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Old 08-30-2006, 08:03 AM
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72 LT1 Timing Problem

The nylon coated cam gear on my 72 LT1 Vette failed. I replaced the timing gears with a dual sprocket set. I am 98% confident that everything is aligned properly. However, I have been unable to get the timing set to the spec of 4 deg btdc. I have been able to get the engine to run at about 28 deg btdc but when I try to advance/retard the distributor, the engine starts to stall. I checked the vacuum which is only about 2" at idle and about 14-15" if I race the engine. Is the low vacuum due to the bad timing or the bad timing due to the poor vacuum. or is soming else going on? I hate to pull everything apart to get to the timing gears again. Any Ideas what to check next?

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Old 08-30-2006, 10:51 PM
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Have you checked for a big vacuum leak?
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Old 09-04-2006, 10:27 AM
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I went back to the beginning and pulled the timing cover. I remounted the balancer with the cover off and confirmed that the gear marks line up when the balancer mark is at tdc. I also made a timing wheel with 5 deg increments that I mounted to the balancer. I went to the intake valves and confirmed that the push rod starts to move around 35 deg btdc (spec in Chilton's was 36 deg). I also tweaked the valve lash to specs of .024/.030 plus .002 for a cold engine.

At this point, the engine runs smoothest at 20 deg btdc. Idle is 1050 - 1100 rpm (spec is 900). Engine vacuum is vibrating between 13.5 and 14.5" (probably worn valve guides).

If I adjust the distributor a little, I can get to 8 deg btdc (spec is 4 deg). idle is 900 rpm. Vacuum, however, drops to 10.5 to 11.5". If I race the engine vacuum drops to 4" and goes up to 22". When I release thottle vacuum drops to 10" and settles between 10.5 and 11.5.

At this point without chasing more major repairs, should I settle for the smoother idle and higher vacuum and adjust down my idle or should I keep it around the 8 deg btdc with the lower vacuum? Or am I missing something else that is relatively simple?
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