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Jeep Service
Jeep Service, Jeep Owner
Category: Jeep
Satisfied Customers: 4839
Experience:  14 years with AMC/Jeep Specializing with pre-1994 Jeep products.
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1976 Jeep CJ7 4.2L I6. Newly machined head with valve job,

Customer Question

1976 Jeep CJ7 4.2L I6. Newly machined head with valve job, weber 2bbl carb, new fuel filter and fresh ethanol free fuel, prestolite distributor with vacuum advance, initial timing set at 8*, vacuum and mechanical advance seems to be working, it fires right up and idles fairly well, but any acceleration at all results in popping through the carb. It does this hot or cold and the more gas you give it, the louder the pop. The popping does seem to be rythmic not random. Adjusting timing does not seem to help.
Submitted: 1 year ago.
Category: Jeep
Expert:  Jeep Service replied 1 year ago.

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

Customer: replied 1 year ago.
The popping through the carb happens the same way, with or without the vacuum advance attached, at the slightest acceleration. "Road testing" consists of idling to the corner and back as the popping sounds like it could be doing a great deal of damage.
Expert:  Jeep Service replied 1 year ago.

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.

Customer: replied 1 year ago.
1 TDC, confirmed by timing mark on damper, rotor is pointed directly at the #1 plug wire.
Expert:  Jeep Service replied 1 year ago.

If the distributor is correct, the cam timing is off.

Customer: replied 1 year ago.
Exactly what does that mean? Are you saying that the timing chain is not aligned properly? Would that allow it to start and idle without backfiring?
Expert:  Jeep Service replied 1 year ago.

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.

Expert:  Jeep Service replied 1 year ago.

The rhythmic popping is indicative of the mixture firing prematurely in all 6 cylinders.

Customer: replied 1 year ago.
I will try and tackle that this weekend to verify. The popping only sounds like one of the six. What else can I check, if the timing chain is aligned properly?
Expert:  Jeep Service replied 1 year ago.

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.

Expert:  Jeep Service replied 1 year ago.

Do you have a compression gauge? That might tell you if you are losing compression.

Expert:  Jeep Service replied 1 year ago.

Or try this:

Expert:  Jeep Service replied 1 year ago.

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.

Customer: replied 1 year ago.
I have a compression gauge that I am borrowing in the morning and will check compression tomorrow. I will also try isolating the cylinder via the plug wires. Pulling the timing cover is not something i'm looking forward to. Also, the head just came back from the machine shop, cleaned, magnafluxed (no cracks), decked, valve job, new seals. Three new push rods (same length as the others) to replace slightly bent ones, and new rockers to replace slightly worn ones. Unfortunately, I did not think to pull the lifters as I believed it to only be a blown head gasket.
Expert:  Jeep Service replied 1 year ago.

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.