Vacum tuning clarification

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Matt Peetz

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Mar 6, 2011
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Location
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My Car
71 Mach1
Ok guys, i could use your expertise. The past weekend i was doing some tinkering in the engine bay and found a bad gasket between my carb and intake. I thought the car was running good but seemed to lack some giddy up. I fixed the gasket and had to re tune the carb and idle rpm. Now i can spin the tires but i still think i can get it to run better. Or i should say i want to recheck everything from the timing to tuning to make sure all is ok.

So i did some research on the net about tuning and vacum gauge kept coming up. I went to the auto store and picked one up and i also got a digital timing gun that gives me rpm's also.

So i am asking if you could give me a run down for dummies on how to use the vacum gauge for tuning along with timing and how to read the vacum gauge.

I attached a pic of my carb set up as i am not sure on which hose i need to hook the vac gauge up to.

I have one that goes to the pcv valve, one to the front of the distributor, and one to the top of the distributor.

So help a dummy out to make sure he is getting the most out of his motor.

Oh, i have an edelbrock 1406 carb.

1zowart.jpg


 
One problem I see with you vac set up is that you have a dual vac distributor (advance and retard) which you have hooked up to your carb vac. Basically you are providing vac to both circuits at the same time. There should be a vac tree mounted in the water jacket. When cold the vac is routed to retard the dist. When it warms up, vac is sent to advance the dist. Look at this diagram, it will give you an idea of how it should look.

If you don't have the vac tree, it's not really necessary. Just plug the retard port (top port) on the dist. Plug one of the carb ports you are using. One is timed vac, the other is constant vac. Others will ring in on which is better to use. I use the constant vac.

71%20351C%204v%20with%20dvcv.gif


As for timing by vacuum. I was told to adjust the timing until you have the best vac at idle. Does it work? I don't know.

 
run the mixture screws in all the way , do not crank down on them just snug and then turn out 2 full turns each Hook up your vacuum guage to the intake manifold port of the carb (sucks at idle).

Set you rpm around 500 - 550 rpms ( too high of an idle and you bypass the idle circuit ) then adjust the mixture screws to obtain the highest vacuum reading on the gauge , then readjust idle to normal . that should take care of it

 
Similar process with the timing, to get the max vacuum at idle. There is a very good animated vacuum diagram over on

SBFTech.com, that illustrates your overall engine health, depending on how the vacuum guage reacts, very handy stuff to know about.

 
Rick

The setup is the way i bought it. I haven't added anything. I saw the drawing you posted and that is why i posted a pic of my set up. It seems odd that i have the two vacs hooked up. I don't have that "tree" also.

It seems that the PO added the carb and intake and didn't really know what to do as just me finding the gasket between my carb spacer and intake ripped, then i fixed it and started car up and it ran like crap so i played with the rpms and mixture screws and got it running better again. But it has me checking everything now as i want to make sure it is running as best as she can.

Thunder,

The hose on the right of carb is the full time vac port. That is the one i want to use for the gauge correct?

What affect is taking place with the full time vac port hooked up to the dizzy?

When i start messing with the 2 mixture screws will i have to adjust the idle speed at all during the process?

 
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run the mixture screws in all the way , do not crank down on them just snug and then turn out 2 full turns each Hook up your vacuum guage to the intake manifold port of the carb (sucks at idle).

Set you rpm around 500 - 550 rpms ( too high of an idle and you bypass the idle circuit ) then adjust the mixture screws to obtain the highest vacuum reading on the gauge , then readjust idle to normal . that should take care of it
Man I wish mine would idle 500-550:D

 
Man I wish mine would idle 500-550:D
Mine will climb a 5% grade in 1st at 500 rpm, like a tractor.

mike
You could pull that boat. Roy on the other hand will be waiting for ya at the top of the hill!! Be sure to pad the bill accordingly. :p

 
Thunder,

The hose on the right of carb is the full time vac port. That is the one i want to use for the gauge correct?

When i start messing with the 2 mixture screws will i have to adjust the idle speed at all during the process?

yea that is the one i used to set mine with , you don't want to adjust the idle until you get the highest vacuum reading on gauge if it will not stay running at 500 or so rpm's adjust to lowest idle that it will stay running and adjust mixture screws . then readjust to normal idle. Also you do not want full vacuum at idle you want the timed vacuum port on dist. ( no vacuum at idle , but vacuum when throttle opens ) if you have vacuum at idle timing is advanced fully at idle speed

 
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Using full vacuum on single diaphragm vacuum advances does give full advance at idle, but also provides three other functions. One, reduces advance when accelerating to reduce the potential for pinging. Two, provides full advance when decelerating to help prevent unburned fuel from entering the exhaust and causing the after-burn popping. Three, when cruising vacuum is higher providing more advance and better fuel economy.

 
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