James W. Frey

Hi Oldsmobile Gurus!
Need a little help here and I apologize if this is a dumb question or has been answered ad nauseum elsewhere in the forum, but I couldn't find anything.
As titled, I've got a 1976 Oldsmobile Cutlass Supreme with a 350 engine and th350 transmission. I've only had the car for about 6 months and am doing everything I can to keep it alive. She runs very well normally, or seems to, from what I can tell.
I've given her a tuneup, new plugs, wires, the works, but I've always been leery of the vacuum system on it, as the previous owner told me he rebuilt the carb and basically went over the upper portion of the engine. As far as I can tell, it's still numbers matching, but I have no idea of the history of the car. There seems to be vac, as I can feel it, but I finally got around to buying a tester so I can read the vac more accurately.
With the vac gauge installed (teed in) between the DTVS and the vacuum advance on the distributor (yellow mark in image),

I'm only getting about 6.5 inches of mercury. Factory manual says that's about right, but seems really low, plus when I increase the throttle, it never changes. I figured I should be getting about 15 inches, right? I've visually searched around the top of the engine area and don't see any leaks or open ports.
Apologies if any of this isn't making sense, I'm trying to get everything down while I remember it...
Any advice is much appreciated, please let me know if I've omitted any details.
james

jaunty75

I don't think you want to measure the vacuum there. You want to measure manifold vacuum as close to the carb as possible. Put a tee in the line right where the "manifold vacuum" arrow points to in the center of the diagram. At idle in park, the vacuum should be 15 to 22 inches of mercury. See the diagram below. When you step on the gas, the vacuum level drops because you're opening throttle to let in air and fuel.

Schurkey

First Guess: Ruptured vacuum advance diaphragm leaking vacuum--reduces vacuum reading.
Second Guess: Faulty thermo-vacuum switch (TVS)
Third Guess: Vacuum leaks somewhere else in the vacuum supply to the TVS. I'd be especially suspicious of the VRV (vacuum regulator valve???) OTOH, maybe that's exactly the purpose of the VRV. Time for you to do more research in the factory service manual.

droldsmorland

The distributor thermal vacuum switch works off coolant temperature. It controls the vacuum advance by opening/closing ports in the thermal switch at different temps.
It advances the ignition when the coolant temp is warm and applies, even more, advance when it's high, 220*f to further cool the engine. This prevents detonation at higher temps.

If you're getting a good raw vac signal (18-20Hg) at the intake or at the port marked manifold vacuum on the carb, that's a good signal. You can tweak the timing and air fuel screws on the Q-Jet to obtain the highest vac signal possible.

Set base timing to 10-12-14* BTDC (with vac can unhook and pluged off). Then hook the advance hose back up using a direct vacuum source off the intake or carb.

The advance canister should bring in 8-10 more degrees at idle, Again, readjust the timing an a/f screws to get the highest possible vac reading.
Road test it. If you hear spark knock back timing off a few degrees. I guarantee it will run better with direct manifold vac to the distributor

The pic is from my 76 service manual.

Vintage Chief

As per jaunty75 - Post #2. Measure "manifold vacuum" first to ensure you have sufficient vacuum at the source.

James W. Frey

Thank you all for the excellent suggestions and support, especially Jaunty75! Great diagram, too, I wish that was included in my FSM!
I will have to wait until this weekend to check all the points you've suggested, but will keep you posted!

many thanks again!
james

jaunty75

It's not in any factory service manual. Just google "engine vacuum readings" or something like that and you should get plenty of hits.