
And timing chain / dizzy are only two yrs old. (<1,500 miles)
Last edited by HeadsU.P.; Aug 26, 2018 at 11:12 AM.
Thread Starter
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Joined: Jul 2007
Posts: 63
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From: Newnan Ga.
Not sure of exact miles. Let's say alot. Speedo was unhooked when I got car 16 years ago. Probably at least 150,000 on this engine. This car has been in the family since 1986. I know the distributor and chain have not been replaced.
Thread Starter
Advanced
Joined: Jul 2007
Posts: 63
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From: Newnan Ga.
I tried it with and without vacuum advance hooked up. When I say it moves around I mean it moves alot just for a second. Mostly stays pretty still but then BAM! Big jump. Way more than a degree.
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Dr. Detroit
Joined: Mar 2012
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From: New Braunfels Texas
Sloppy timing chain or the springs on the advance weights under the distributor rotor have come off, or are weak.....
Very unlikely a timing light could make it appear as it is jumping around. Try your light on another car or borrow a light from a buddy. I do not use dial back lights unless I have to as the dial back feature in my experience can make for incorrect readings as it is a delay controlled by a rheostat. I use a crummy $35 Craftsman light that I bought twenty years ago and it is accurate......it has been back to back checked with a Blue Point strobe.......simple.
Anyway.....pull the cap and move the rotor by hand....it should move a bit with some tension and snap right back......if it does not, you have found the problem.
Good luck,
Jebby
Very unlikely a timing light could make it appear as it is jumping around. Try your light on another car or borrow a light from a buddy. I do not use dial back lights unless I have to as the dial back feature in my experience can make for incorrect readings as it is a delay controlled by a rheostat. I use a crummy $35 Craftsman light that I bought twenty years ago and it is accurate......it has been back to back checked with a Blue Point strobe.......simple.
Anyway.....pull the cap and move the rotor by hand....it should move a bit with some tension and snap right back......if it does not, you have found the problem.
Good luck,
Jebby
Race Director
Joined: Apr 2003
Posts: 19,345
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From: Tampa, Florida
Had the same issue. Turned out to be too vertical movement in the shaft. Shimmed it up as tight as spec would allow. Alll good after that!!!!
Last edited by jim2527; Aug 26, 2018 at 08:10 PM.
Drifting
Joined: Aug 2013
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From: perth Aus w.a
2020 Corvette of the Year Finalist (stock)
C3 of Year Winner (stock) 2019
Had same issue on my l68 and it was a worn timing chain ,I agree with forman on a good way to check for wear on your timing set .Mine was getting about 6 degrees of movement .Which meant I could turn the motor 6 degrees before the pointer would move on the distributor .
Le Mans Master
Joined: Mar 2004
Posts: 7,454
Likes: 1,480
From: Little Rock AR
Check for slop in the ignition system first but there can be other causes. See if it still scatters at high RPM or just low RPM. Help you know where to look.
Multi spark ignition will cause many timing lights to act that way even when everything else is working fine. Setting the timing at the top of the advance curve will avoid the problem with multisparks. Not a bad way for a single spark ignition either.
Multi spark ignition will cause many timing lights to act that way even when everything else is working fine. Setting the timing at the top of the advance curve will avoid the problem with multisparks. Not a bad way for a single spark ignition either.
Le Mans Master
Joined: Aug 2017
Posts: 8,336
Likes: 2,809
From: Cool Northern Michigan
Check for slop in the ignition system first but there can be other causes. See if it still scatters at high RPM or just low RPM. Help you know where to look.
Multi spark ignition will cause many timing lights to act that way even when everything else is working fine. Setting the timing at the top of the advance curve will avoid the problem with multisparks. Not a bad way for a single spark ignition either.
Multi spark ignition will cause many timing lights to act that way even when everything else is working fine. Setting the timing at the top of the advance curve will avoid the problem with multisparks. Not a bad way for a single spark ignition either.
Some timing lights don't do well with the 50,000V systems. The gun can not process the sparkplug signal fast enough and relay it to the vibration damper.
Last edited by HeadsU.P.; Aug 27, 2018 at 07:40 PM.
Melting Slicks
Joined: May 2009
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2024 C3 of the Year Finalist - Unmodified
My 69 BB had 56k when I bought it. The nylon coating on the timing gear teeth was mostly in the pan and the chain was flopping around. The lower bushing in the distributor housing was worn and causing erratic timing. Changed the housing and the problem was fixed.