Engine guys I need some help

[Old School]

I have a 74 charger with a 400. It is very sick and I can't figure out what is wrong. It ran great for about 400 miles
then all of a sudden it just started running like crap. It doesn't like to start for one, it used to just pop right off.
It also likes to backfire out the carb. I check the compression and all seems fine, I think. They are all at 130 to 120.
I checked the timing nothing wrong there. I can't seem to find any vacuum leaks. I did hook up a vacuum gauge
to it and got some bad results. The needle was like a jack rabbit back and forth between 9 and 11 lbs. all the time.
I have the screw for the idle in all the way just to keep the car running so I can check some things. Oh ya and run rich,
once you get it started you got about five minutes till your eyes start to burn. What is my problem?

Thanks

[RECHRGD]

What kind of carb?  If it's a Holley, could be a blown power valve.  Bob

[firefighter3931]

OS, it sounds like your original problem was/is a vacuum leak. If the car has power brakes it could be the booster...get the vac guage on it and pinch the hose to see if the idle vac increases. Check all the hoses agiain. If the engine is stock then 9-11in vac is way low.

Timng ; make sure the weights aren't sticking....and you have enough base timing. Where is it set now at idle and where does it stop advancing and how many total degrees of advance ? When you get the timing light on it blip the throttle and make sure the timing retards back to the base setting as the engine slows down.

Carb ; as Bob said it could be a blown PV (holley) or it could just be rich because you're now idleing on the main circuit. If the idle speed exceeds 1000-1100 rpm...you are now using the carb's main circuit instead of the idle circuit to keep it running.


Often what happens is while trying to fix one problem, you will create others.



Ron

[Old School]

The carb is a ThermoQuad. I have swithed it out with another one that I have and the result was the same.
The engine is stock except for a larger cam. 268/280 and .477/.480.

[craigandlynda]

always start w compression test....once satisfied, review all ignition parts, particulary cap rotor and wires...make sure wires aren't shorting out anywhere, and no carbon tracking in cap...if all there is ok, and this happens w either carb, look for vacuum leak..plug off anything that uses vacuum, see how it runs...then add vacuum loads one at a atime...this'll get you started...