<VV> vacuum modulator

[James Davis]

Surging is usually caused by a lean carburetor condition (vacuum leak 
or too low float level) or by too much vacuum advance.  Disconnect 
the vacuum advance hose at the distributor and plug it (golf tee 
works well) .  Drive the car normal with the vacuum hose 
disconnected.  If the surging goes away it is too much vacuum advance 
and Matt N. has the fix for that.  If that doesn't fix the problem 
,  it is time to check for vacuum leaks  and the carbs float level

The Corvair PG modulator only controls the harshness (abruptness) of 
the shifts.  It has no effect on when (MPH) the transmission 
shifts.  If the diaphragm in the modulator has failed, it can let the 
engine suck fluid out of the transmission.  When this happens, there 
is a noticeable drop in PG fluid level and usually noticeable engine 
exhaust smoke.  Also the transmission will shift harshly at part 
throttle.  A vacuum leak anywhere from the carb balance tube to the 
modulator could cause the modulator not to work properly.

You say there are no leaks under the car.  Put a piece of clean 
cardboard under the transmission.  You might weight it with a 
brick.  Start the engine and place the PG in drive (holding it firmly 
with the brakes).   Rev the engine slightly (1,000 to 1,200 rpm) for 
no more than 30 seconds (don't want to overheat the 
transmission).  Shut  the car down and look at the cardboard.  If you 
have a small red puddle under the drivers side of the transmission, 
you have a bad governor "O"-ring.  The leak will only show up when 
the PG is in gear.
Jim Davis


At 10:08 AM 10/23/2006, Dave Schneider wrote:
>The car: 1965 Monza with 110HP/Powerglide  The problem: engine surge 
>at constant speed and transmission fluid loss (transmission also has 
>delayed shift when fluid gets low), no leaks under car, no noticible 
>smoking. Could a bad or going bad vacuum modulator be the cause of 
>this? Vacuum hoses are all new.
>   Thanks for any help.
>   Dave
>
>
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