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Joined: Dec 2003
Posts: 2,736
Shop Shark
Thanks for the link to the site John. Very interesting material there that we can all benefit from.

1953 Chevy 5-window 3100
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Engine & Driveline Moderator

If you can't make seventy by an easy road, don't go. ~~ Mark Twain
Joined: Jan 2014
Posts: 86
Shop Shark
I had a 1975 plymouth VALIANT that had a light on the front fender that would lite up when you pushed down on the gas to much (DRIVER-SIDE).I asked the dealer at the time about the lite and his reply was (GET THIS)If you are using to much gas to drive the car it comes on to make you back off on the gas pedal to conserve fuel.That lite drove my wife crazy.She believed if a lite comes on there is something wrong.I cut and taped the wire that went to it.

Joined: Aug 2012
Posts: 568
Shop Shark
understanding the vacuum readings video

"As I lay rubber down the street, I pray for traction I can keep, but if I spin and begin to slide, please dear God protect my sweet ride." -Amen

56 Chevy 3100
Joined: Sep 2008
Posts: 882
Shop Shark
What most of those vacuum gauge tutorials omit is that at idle you might see a 1-2in drop on the guage. Its worse with a 4 cylinder. I was chasing the needle on my L134 in the jeep and it turns out that, at idle, a 4 cylinder will bounce 1-2 points.. is a valve timing issue. 6's and 8s don't bounce much, if at all at idle.

Joined: Feb 2012
Posts: 537
Thanks for the continued ideas. I'm on the road this week. I'll check out the tutorial video and play around on the truck this weekend. I'll keep you posted.


Joined: Dec 2008
Posts: 1,708
The position of the vacuum tap matters.
A tap too close to an individual throttle disc makes the gauge "nervous" at low speed.
The more buffer (plenum) volume between the tap and the disc the steadier the reading will be.
Be sure to get the spark right before making mixture adjustments - more initial advance has a big effect on idle vacuum!

Joined: Sep 2008
Posts: 882
Shop Shark
A lot of the time you don't get a choice. My L134 had two taps by my GMC270 only has one.

The at idle bounce is not an indication of an issue (although a lot of the tutorials will say that a bounce at idle is an issue) but if its only 5-10in drops, not 1-2in ticks.

Taking the motor even slightly off idle stops the bounce and does not interfere with other inductors.

Joined: Feb 2012
Posts: 537
I finally got a chance to play around with the vacuum gauge this weekend. The first thing I did was zero the gauge. I figured out that the rubber cover and plastic face plate on the gauge were removable. There is a small flat head screw on the face of the gauge that can be turned to adjust the needle. Out of the box from HF, mine bottomed out at 10 instead of 0. I saved myself having to buy a new gauge.

I hooked up the gauge to a port on the intake manifold and set about checking the timing. I was using a cheapo HF timing light so it was a bit challenging. I unhooked the vacuum line from the advance on the distributor and plugged the line. I had the timing right where I thought it was good. The problem arose when I hooked the vacuum line back up to the advance. As soon as I got the line even partway on the nipple the engine would instantly die. Hooking the vacuum line to the advance killed the engine every time. I had to call my man Joe H to have him help me figure out the issue. I've got it running pretty good now with the advance hooked back up, but I'm not totally convinced I solved my issue.

I got a long piece of vacuum line from my FLAPS and ran it through the firewall to the cab. I hung my vacuum gauge from my GPS mount and was able to read it while driving. It made for some very interesting driving. It's amazing how little throttle will make the vacuum drop. I found that if I could hold the vacuum above 10 inches while driving around the local area it seemed pretty economical. I then took it out on the interstate for a test drive. The first thing I discovered was how non-aerodynamic my Burb is. There was about a 20 mph wind that was blowing west to east parallel to the highway. I tried to keep the vacuum at or above 10 inches while driving. On the flats or downhill it would go as high as 15-17. I had to baby it going uphill to keep it above 10. Running with the wind I was able to maintain 60-65mph consistently without going below 10. Running into the wind it was more like 50-55 without busting 10. It was amazing how much difference the headwind made.

I welcome any thoughts/suggestions on my vacuum advance-timing issue and any interpretations on my driving gauge readings. Should I try a scientific experiment on timing settings. I'm think of finding a long hill that I can fun up and down. If I pick a speed and document my vacuum readings can I play with the timing in order to try and get the vacuum as high as possible at the given speed on the same hill??


Joined: Sep 2008
Posts: 882
Shop Shark
Mechanical timing is only going to get you so close... and as things wear the timing will shift yet again so I would really aim for the best average of all speeds.

The advance setting that works on flat ground may ping the motor up a hill... that's why the vacuum advance helps so much... on flat ground, high vacuum, lots of advance. Up a hill low vacuum, a little less advance.

You can get a 'fuel economy' vacuum gauge and drive that.. its pretty much what you want to do but in a nice color coded gauge.

You *could* look at the old Model T' and A's... they had manual spark advance so you could dial in the motor.... but with vacuum advance and mechanical/spring loaded dist weights much of the 'easy stuff' is already taken care of.

Joined: Feb 2012
Posts: 537

Hooah. Thanks. I don't want to make this super complicated. I'll look for a fuel economy vacuum gauge.


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