Hello, Today I was checking a Ladies Corvette, 1971, 350, Quadrajet, Auto. The Lady said she had the Carb. replaced and its giving her trouble. I found a Vaccum port uncovered and pluged it, lowered her idle from 1200 rpm to 600 rpm. Set the idle mix screws at 3 turns out and with a Vaccum gage the manifold negitive pressure was 16in Hg but when I opened the throtle the Vaccum goes to 20in Hg and I know, it should go to 0 (no Vaccum). Any ideas? Thanks, Curtis

Just getting in to the car and determined that I have a vacuum problem. When I unplug the service line from the boss on the intake the engine runs perfect. Knowing that the service line has bad component(s) somewhere, where is the best place to start looking? vacuum accumulator tank, headlight motors, wiper door motor? All these items do function, albeit the headlights open very slowly and one at a time.

Also, what is opinion on installing an additional vacuum accumulator in the system?

Bill Folse
'70 LS5 Coupe

Domino said: Just getting in to the car and determined that I have a vacuum problem. When I unplug the service line from the boss on the intake the engine runs perfect. Knowing that the service line has bad component(s) somewhere, where is the best place to start looking? vacuum accumulator tank, headlight motors, wiper door motor? All these items do function, albeit the headlights open very slowly and one at a time. Also, what is opinion on installing an additional vacuum accumulator in the system? Bill Folse '70 LS5 Coupe

i dont think adding another canister is going to help..

there isnt much room and the stock one is pretty big..

there are 5 things that will run off your manifold vacuum

wiper door
headlights
brake booster
ventilation and heater controls
and vacuum modulator (if you have an automatic)

i ssume when you say you disconnect and it runs better that you are plugging the manifold..

i would disconnect each system one by one to find the leak..

but you should look at the vacuum diagram and make sure you have each component if your check vavle is working it shouldnt be lights, ventilation, or wiper door..

but check its function..

http://www.vetteprojects.com/cthulhu/misc/vacuumlines1.jpg

this diagram will help

Once you use Ben's diagram and you are certain there are no leaks, you may find three turns on the mixture screws is not the best setting. Turn them in or out a 1/4 turn at a time. Peak the vacuum and rpm. Then turn them in as far as possible without dropping vacuum or rpm. But be sure there are not other leaks first. Timing must also be set first.

kstyer said: Once you use Ben's diagram and you are certain there are no leaks, you may find three turns on the mixture screws is not the best setting. Turn them in or out a 1/4 turn at a time. Peak the vacuum and rpm. Then turn them in as far as possible without dropping vacuum or rpm. But be sure there are not other leaks first. Timing must also be set first.

i usually use 1.5 turns to start out with and work from there..

and when i do it .. i usually set my initial timing by peak vacuum, readjusting the idle up or down as necessary to set it where i want it (usually a hundred rpms under the factory default if possible) then adjust the idle mix to peak vacuum and readjust idle back to my spot.. then turn idle mix in and out again to try to get it to increase vacuum again. i repeat this until the car is at the correct idle and turning a 1/4 turn in or out decreases vacuum.

then goto next screw..

then once that is all done I readjust timing for peak vacuum.

using vacuum readings to set ign timing can get you in trouble as most cases your ign timing can be overly advanced at idle and it gives a good high reading----but it's much to far advanced for loaded conditions---set the timing with a light first and then go after the mixture and idle speed------marty

mwspecht said: using vacuum readings to set ign timing can get you in trouble as most cases your ign timing can be overly advanced at idle and it gives a good high reading----but it's much to far advanced for loaded conditions---set the timing with a light first and then go after the mixture and idle speed------ marty

this is the case if you havent tuned your timing curve..

ideally you have your idle timing peaked. and you use springs of different strengths to change the rest of your timing curve..

this gives you the best off the line performance and through curve tuning you can keep your timing near the peak all the way to the top..

if your not looking to tune your performance to the max.. then by all means.. set your timing by picking some arbitrary value and setting it with a light..

my 350/350 69 vette had factory timing of 8btdc if I remember right.. and I ran it at around 15 btdc.. with just a lightly stronger set of ramp rate springs.

Sounds like you have the vacuum gage on a port thats either reading timed spark or venturi vacuum. But yes it should drop.

Connect to a manifold vacuum and you will see the drop.

Hi Curtis
These guys are right.There are two kinds of engine vacuum.You are correct in thinking that manifold vacuum drops at wide open throtle but at the same time,as engine speed increases,more air is flowing,at a higher velocity thru the carb venturi creating a low pressure area that pulls more gas out of the float bowl so the engine can accelerate
.The vacuum advance on your distributor also uses this increasing vacuum to advance the timing to further aid the acceleration of engine speed.Hope this helps.
Steve