Thread: How do you measure vacuum?

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When I rebuilt my carb (670 street avenger, engine 60 over 454, cam RV) I adjusted the idle mixture screws with a vacuum cage plugged to the intake port for the transmission. I adjusted the screws according to the best vacuum/highest reading at idle. Was that the correct way or did I luck out "and find and acorn"? Truck is running fine. I saw something the other day posted saying the adjust with the car in gear. Gage I was using was one that is also used to bleed brakes.

I always use a vacuum guage just as you did. I suppose there are other ways but thats how I learned.

You'll be fine with doing it as you did. Putting it in gear, setting the e-brake, hopping out to blip the throttle is a fine way to see your vehicle go winging through the back of the garage - maybe with you hanging on to the throttle linkage for dear life.

Usually, smooth idle, 15-21 inches of vacuum with a stock to mild cam, and none to very little needle movement at idle is what you are looking for. An ~4" gauge is best, but a small, 2-1/16" will usually get you close enough

Without vacuum gage, you can adjust for the leanest, highest idle RPM and back off 1/4 turn on both sides. That will fatten it up enough so as to idle good in drive. Definitely do not attempt to run adjustments in gear, someone was pulling your leg there.

Thanks guys. Thought I was right!

I just picked up a nice guage off eBay. It measures pressure and vacuum, came with adapters, I think I got it for under $20, and it already paid for itself. It showed me the fuel pressure was fine, 7psi, and manifold vacuum was low. I sprayed carb cleaner around the manifold tll I found the leak.