Causes for Timing shifting on SBC 305.

[Motor City Impala]

I have a bone stock 1987 Caprice with a 305. I have tired almost everything the service manual spells out for adjusting timing. what is really happening is that with the EST unplugged from the distributor and the car in park....I have checked timing and noticed it does not stay at 0 Deg. I am not touching the throttle and I have all vacuum ports plugged. The distributor is locked down and yet the balancer keeps shifiting from 0 to 8 to 4 to 10 to 6 within a split second. What would cause such a rapid shift in timing? I am under the assumption that this is what is causing my Spark Knock.

Any help would be greatly appreciated. At this point I am going to accept a spun outer ring on the balancer or too much slack in the timing chain. And just because the idle speed moves from 580 to 620 in my opinion is not enough to warrant this kind of timing shift.



Todd

[astroracer]

How many miles on that old dog? Sounds like a loose timing chain to me... Probably lost all but a few of the nylon teeth.
Mark

[MrQuick]

since it changes I'd rule out the balance and timing chain gear...unless the balancer is super lose. I don't see it. Timing gears they tend to stay due to rotation.

Is this the small or the large cap? I pull your distributor and check the gear for wear, corrosion in the cap, carbon tracking, the ignition pick up for deterioration from getting oil soaked, a broken advance spring or sticking weight. If your wires are old you can get spark scatter that way too.

How is the idle? choppy?

[Yoda4561]

Typical causes are timing chains with excessive stretch/slop, broken timing gear, mechanical advance springs too light/worn distributor, the harmonic balancer outer ring slipping on a rubber/steel balancer. If you set timing on a bad balancer it would explain being way off on the timing. Also be sure that the timing light isn't picking up any stray signal from other plug wires.

[Motor City Impala]

Being a bone stock 305, Idle is very smooth. Small cap Hei / Divorced coil. I was using number one cyl for spark timing. The Distributor is the GM ESC style with no advance weights. All advance is Computer controlled.

I was using a Mac Adjustable Timing light with a default setting of 10 deg. Since I was uncertain of how to work with it even after reading the manual I went to my old timing light and did not observe the shift in timing. My assumption is the the Mac light is much more sensative to slight shifts than my old timing light. However I have noticed a very pronounced vibration coming from the front accessory drive. It is very noticeable with the a/c on. So noticeable even that I get stares from other drivers. I is very bad at idle in drive; gets worse at initial acceleration and then smooths out and goes away. It is real bad on decel. It does this at idle in park as well.

I am thinking that the timing shift and the vibration are tied together, and it is most likely the balancer. I have tightened EVERY fastener on the front of the engine 2-3 times now. Balancer bolt and balancer pulley bolts are all tight. I can not see any cracks in the pulleys. I also pulled the belts off the a/c comp and p/s pump to elminate tham as possible suspects. The only belt I have not taken off is the Alt. belt. I highly doubt any of those contributed to the timing shift though.

[MrQuick]

how is the balancer? is it wobbling? can you spin the outer ring? I've seen a broken flywheel play funkyness with timing. Your saying the reading with the light on it is bouncing around right? edit, if it went away with your other light i dont think you have a problem.

but the vibration, if you think its a drive take all the belts off and run it. what motor mounts are you running?

[Motor City Impala]

Stock 25 year old engine mounts. I havent tried to spin the outer ring. I have already had the p/s and a/c belts off along with the fan blade/clutch. Guess I need to go all the way and take the alt belt off too. I think its getting worse.

[astroracer]

MrQuick may have hit on it. Check your motor and trans mounts. If one or more is split or broken that will cause some vibration.

[Motor City Impala]

Pulled all the belts off. No noticable heavy vibration...then again I have it on car ramps. Havent checked timing yet. Tried to spin the balancer outer ring... it wont move. It does not appear to wobble. The rubber in the ring is split all over and several very noticable cracks in the rubber. I am assuming the spilt/cracked rubber is justifiable cause for replacement of the balancer. I am assuming the worn rubber is allowing a rubber band like effect on the outer ring. Hence why I see a minute amount of timing variance with the one light over the other.

[LS6 Tommy]

Some dial back timing lights respond funny on MSD systems, but I've never heard of it happening on a stock HEI, so the MAC light may just be bad. If the elasotmeric ring on the balancer is bad but you can't move the inertia ring by hand, the timing wouldn't move around at a steady idle. It would be completely different, but steady, each time you check it after revving the engine. The balancer should go, regardless. Other than the previously mentionted distributor wear issues, it could also be bad distributor/cam gear or too much cam walk from old age.

Tommy