Low vacuum at idle, Chevy 305

Stuball · Post by **Stuball** » Fri Jul 08, 2011 7:59 pm

Im a bit stumped by this one.

Got my van fired up after its been sitting a few months, have new set of gauges all hooked up including a vacuum gauge.

Whats confusing is with engine warm, all running smooth etc, its only reading about 5 in.hg? (idle speed is 900rpm) as you open the throttle it increases. Ive had a good check of the pipe to the gauge and doesnt seem to be leaking.

Running a holley 600 double pumper, has 3 what i assume are full vacuum ports, 2 small ones on the front of the base (ive tried both), and large one at the back for brake servo.)

Sure it should be reading nearer 18-20 at idle?

Any thoughts?

Stu

JSF55 · Post by **JSF55** » Fri Jul 08, 2011 11:02 pm

It's quite a "built" engine Stu? have a read here, i tried a few of the ideas trying too set the idle on my edelbrock
http://www.centuryperformance.com/tunin ... g-148.html

Stuball · Post by **Stuball** » Fri Jul 08, 2011 11:26 pm

Cheers for the link, given me a few ideas.

Engine is only in a very mild state of tune.

Im now wondering if both ports
on the front of my carb take a signal from before the butterflies (hence my vacuum increases with rpm). May have to try somewhere else!

Stu

DaveEFI · Post by **DaveEFI** » Sat Jul 09, 2011 7:26 am

If it's a feed for a dizzy vacuum unit, some are arranged so you don't get the true vacuum at idle, to prevent advance at idle. For emission reasons. So to measure the true vacuum at all times the take-off needs to be some way from the butterfly.

ChrisJC · Post by **ChrisJC** » Sat Jul 09, 2011 8:07 am

Stuball · Post by **Stuball** » Sat Jul 09, 2011 12:47 pm

Well i found yet ANOTHER port on the carb (how many does it need !?)

This one seems to be full vacuum. Getting a reading of 18-19 in.hg at idle now. Cheers for the help

STU

mgbv8 · Post by **mgbv8** » Sat Jul 09, 2011 4:26 pm

The port that shows little vac at idle but more vac as the engine rpm rises is the one I connect to the vac modulator on the auto box.

DaveEFI · Post by **DaveEFI** » Sat Jul 09, 2011 4:39 pm

On my SD1 EFI auto, the gearbox vacuum connection is next to the servo one - on the opposite side of the plenum to the throttle.

mgbv8 · Post by **mgbv8** » Sat Jul 09, 2011 8:17 pm

DaveEFI wrote:On my SD1 EFI auto, the gearbox vacuum connection is next to the servo one - on the opposite side of the plenum to the throttle.

Mine is a chevy box so may work different. Should have mentioned it was not a rover box

DaveEFI · Post by **DaveEFI** » Sun Jul 10, 2011 8:18 am

mgbv8 wrote:
DaveEFI wrote:On my SD1 EFI auto, the gearbox vacuum connection is next to the servo one - on the opposite side of the plenum to the throttle.
Mine is a chevy box so may work different. Should have mentioned it was not a rover box

Can't see why. Low vacuum causes the box to change down - would you want this when you release the throttle?

BTW, what box is it? Chevy is GM, and it's a GM box on my SD1.

jefferybond · Post by **jefferybond** » Tue Jul 12, 2011 12:42 pm

5in of Hg at idle is fine! That's quite a good vacuum (lower is more vacuum, 0in Hg = perfect vacuum), and means that the engine is idling efficiently with the throttles almost shut.

If you have a wild cam, the efficiency at idle is very poor, so you need a larger throttle opening and therefore have *less* vacuum (higher number).

You mentioned 18-20in Hg. That is about 2/3rd of atmospheric pressure (30in Hg), so therefore a relatively poor vacuum.

Jeff

sidecar · Post by **sidecar** » Tue Jul 12, 2011 1:07 pm

jefferybond wrote:5in of Hg at idle is fine! That's quite a good vacuum (lower is more vacuum, 0in Hg = perfect vacuum), and means that the engine is idling efficiently with the throttles almost shut.

If you have a wild cam, the efficiency at idle is very poor, so you need a larger throttle opening and therefore have *less* vacuum (higher number).

You mentioned 18-20in Hg. That is about 2/3rd of atmospheric pressure (30in Hg), so therefore a relatively poor vacuum.

Jeff

Er I'm not sure that's right or if it is we are looking at this from opposite sides of one atmosphere. My understanding of such matters is how much the engine could suck a column of mercury round a U bend and up the side of it. (Well really how far the atmospheric pressure could push the mercury round the bend of a U tube due to the vacuum on one side of the tube). Therefore 5 inches of mercury is less vacuum than 10 inches. Absolute vacuum being 29.92 inches.

5 inches is not a very high reading for tickover, it could be due to a badly setup or worn engine but it could also be due to a wild camshaft. My mates 7 litre SBC only pulls around 5 inches at tickover due to a wild cam, as soon as the revs rise above tickover the vacuum goes up. (a lot!)

Most cars that have a vac gauge fitted will read a lower number when the throttle is booted due to a low vacuum and will show a high reading when cruising about with the throttle just cracked open.

EDIT...Just read the whole the thread, the OP has found the missing inches!

DEVONMAN · Post by **DEVONMAN** » Tue Jul 12, 2011 1:26 pm

sidecar wrote:
jefferybond wrote:5in of Hg at idle is fine! That's quite a good vacuum (lower is more vacuum, 0in Hg = perfect vacuum), and means that the engine is idling efficiently with the throttles almost shut.

If you have a wild cam, the efficiency at idle is very poor, so you need a larger throttle opening and therefore have *less* vacuum (higher number).

You mentioned 18-20in Hg. That is about 2/3rd of atmospheric pressure (30in Hg), so therefore a relatively poor vacuum.

Jeff
Er I'm not sure that's right or if it is we are looking at this from opposite sides of one atmosphere. My understanding of such matters is how much the engine could suck a column of mercury round a U bend and up the side of it. (Well really how far the atmospheric pressure could push the mercury round the bend of a U tube due to the vacuum on one side of the tube). Therefore 5 inches of mercury is less vacuum than 10 inches. Absolute vacuum being 29.92 inches.

5 inches is not a very high reading for tickover, it could be due to a badly setup or worn engine but it could also be due to a wild camshaft. My mates 7 litre SBC only pulls around 5 inches at tickover due to a wild cam, as soon as the revs rise above tickover the vacuum goes up. (a lot!)

Most cars that have a vac gauge fitted will read a lower number when the throttle is booted due to a low vacuum and will show a high reading when cruising about with the throttle just cracked open.

EDIT...Just read the whole the thread, the OP has found the missing inches!

My vacuum gauge goes to zero inHg at WOT and idles at About 20 inHg at idle.

Cheers Denis