Thread: eratic vacuum

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Hi all, i did my valves over again, this time at a 1/2 turn 1/4 turn seemed to loose, but when i hook up a vacuum gauge on my carbs ( full vacuum) the reading is bouncing between 14-18 and engine stumbles. Any suggestions?

What cam do you have? From your last thread we never got that info. It could be useful to know that for diagnosing the problem.

mild 204/214 edelbrock 420 lift

have you checked for a vacuum leak?

yea no vacuum leaks, the needle is a little bouncy and goes from 14 -18 and the idel is not to steady motor has slight stumble, i think i still hear some clacking faintly from the valves could i need to go more than the 1/2 turn?

If you're running stock steel rockers, you may have wear causing the tip of the rocker to not contact the valve stem squarely causing a light click. Some roller rockers have a tendency to make a bit of ticking noise. What are you running for rockers?

stamped steel, this is my first time adjusting valves, i folowed the procedure to the letter, i would have thought they were perfect but now when its running i can hear them clattr slightly, and i don't get the vacuum reading.

if i bring the rpm up to 1500 -1800 then the motor smoothes out, (not sure of the vacuum reading at that rpm)

If tere is even a slight bit of wear between the rocker tip and/or the valve stem tip, you will get some slight ticking. Tightening the valves won't make it go away just make the run condition worse. As far as the vacuum surge, one quick check is to take your hand while the engine is running and slight lower it over the primary side of the carb and creat a choke style condition. Raise and lower your hand to restrict the air flow ever so slightly and watch the vacuum gauge. A lean idle circuit can cause this stumble and fluctuating vacuum reading.

so by using your hand when you place it over the primarys the vacuum should fall and return when you remove you r hand ? if this happens the problem is whith the idel mixture?

Most likely. The idle circuits in all carbs are a big part of part throttle to 1800-2000 rpm range. Give it a check.

Thanks, iv'e had enough of this car for one night but tomorow it will be the first thing i do.. thanks again for your help..