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Old 10-28-2004, 08:49 AM
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SBC Timing gremlin

SBC Help
I have a problem with my timing moving as rpm's increase.

At idle I have 36 degrees, when I increase the rpm to 4000 my timing reduces to 28 degrees. I have no vacuum or mechanical advance, both are locked out.

As the engine returns to idle, the timing slowly creeps back up to 36 degrees.

The distributor is and HEI, I have tried a unilite as well and the same thing happens.

My question.. Does this sound like a strentch timing chain? Although its a new motor, less than 2000 miles on it, I believe that is the problem. I included the engine specs.

383
cam 260 duration @ .50 625 lift
10 to 1 compression
220 cc 2.02/1.6 valves

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Old 10-28-2004, 09:34 AM
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Could be, you can check by pulling the cap off, and turning the crank by hand. Watch to see if there is a significant delay in rotor reaction. Turn the crank in both directions, to make sure you have seen all the slack that may be in the chain. With the timing retarding under load, I'd say that timing chain is a likely suspect.
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Old 10-28-2004, 09:45 AM
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I would say timing chain as well.
But one other thing. This happened to me.
The gear at bottom of the distributor shaft was loose.

Before tearing into the timing chain. Pull the dist and check the gear for wear and also for movement. The roll pin should hold the gear tightly to the shaft with no movement.
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Old 10-28-2004, 09:55 AM
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Do you still have the module in the distributer? If so then they have a certain amount of dwell built into them and without the mechanical advance the timing will retard at higher RPM.
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Old 10-28-2004, 10:59 AM
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Questions..Answered

Okay, I have an aftermarket hei, and tried multiple modules. The shaft and gear are new, with little to no wear.

I plan to pull the timing cover this weekend and rotate the engine back and forth to observe the "slack. How much is normal?

Is it strange that the retard only happens if I rev the engine fast or at hight rpms?
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Old 10-28-2004, 11:08 AM
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Read my preveious post.

If you want to keep your timing locked out then I would recommend installing an aftermarket ign box that will fire off the magnetic pickup in your dist.
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Old 10-28-2004, 12:00 PM
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You said the distributor gear was new, but look at it closely. In a 389 Pontiac I had installed a brand new Engle cam, Accel (point) distributor, bronze gear, all brand new. After about two months the thing would idle great but as soon as I attempted any power runs it would fall on it's face, kick, buck and raise cane.

I found the roll pin had broken and the gear wasn't secured. It felt tight looked tight, the roll pin ends stayed in the gear so I assumed it was good. I grabbed the gear and turned the rotor. It even had some resistance turning but a simple roll pin was the problem. I haven't got a clue what caused it to shear to this day.

Larry
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Old 10-28-2004, 02:04 PM
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I have had the gear on and off, the pin is good. I just am baffeled to the timing retarding by 10-12 degrees. It is a big solid roller, rpm up to about 7600, but still baffles me. But after replacing the distributor with others and seeing the same problem I am getting more comfortable with pulling the front of the motor apart.
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