Low Vacuum

[gophersnake]

For as long as I've had this truck -- going on 27 years -- the manifold vacuum has been way less than I'd expect. For several years now I've kept a vacuum gauge permanently connected to the engine and strapped to the steering column where I could read it even while driving. A typical reading, idling or cruising, is 5 or 6 inches. The only times I see a vacuum as high as 15 inches are when I'm decelerating from highway speed or going downhill.

I'm still using the same Bendix Stromberg WWC carburetor, though it must have at least its 8th set of gaskets by now. The only weirdness I'm aware of in the carb is that the throttle discs aren't a very close fit in the bores and (lately) that the throttle shaft has a bit of slop where it fits through the throttle body. I doubt that either of these would be enough to account for the low vacuum. I check the power valve and its vacuum piston at every rebuild but there never seems to be anything wrong with either one.

The ignition timing is usually right on spec. Now that the vacuum advance in my distributor has quit working, I've even bumped the timing up a few degrees to compensate -- at least till I have to replace the distributor for some other reason.

One obvious possibility would be that the valve timing is off. I checked for that 11 years ago, though, and even replaced the timing chain. It made no difference at all in the vacuum readings.

The truck has been on the sluggish side for years. It'll maintain freeway speeds fine on the level but tends to slow down on uphill stretches, even at full throttle. Gas mileage is pathetic. I used to get 9 or 10 mpg when I was doing more than half of my driving on the freeway. Now that it's almost all on city streets, I count on no more than 5 mpg.

I've wondered occasionally if a different model (or even make) of carburetor might not work better than the one I have. So far, though, I've never come across a replacement for the WWC. I don't even know where to look.

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'68 GMC C2500 with 351 V6, Turbo Hydramatic 400A.

My other car is an '80 Honda Civic.

[mnunn454]

If you have power brakes, how do you get it stopped?

Seriously,

Manifold Gasket Leak?

Cam timing way off?

Crushed tailpipe?

Worn out cam lobes?

Huge vacuum leak in a hose somewhere like the vacuum brake booster?

Keep us posted. I'm curious now.

[Sport/Truck]

While it’s running spray some carb cleaner around your carb, hoses, intake manifold etc. Wherever there’s a leak the idle will change when the cleaner enters the opening.
I tend to think your carb is the culprit. Is it possible to bolt a q-jet on to see if anything changes? (I don’t know what fits your WWC setup.)
s/t

[orangeuavol]

Where are you taking the reading from? (where is the hose connected?) Without the vac. advance you are only getting crank and mech. which is nowhere near what you would have with the vac hooked up.

[gophersnake]

(Combined questions from mnunn454 and orangeuavol)

If you have power brakes, how do you get it stopped?

It's not a problem when I'm coasting to a stop because that's when the vacuum is highest. First thing in the morning, maneuvering out of the parking lot with the choke on, I do have to push harder on the pedal.

Manifold Gasket Leak?

Unlikely. The vacuum was low both before and after I had the manifold off. When I eventually try Sport/Truck's suggestion (the spray carb cleaner) I'll make sure to include the manifold joints.

Cam timing way off?

When I checked the timing chain, the marks lined up exactly like they were supposed to. The chain itself felt fine but I tried a new one just for luck -- and of course carefully lined up the marks. The vacuum was no better afterwards.

Of course if someone at GM happened to screw up and, say, put the mark in the wrong place on the sprocket, it might take me quite a while to figure it out.

Crushed tailpipe?

It's been that way through three tailpipes and at least three mufflers. The present tailpipe isn't crushed, nor were the others. One of those tailpipes may have been a little undersized, 1.5" instead of 2", but when it was replaced the vacuum didn't seem to change any.

Worn out cam lobes?

They didn't look particularly worn when I got a look at them (1996). The vacuum had been low for many years and miles by then.

Huge vacuum leak in a hose somewhere like the vacuum brake booster?

I had the spark plugs out today. Except for one that was fouled they all looked pretty normal, from tan to gray.

Some 20 years ago one of the PCV valves blew out. That was a huge vacuum leak. I drove home mostly with the accelerator pump.

Where are you taking the reading from? (where is the hose connected?)

There's a fitting screwed into the back of the manifold. The brake booster and transmission vacuum hoses are connected to it. There used to be a third hose for some vacuum-operated vent controls. That's where I've had the vacuum gauge connected ever since I got rid of the air conditioning.

Without the vac. advance you are only getting crank and mech. which is nowhere near what you would have with the vac hooked up.

I'm on my second distributor. When the vac advance in the first one was still good, and again when the vac advance in the second was still good, the manifold vacuum was still low.

(I do have the vac advance line plugged, by the way.)

----------------------------
'68 GMC C2500 with 351 V6, Turbo Hydramatic 400A.

My other car is an '80 Honda Civic.

[gophersnake]

Sport/Truck: I don’t know what fits your WWC setup

I have no idea what other carbs have that footprint or could somehow be adapted to it, but here's what it looks like. The rear mounting holes (top of picture) are 3-3/16" OC, the fronts 4-1/2".

Right now, about the only way I could get access to another carb for testing would be to buy it at a wrecking yard. I've considered trading mine in for a rebuilt WWC but I'll probably wait till it gets a little worse. Its latest (homemade) gaskets are only about 6 months old.

[68C15]

it kinda sounds to me like burned valves (leaking compression).

keep in mind a 351 isnt excactly the most fuel efficient engine out there. top that off with you probably have some steep gears in back.

you could pull the heads and do a valve job but then it will likely start smoking past the rings.

what it all boils down to is its time to spend some money, big money.

[68speedalert]

You haven't said how it idles.

Experience tells me that if it is idling with that low of vacuum, it should be like CRAP!

Have you verified it with another gauge? Is that in park or in gear?

Does covering the top of the carb (choking engine) make it run smoother and with higher vacuum?

What is the engine compression?

[stsalvage]

well i'm starting to run into that problem my self in my 1968 Gmc 3 door suburban and my 1965 3/4 ton work truck with a 400K miles on it the suburban has a lack or vacum but it has a miled to high cam in it and its got 12 pounds my 1965 3/4 ton has blow by and compression in 2 cyclenders are at 100 and one is at 90 psi

Now latley i've been told the HEI with the big cap if your lucky you can get 150 k in miles out of them then they are wore out

the one in my 1965 3/4 ton has over 400k mules on it after 2 eng swaps the suburban has over 200K miles I've been told to buy a rebuild but they say it will only last me 50-100 miles mabey before i have to ajust the timing diffrently

so the HEI was a nice tool but been told to go back to the old distubitor and run the solid state in it