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Author
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Topic: Ignition timing question?? Why does it bounce?
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70coupe
Gearhead Posts: 483
From: Toronto,Ontario,Canada
Registered: Aug 2000
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posted 05-20-2002 05:23 PM
  
I was wondering what might cause my timing mark to bounce once its gets over 4000 with a timing light? It stays nice and steady all the way to 4000 then it starts to jump up and down.
I pulled the distributor to check the gear and it seems to be fine. I also looked at the cam gear and it looks fine. Any ideas?
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MAV
Gearhead Posts: 237
From: Macon,Georgia, U.S.A.
Registered: Mar 2002
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posted 05-20-2002 09:01 PM
I`m guessing a slack timing chain
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Dusty Kiser
Gearhead Posts: 184
From: Bethel,Oh USA
Registered: Mar 2002
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posted 05-20-2002 09:34 PM
quote:
Originally posted by 70coupe:
I was wondering what might cause my timing mark to bounce once its gets over 4000 with a timing light? It stays nice and steady all the way to 4000 then it starts to jump up and down.
I pulled the distributor to check the gear and it seems to be fine. I also looked at the cam gear and it looks fine. Any ideas?
70Coupe. Timing mark moving around can be caused by many things. How good is the timing light and {inductive?}pickup? Play due to wear in distributor shaft. Not just side play but end play. Exdessive end play can cause the dist drive gear to walk up and down the cam gear and cause notable change in timing. A defective cap, rotor or wires. For example, your plug wire insulation is barely adequate at idle. As rpm increases and cylinder pressures and fuel mixture changes resistance can go up and cause voltage leak through the insulation. An inductive light pickup can pick up this weaker signal and fire the light trigger. As already mentioned loose timing chain or excessive end play in the cam. Mechanical advance in a stock distributor can get errattic with wear. Rubber failing in the harmonic balancer can contribute to timing mark moving around especially if engine has reached a severe harmonic stage. I could go on, but you get the picture. Anything remotely related to the mechanical or electrical functions of your ignition or test equipment can cause a jumpy timing mark!
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Fordwiser
Gearhead Posts: 442
From: Metamora, Illinois
Registered: Dec 2000
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posted 05-20-2002 09:37 PM
If heard if you have the really lite spings in your centrifical advance they can't control the weights as well and can cause the timing to bounce around some. This is one of the reasons people lock the distributor.
Roger
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70coupe
Gearhead Posts: 483
From: Toronto,Ontario,Canada
Registered: Aug 2000
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posted 05-20-2002 11:19 PM
The distributor that I'm currently using is a Mallory comp 9000. I know some people dont like them but I've never really had a problem with it. I dont want this to turn into a MSD vs Mallory war. I'm just looking for facts which is what you guys have given me. I really appreciate the posts and will start by checking everything over and maybe re-installing my gear drive. I know the dampner is good because I found top dead center and marked it with a white line from side to side and the line is still together. Is this problem something that I should really worry about if the car is running well and the cam and distributor gear look fine?
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