Anybody have any idea why a late 70's Q-jet would be getting a full vacuum signal at idle from the vacuum advance port?Last time i had "old faithful" on my car it was doing some weird stuff,and this is the only clue i found.Could it have a crack somewhere or possibly the wrong gasket?

 

Full vacume at the port you hook the advance up to? Wich port are you connecting to?

 

You're talking about the one on the bottom front, pass front corner of the carb sticking out at a slight angle? That's not always a ported vacuum source. GM liked to fiddle around with the functions of the vacuum ports from time to time and from model to model.

QJets are like a box of chocolates. You never know what you gonna git.

I actually trace the ports myself when I'm rebuilding them so I know what each one does or is supposed to do when I put it on the motor.

Also remember that the ported (off-idle) vacuum port will often start supplying vacuum at very slight amounts of throttle- enough that even while on high idle (choke on) you'll often get vacuum at that port.

 

I thought the one at an angle is where you hook up the aircleaner?

 

The port i'm talking about is the one on the driver-side at the front of the carb, about half way up (the one your supposed to use for vacuum advance.)I've used this carb for years with no problem,put a kit in it when it needed it with no problems.I was thinking about running it again because it gets better mileage around town than my holley,but not unless i can figure out why it's doing this.When i ran the carb last,the idle was probably 800-900 rpm,i thought i was having some timing issue,i pulled the hose off the port and the idle stumbled like it leaned out.I stuck the vacuum gauge on and found full manifold vacuum.I tried the vacuum advance on another ported source and got it running decent,but it still didn't act like itself.When starting it cold,you had to keep pumping the throttle for several seconds to keep it running,it never did this before.I thought maybe these problems were connected somehow.Now you've got the whole story.I hate to give up on this carb because i spent alot of time getting it tuned to my engine and it's hard to beat ,i've had an Edelbrock performerand a Holley on my motor,but the q-jet is better at both gas mileage and full throttle power.

 

Thats the port I use for vaccume advance. Heres what you do if you think you have a vaccume leak, if it has a vaccume leak it will Idal ruff, while it is idaling spray the carb down with carb cleaner around the outside, if the idal picks up look where your spraying thats where the leak is. Check to see if your vacume advance is working, sometimes the diaphram can give out, if the diaphram is good, pull the cap off and see if the weights are moving.

 

Wait how long did it set off the engine?