Low Vacuum - The 1947 - Present Chevrolet & GMC Truck Message Board Network

gophersnake · 07-01-2007, 04:22 AM

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.

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

My other car is an '80 Honda Civic.

07-01-2007, 04:22 AM	#1
gophersnake Registered User Join Date: Jun 2007 Location: California (northern) Posts: 20	Low Vacuum 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. ---------------------------- '68 GMC C2500 with 351 V6, Turbo Hydramatic 400A. My other car is an '80 Honda Civic.



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