vacuum readings from Smith vacuum gauge

[sujitroy]

I got a Smith's vacuum gauge off a parts car. I use it on the Stag once in a while when my car doesn't seem to idle right. During idle it the needle seems some between drive and idle, but during acceleration, the needle points to decelerate .
What could be going on here?

I've taken the vacuum off the feed to the brake booster and ac/heater tank.

I'm going to check my carbs diaphragms this weekend. What's the typical life expectancy of carb diaphragms?
The last time I had a rough idle situation, the carbs needed balancing.

Sujit

[trunt]

I found these very useful to understand what you should see.

https://www.youtube.com/watch?v=SdlNwm8OHco

https://www.onallcylinders.com/2015/...gine-problems/

[barkerwilliams]

Could you also borrow a second gauge, even one of the cheap hand helds just to get corroboration that the gauge is "about right"?

Alan

[Klaus Schlueter]

Terry, do you have personal experience with the vacuum meter as it is decribed in the you tube film? Sounds very easy to find missettings or mechanical problems.
Klaus

[trunt]

The only experience I have is on my turbo mini, after a head rebuild. It had a boost gauge which also showed vacuum, the engine was stumbling intermittently and when that happened the gauge would pulse rapidly... it put me on to the cause:- sticking valves, the machine shop had set them too tight.

After I found those links I looked and a decent looking gauge like the one in the video is on Amazon for $16 - I ordered it!

its also useful for fuel pressure.., maybe checking the vacuum advance.. I thought it would be a nice addition to my toolbox.

Terry

[jakesmig]

My millstone has a 3.9 RV8 fitted with a 270 cam, Monarch headers and matched ports to the inlet and header manifolds.

Ticking over at 750 rpm the vacuum gauge indicates a reasonably steady 16.5 inches, but on application of slight throttle it increases to a steady 21 inches.

Further application of the throttle produces the anticipated reduction in vacuum pressure as the engine revs rise.

I am not surprised by the 'apparently' low 16.5, that is normal with a sportier cam, but the rise to 21 on slight throttle is puzzling me.

Anyone out there got an explanation?

Cheers, John.

[MandM]

I found this useful

TRS Magazine - Dealing with engine vacuum (therangerstation.com)