<VV> low vacuum findings

[iscdirector]

Ahhh, the Corvair urban myth, no the camshaft does not have enough overlap to put the readings on the vacuum gage into the sick engine range. GM designed the engine to operate with a auto trans and the modulator on a Corvair powerglide needs at least 15 HG of vacuum to operate. This is what I am seeing on many Corvairs. Also was the needle on your gage unsteady? Sort of almost vibrating? I don't like adjusting the rockers with engine running but in this type of case it is the only thing that will do. Hook up your vacuum gage under the car while you adjust the valves and set the rockers at zero lash, just where they will run without clicking and see what happens on the vacuum gage. My bet is it will come up to 16 or 17 and the engine will smooth out. Then move the vacuum gage up top and set your carbs for a normal idle and adjust idle air screws for max vacuum. Feel the exhaust coming out the tailpipe at idle, it should be a steady flow of hot air, no puffing.  Also running
 the timing as far advanced as you can get away with without spark knock will raise vacuum readings some more. Then when you drive your car you will think you have a new one. Less pressure on the gas pedal to sustain cruise speeds, no more annoying thump when the trans downshifts as you come up to a stop and no more thud when you shift from neutral to drive (sound familiar?). Many corvairs have been driven around for years with the engine not producing enough vacuum to operate the modulator and I believe this is what is causing so many pinion gears to come loose on the pinion shaft in powerglide diffs. Also broken e-clips in the valve body and broken band servo cushion springs. Just be sure that you check the modulator to be sure it is good, as many cars run for years with one that is defective with no symptoms other than the ones already described. The issue I am describing is subtle and basically these cars run pretty well but they don,t have the smoothness and
 performance that GM designed into them. I have done this on mine recently and just took it to a local cruise today and I am still in disbelief of how much improvement it has made. Now we just have to discover what is needed to get motors like this to run right with the rockers set the way GM calls for. 

william weissheimer <willweissheimer at sbcglobal.net> wrote:   Hi Dan,
Just wanted to share the findings on my 110pg, 64 carbs, 12 deg. BTDC, looks like 
sbc lifters set at 1/4 turn:
11" hg @ 600 rpm in neutral
8" hg @ 500 rpm in drive
Vacuum gauge was attached to balance tube modulator nipple.
Engine runs well off idle, but does idle rough in drive.
I thought the rough idle was the result of not having the carbs set up properly as I'm 
not a Corvair expert. Did do a carb kit and shady tree carb balance with vacuum 
gauge.
Maybe worn guides? 
Valve overlap as stated earlier sounds logical.
Will 
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