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Discussion Starter · #1 ·
I just put a brand new cap, rotor and wires on my 87 caprice w/ a 305. I did this because a few of my wires got caught up in the headers and got a little too fried.
After doing that the motor still has a little stumble so I decided to time the motor. when looking at the timing mark it is bouncing around so I cant get the timing set correctly. It is set relatively around zero but its bouncing around too much to get it right. Motor is running OK but could be a little better.
What causes the mark to bounce around like this?

Thanks
 

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Checking timing chain wear is pretty simple.
Remove your distributor cap and turn motor over by hand until your timing mark on the motor pulley is aligned with timing mark on timing cover.
Now turn the motor by hand in opposite direction while watching the rotor on the distributor. Once you see the rotor begin to move look at timing mark to see how many degrees it took to begin to move. If my memory serves me correctly anything 5 degrees or more indicates a stretched worn chain.
 

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aka Duke of URL
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Once you establish the chain is within wear limits, watch the indicator with the vacuum advance disconnected (approx. 550 RPM to keep mechanical advance from cutting in). If it is steady (no large RPM fluctuations), then the mechanical advance portion of the dist. is OK. Reconnect vacuum advance. This should appear fairly steady at idle also. If fluctuates, suspect unsteady manifold vacuum (trace source). Try to move distributor shaft from side to side. If movement is noted, suspect bad shaft bushings.
 

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Also you need to check the wiring to the distributer, if your ground is not good it can cause the timing to jump, Double check it , clean the ground and put a star washer on it just to be sure.

30dee :thumbup:
 

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my dad had a standard 4 pin HEI firing his '72 C10's 350, bought the HEI from a car show, brown box unlabled for $100. dropped it in and the timing was very irratic with the light, bounce all over, eventully still timing the engine ( after 1hr of run time or so as my dad " power times " his engine by driving it up our hill which is approx 2k' and a steep angle, he adjusts timing until he gets the fastest up the hill in 2nd which is around 50 MPH ) we were in the driveway adjusting the timing at that time, so it was no big deal, he bought a new modual from the parts store and put it in, fired right back up and the timing was dead on. evidently the modual was bad.
 

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aka Duke of URL
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cutsupreme said:
There is no vacuum advance on this distributor
:pain: DOH!

I need to keep my mouth shut on GM. If fully electronic, I would suspect the ign. module, stator, or maybe even the ECM. I even had an OLDS once that had a tooth damaged at the stator.

There has to be a diagnostic sequence in the appropritae year shop manual.
 

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aka Duke of URL
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As I remember... :pain: ... (and I used to work on these things)

Some had a wire disconnect like FORD and some you had to connect two pins (by a jumper wire or special key in the diagnostic port. Usually, the wire disconnect was along the right firewall in the engine harness.
 

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Tan wire w/black tracer. Must be unplugged b4 you start the car. Also, KULTULZ, you were correct on checking the dizzy, these type of GM HEI's has issues with the reluctor magnets cracking, causing an erratic/intermittant/random missfire, or sometimes even no start. After haveing lots through the door, I learned to look at this right after I checked plugs, wires, cap and rotor. I suspect this, along with worn bushings, is the culprit.
 

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cutsupreme said:
I just put a brand new cap, rotor and wires on my 87 caprice w/ a 305. I did this because a few of my wires got caught up in the headers and got a little too fried.
After doing that the motor still has a little stumble so I decided to time the motor. when looking at the timing mark it is bouncing around so I cant get the timing set correctly. It is set relatively around zero but its bouncing around too much to get it right. Motor is running OK but could be a little better.
What causes the mark to bounce around like this?

Thanks
Did you ever check the timing gears/chain?
How many miles?
 

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Discussion Starter · #20 ·
I bought a new distributor and installed it today.

The timing mark is still bouncing around.

I did the timing chain test and it has about 3-4 degrees of play.
Is this enough to be a worn chain??

Thanks
 
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