Vacuum Gauge Readings.

[Bill B.]

I just hooked up a vacuum gauge to my 327/300 and I'm seeing something I'm not sure about. I tee'd into the vacuum hose at the distributor so I would have vacuum advance and I'm reading 10in.hg. I know it's supposed to be closer to 17in. I unhooked the hose from the distributor but did not plug it and got 8in. hg. I then plugged the hose and saw a bit over 17in. I went ahead and adjusted the carb.(Carter AFB) and had readings of 18in. hg. I went through the whole procedure again and the readings were the same with vacuum hooked up to the distributor. My question is: should adjustments with a vacuum gauge be performed with the vacuum line pulled and plugged at the distributor or do I have a problem? I looked in the archieves and couldn't find any info pertaining to my particular problem so any advice would be appreciated. Thank you in advance.

Bill

[Douglas L.]

Re: Vacuum Gauge Readings.

Might want to check for a ruptured diaphragm in the vacuum advance canister.

[Duke W.]

Re: Vacuum Gauge Readings.

The vacuum gage should be teed into the VAC signal line. If you remove the VAC signal line and connect it to the vacuum gage, which leaves the VAC disconnected you have changed the engine configuration and your readings are invalid.

Vacuum readings depend on engine idle speed, and without stating the speed at which you are measuring vacuum, your readings are meaningless.

As suggested, you should use a vacuum pump to pump down the VAC to at least 20" and make sure it holds. Also check that the link starts moving at the vacuum value specified in your shop manual or AMA specs and that it pulls smoothly to the maxium amount as specified. It could be sticky or leaking.

A misbehaving VAC will cause idle speed/stability problems as will an incorrect replacement such as the B1, which does not meet the Two-Inch Rule for any Corvette engine.

Duke

[Bill B.]

Re: Vacuum Gauge Readings.

Thanks for the replies Douglas and Duke. I found the vacuum can is bad. I suppose I should of troubleshot a bit more and I would of found the problem myself. The help I've received on this forum is great and thank you again.

[Bill B.]

Re: Vacuum Gauge Readings.

006.jpgI just changed the vacuum can and the new can fixed my problem. The can I pulled out has this marking on it and I'm wondering if anyone can tell me what application this is for. Thank You.

[Duke W.]

Re: Vacuum Gauge Readings.

The stamped ID does not correspond to the OE or current replacement nomenclature, so I can't tell the source or find the specs. It looks like it's been there for a long time.

Assuming your 327/300 has a manual transmission, the best current replacement would be stamped B22. It's manufactured by Standard Motor Products and is available in about half a dozen brands, all of which have their own part number. The NAPA part number is VC1802.

Duke

[Jim D.]

[Bill B.]

Re: Vacuum Gauge Readings.

I installed the B22 yesterday and I'm back in business. Thanks for the help.

[John D.]

Re: Vacuum Gauge Readings.

I meantioned this long ago. One time I bought 2-3 dozen SR replacement VAs from Chevy. At the time they were the replacement numbers for the 201. Inside the boxes were all B1. I was too dumb to know in the early 80's that a B1 vacuum advance was/ is only good for maybe a paper weight. The spring is heavy enough for a front coil spring. JD