Issues reviving 69 396 from long term storage

[DaleR]

I have a ’69 396 BB engine in a 69 Camaro that I'm working. The engine was totally rebuilt 10 years ago and I installed new rod bearings, rings, mild performance cam, new timing gears, and ground the valves and seats. When the engine was fired up it ran like a fresh motor, not issues. It had a stock points ignition and a 4 barrel Qjet.
The engine then sat for 8 years due to unexpected issues in life. I rebuilt the carb to insure no stale gas/varnish issues. Upgraded to a new HEI distributor and new wires. I insured that the distributor was installed correctly ( #1 at TDC with rotor pointing to #1 tower). Fired it up and the vacuum gauge holds steady around 11 inches. It backfires through mufflers. On acceleration it backfires through the carb and mufflers.  Adjusting the timing does not improve the symptoms. Adjusting the valves did not correct it. Ran a compression test, cylinders came in at 110 psi with minor variance. Cylinder leakage test show no major issues. Any ideas on the root cause?  I suspect sticking valves. Any ideas out there?

[HawkX66]

I'd look closer at your distributor and triple check your firing order. Are you getting enough juice to your coil? Could your timing chain have slipped a tooth somehow? The latter sounds unlikely in your scenario.

[cook_dw]

Very odd you are getting a lean and rich condition at the same time.  Pull the ignition module and have it tested.  Also like Hawk said double & triple check the firing order.  Also double check you dont have the dizzy 180* out..  These are just a couple of the first things I would look at.

[Mike S]

 There is a power valve in the Q-jet that controls the mixture in the primary side. You can get to it by removing only the top portion of the carb. If it is stuck then you can either have a very rich (stuck open) or very lean (stuck closed) condition relative to the throttle position depending on which way the brass piston is sticking in the bore.
  Does the exhaust have a rich smell or burn your eyes?

Mike

[DaleR]

Brand new HEI distributor with new plug wires and plenty of juice to it. Double double checked correct installation and firing order. So I tend to think ignition as OK for the moment. Looking into other possibilities. The exhaust is running rich regardless of mixture screw adjustment. When I get a muffler backfire I get a drop on the vacuum gauge, indication of a sticking valve? need to pull plug wires and see if I can isolate the cylinder that's acting up. Thinking I have multiple problems here.

[Mike S]

I suspect a stuck open power valve in the carb. I had that happen after sitting for a few years. Unless you have a collapsed lifter (usually permanent) I would approach it from the KISS principle and check the Q-Jet.

Mike

[ko-lek-tor]

Couple of other thoughts from the King of letting things sit a long time and then resurecting (me). Cracked dizzy cap, especially since you changed out that component, you say, I'd really give it a look over. Mouse nest in exhaust and mud dobber nest in intake have been some issues I have seen. If air cleaner on whole time, hard to imagine dobber nest (will cause lean running)and mouse nest probably would not cause backfire. I had some valves get rust (I suspect rusty on seat) and caused some backfire, just drive it on a long drive 20 miles or so. Sticking valves, you mention can cause your problem. Hook a vacuum gage up with a long hose into driver compartment as you drive and monitor when it acts up, needle will bounce around(could be another valvetrain issue too like spring). All these steps are non-invasive in keeping with the KISS theme. Then, dig into carb when all these check out.

[JKZ27]

Where did the HEI distributor come from?
How old are the plugs?

[hotrod68]

  Worst comes to worse, the cam could be going flat. Just a thought. Good luck!

[L78 steve]

The power valve can be checked simply by inserting a small screwdriver down the vent tube.  With the engine not running the valve will be up and you will be able to push it down with little force. The screw driver blade will need to be positioned parallel with the firewall to engage the T on top of the needle hangar.
If that is not it may be firing order.