Vacuum lines for '69 Firebird P350
Stephen "KNT2000"
I just had my '83 T/A's 305 CrossFire engine rebuilt. When i got it
back, it had the same problems that you are experencing. I changed ever
possiable thing!!!??? I even went as far as to change the ECM
computer!! Still, nothing.... The car just didn't want to down shift,
always want to try to excellerat in the current gear. Finnaly i had the
Timing "bumped" up to the farthest point to where it would not "ping" -
you want to watch that you don't set it too high...
Now, the car runs perfect - no more hesitation and has a quicker
responces when "flooring" it....
Let me know what happens
Stephen
Mark Canning
David S Chesler wrote:
> My '69 Firebird developed a hesitation when I hit the accelerator.
> (It's already geared for high speed, not acceleration, so
> this is most embarassing at red lights.) The 3-speed automatic
> was just rebuilt, and one problem is that it's not kicking down
> the way it ought to.
> And it could use some more tuneup (but the carburetor was worked
> on last year; my mechanic just adjusted timing and such and things
> are better.)
> My mechanic was suggesting that my vacuum lines were messed up.
> In particular, he was unhappy that the distributor vacuum advance
> was getting ported vacuum instead of manifold vacuum (or did
> he say vice versa.)
> This is a Pontiac 350cid V8, generally as it came from the
> factory, rebuilt about 13 years ago. (Although the water
> pump inlet may match the Chevrolet 350 configuration.)
>
> 1) Any suggestions for what might be causing that hesitation?
>
> 2) We couldn't find a vacuum line diagram in the shop manual.
> Anybody have one, or know where to get one?
>
> The car has a transplant power brake unit, and an aftermarket
> vacuum gauge. The mechanic just ripped off all the lines that
> didn't seem to be doing anything (including two that came off
> the manifold and then joined together), plugged loose ends,
> connected the distributor as he saw fit, and says he got the
> heat riser valve working. The brakes and gauge still work.
> --
Assuming the hesitation is vacuum related and not carb related (accelerator
pump circuit) then:
Distributer vacuum should be ported not manifold.
If you've attached the distributer to a manifold vacuum source then the
distributer will be at full vacuum advance at idle (not total - since total
is base + vacuum + mechanical) probably somewhere around 20 degrees BTDC
assuming vacuum advance adds 16 to 18 degrees. When you jump on it,
manifold vacuum goes away (at least decreases significantly) and your
distributer retards back toward the base timing level. If the car was
timed without pluging the vacuum advance and the timing was set at base
(somewhere around 4 degrees BTDC) when you jump on it timing drops back to
16 degrees ATDC which will cause a big hesitation.
Plug the vacuum advance line to the distributer, time the car to base
levels and find a ported vacuum source to reconnect the distributer to.
Kickdown of the transmission is controlled by the accelerator linkage,
normal upshifting and down shifting is controlled by a combination of the
vacuum modulator and the governor assembly in the transmission. The vacuum
connection to the transmission modulator should be from a manifold source.
If a 69 Firebird trans linkage is like a 69 Camaro's then there is a cable
attached to the bottom of the carb throttle arm below the accelerator pedal
linkage. There is a bracket between the carb and the firewall with
elongated holes the the kickdown cable is attached to the bracket. Loosen
the bolts enough to move the linkage, slid the linkage back on the bracket
to make the trans. kick down sooner, move it forward to make it kick down
later. Tighten the bolts and then check to make sure the throttle opens all
the way (if you move the kickdown linkage too far back it can keep the
throttle from opening all the way)
Thers not a lot of vacuum lines on a 69 Firebird. (I'm familar with a 69
Camaro so take this for what its worth)
Power brake booster - Manifold Vacuum
Transmission Modulator - Manifold Vacuum
Vacuum Gage - Manifold Vacuum
Distributer Advance - Ported Vacuum
PCV Valve - Manifold Vacuum (don't connect PCV and Brake booster to same
connection since the flow through the PCV system can reduce the amount of
vacuum to the booster)
From the factory with a (Rochester carb) there is one fitting on the
manifold behind the carb that supplies the brake booster and the trans
modulator. The PCV line goes into a fitting on the front center of the
card between the idle screws. The distributer is connected to a small port
on the drivers side front of the carb outboard of the idle screw. The
temperature modulator on the air cleaner attaches to a connection on the
passenger side of the carb (mines a 2" long 1/8" tube) aligned with the
primary venturies. (this connection may be thermostatically controlled -
manifold vacuum when cold, no vacuum when hot).
Your vacuum gage should probably be connected with a "T" in the modulator
line since neither device uses any vacuum and one won't affect the other.
Hope some of this makes sense. Good luck
Mark Canning
1969 Indy Pace Car Convertible
RPO Z11