Join Date: Nov 2005
Location: Kitchener, Ontario Canada
Vacuum readings- 1969 LM1 350...normal?
Tuning up my bone stock 69 Impala which yes is not a Camaro, but has the same (very same) LM1 255 horse 350 with turbo 350 trans so though I would seek info here.
At idle, and at all rpms with no load, vacuum readings are all very good for this motor and rock solid. But as soon as you drop it in gear, the needle will vibrate but no more than about 1hg. It is constant vibration, not intermittent or rythmic. If I punch it to WOT it will drop to about 2hg, but vibration is worse, about 3-4hg in the otherwise same vibration on the highway. (Again, only in drive, not in park with no load)
I've read several really good tech articles about vacuum signal and what it may indicate, and all my readings seem to check out, but nothing I've read talks about signals under light, moderate or full load at WOT. Is this vibration normal or indicating another issue?
BTW to check this I used a vacuum guage connected
to manifold vacuum on a 3 foot hose so I could temporarily mount it.on my cowl and observe the readings under various driving conditions.
I'd be happy to hear anyone's thoughts on this. The carb, like the engine is a 7029202 quadrajet, 100% to factory specs inside and out (jets, rods, etc) timing is 12degrees advance at idle and 36 degrees at about 2900 rpm where it peggs out, measured with vacuum advance disconnected of course. It seems to run, idle and accelerate great, just can't figure out the vibration. Motor has a new timing Chain and gears, but otherwise never rebuilt and 130000 miles on it
Last edited by 69chevyguy; Jul 5th, 15 at 06:32 PM.