I have been trying to get my 1979 Z28 350 to run correctly for months. The engine was rebuilt by my father a few years ago but never ran. He gave the engine to me but it has been one problem after another. First the rocker arms would not adjust properly and ticked all the time, it turned out to be the valve springs so I put on a brand new set of chevy heads (small valve) that I had and finally fixed that problem. Now the problem is it "pops" through the exhaust at idle no matter where the timing is or the mixture. The popping only gets worse if you give it throttle and the engine boggs when given throttle in gear. I've heard it could be the cam, which was new after it was rebuilt (and my father conveniently forgot the cam specs), or posibly the fuel delivery. I have rebuilt several engines myself and never had these problems, I'm about to scrap the whole block, any help would be appreciated thanks.
The particulars:
Block, Crank, Mains, Rods, Pistons (.030) All stock '79 parts rebuilt
Heads (including valves, springs, rocker arms, and push rods) Brand new
never ran before this GM small valve
Cam and lifters brand new Crane (unknown specs on cam but know it is only
a mild cam, which doesn't help moch I know)
Intake '68 Chevelle/Camaro AFB/Holley square bore cast iron
Carb New Edelbrock 600 CFM AFB style
Distributor Non HEI Breakerless type ignition uses external coil
Fuel Pump New '68 350
Plugs Bocsh Platnum '79 Z28 gap
If you need to know anything else just ask and I'll post a reply. Again thanks for any help at all I am really at a lose here, thanks again.

 

popping out the exhaust could be an exhaust leak, a valve adjusted too tight, timing or carb.

I would first verify the firing order is correct, then warm it up and readjust the valves. Then try to set the timing by ear, just move it back and forth and set it where it runs the smoothest.
Then put a timing light on it. The cam may be off a tooth and your timing could be way advanced or retarded.

 

I just readjusted the valves this morning, and it doesn't matter where the timing is it pops more or less but it never stops. Do you think I should try moving the distributor a tooth or more in the direction it seems to pop less? I can get the timing mark to line up now but it pops plus with out knowing the particulars of the cam specs I can't figure out where the timing should be. I f the exhaust didn't have enough back pressure could that cause this problem?

 

A ball park starting point is about 8-10 deg adv. If you can't get enough adjustment you can pull it out and move it a tooth.
Backpressure.. uhh no. Open header cars don't pop...
How are you adjusting your valves?

 

Initially I adjusted the valves the long drawn out way by manually turning the crank and adjusting each valve in turn, this morning I used the quick crank it up and adjust the rocker out until it taps then back in a half turn (and I might add that all the rockers appeared to be adjusted correctly when I adjusted them this morning). I didn't think the back pressure would've been a problem but I'm grasping at straws here.

By the way 8 degrees is what it is running at now and it is popping.

 

Pull the disb and make sure your timing is right. Sounds like its off, maybe by one tooth as stated before. If your disb is off it will never be right until you reset it. But make sure your at TDC on #1 and set disb back at #1 position. Adjust your starting timing at 8-10 as stated before.

Also check your plugs, make sure your not running too rich. Too rich will also cause bogging and possible poping due to too much fuel being dumped. On the Edel carb adjust ur mixture screws all the way in and back out at 2 full turns per screw. That should be enough to get you started again. Then you can properly adjust the carb.

 

Are you sure the cam isn't in 180* out? Dots together on the timing chain means #6 at TDC (and #6 valves should be closed).

Easy way to check- roll the motor to 0* timing mark, #1 valves should be closed, and rotor pointing to #1 terminal. If #1 valves are open, then rotor should be pointing to #6.

1-8-4-3-6-5-7-2? Odds on driver's side, #1 at the front.

Only two things I've ever seen give random popping.

 

Forgot about that too. . Reset teh disb and check the carb. Then go pull the front off and check timing chain.

 

The timing gears are correct I know because before I put the engine in I changed the pan and timing cover and was sure to verify the correctness of the timing chain. And if the distributor was 180 degrees out it wouldn't start, atleast thats been my experience. I will try changing the placement of the distributor tomorrow to see if that clears it up. I was under the impression that even if the distributor was off a tooth (or more) the distributor may have to be turned more to get the correct timing but as long as the marks met up it will work fine, that is not the case? Thanks for the ideas.

 

who built the engine and did they do anything to the heads if they didnt lap the valves properly you may have one sticking open.
or your intake may be sucking air at the heads. with the engine running take a propane torch and go around the intake near the heads and see if the engine changes tune if so replace the intake gaskets.

 

You say the engine was rebuilt and never fired up. When you first started it, did you run it at 2000 rpm to break in the cam and lifters? If you didn't, the cam is probably shot by now and causing you your grief. Since this is a '79 engine, it is a flat tappet cam and must be broken in initially or it's almost guaranteed to wipe out a lobe. Since you've done most things ignition wise already, you might as well pull the cam and see what the lobes look like.

 

If you're measuring 8* initial (I assume that's with the vac advance disconnected, right?), then you're correct- the distributor can't be off one tooth, as your timing is where you want it.
You might also try gapping the plugs down to .035-.038" or so, from the stock .045".

 

Agree with jzajac1. If cam wasn't broken in right, it could have lost a lobe which causes backfires. Easy to see. The rocker on any lobes that are worn will be adjusted lower than the others. The nuts will be farther down on the stud.

You say mixture is ok. So do the idle mixture screws have the effect they are supposed to when you adjust them? Vacuum leaks can cause pops. But since you say the engine bogs when you give it throttle, sounds more like a fuel pressure problem. Put a gage on it. If low, line or pickup in tank might be restricted. IS the float level ok?

 

I would think if the lobes were burnt off the cam it would pop through the intake as well

 

The heads were brand new GM assemblied units off a crate engine, the heads had never been run and where received about two weeks prior to being installed from GM. The crate they came on is going in another car and heads where replaced with aluminum ones, rather then spend the money getting the '79 heads redone I just put the crate heads on because I had no plans for them. The crate heads are off of GMs base model 350 Carberated Crate engine (they are small valve heads).

I recently swaped intakes to the one I have on the motor currently, all new gaskets of course, same problem presisted (aka the popping happened with both intakes).

The engine was broken in correctly despite the initial valve adjustment problem and all rocker arm nuts appear to be in the same relative position on the studs.

Yes, the ignition timing I quoted earlier was initial with the vacuum advance disconnected.

I bought another set of new plugs and wires I'm going to put in to see if that does anything despite the fact that the ones in it are new. I will try regaping the plugs if it doesn't make a difference. If still no difference I will look into the fuel pressure, the line from front (pump) to back (tank) is the original one perhaps it is the problem. Again thanks everyone for your input.