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Hello. Welcome to Just Answer. Please allow me to assist you. Please remove the vacuum from the advance unit and road test, then advise. Thanks
If you are getting popping, your cam timing is incorrect. Set the engine to TDC. Pull the cap off and examine where the end of the distributor rotor is. Advise exactly where it is in relation to the #1 cylinder tower inside the distributor cap.
If the distributor is correct, the cam timing is off.
Correct. the crank-to-cam timing via the timing chain is absolutely critical. When the mechanical advance moves the distributor base plate, the spark happens at a different instant in time. What you have, on acceleration, is the spark occurring before the piston reaches TDC on the firing stroke. The intake valves are being allowed to move to the closed position as the piston starts upward. If the cam timing is off 1 tooth, the vehicle will start. As the spark occurs (in this case, at the right time via distributor placement, but, at the wrong time with regard to valve placement), the mixture fires in the cylinder. If the intake valves are not fully closed, the compression of the mixture igniting takes the path of least resistance. In your case, it is blowing past the slightly open valve into the intake manifold. This is the popping. The recommendation I have for you is to pull the timing cover and recheck your cam timing.
The rhythmic popping is indicative of the mixture firing prematurely in all 6 cylinders.
Burnt or otherwise damaged intake valve. Bad intake valve seat. Rockers & pivots in the head incorrect. Incorrect push rods. Anything that allows either compression to slide past the valve (if it is fully closed) or something that is not allowing the valve to close completely. Popping back thru the intake is not something that we could call common on the 4.2/258s.
Do you have a compression gauge? That might tell you if you are losing compression.
Or try this:
Get someone to help you. Have them hold the idle fast enough to make it pop. Using insulated pliers, pull off the spark plug boots one at a time. When the popping quits, you have found the cylinder that is the problem. If it pops on all of the cylinders, the cam has to be off a tooth.
a sticking lifter could cause this, but that would be rare. I am thinking more of a valve issue. I would not pull the timing cover off unless you have pops on all 6 cylinders.