1974 Nova Backfire HELP!!

[Pvt Parts]

Take the exhaust loose at the Y pipe and look into the pipes with a flashlight. Sounds like the inner exhaust pipe may be collapsed. ie fouled plugs (all), no power, backfires under load but idles OK.

[Alan Roehrich]

Valve springs?

Valve sticking in a guide?

TDC on balancer wrong?

Wrong timing tab?

Timing set machined wrong?

Compression test?

Leakdown test?

Actual HEI spark test at the spark plug end of the wires?

Vacuum reading at idle?

Those 292 magnum cams just plain suck. You need 11:1 just to make one run decent. If you do not have at least 11:1, it needs to be advanced a full 4 degrees, verified. It needs hot extended tip plugs, a ton of initial timing (like 20 degrees at 650-750, and around 44 total, all in by 3000) a 33-36 squirter (with the high flow screw), an aggressive accelerator pump cam, and maybe even a 50cc pump. If it's not a vacuum secondary, it needs all that on the secondary side, too. They're usually better off with 4 corner idle circuits, and 0.060 holes drilled in all 4 throttle plates. Try about a 4.5 power valve.

Even if you have an honest 11:1 or better, you're usually better off ditching that cam. For a hydraulic flat tappet, in a small block, anything more than around 236-238 degrees is almost always a joke, there more for sound than power. At that point, I go solid lift. I never use a shelf cam that has more than around 236 @ .050" in anything, at that point, it is serious enough to warrant a real custom grind.

Set the valves at no tighter than zero lash plus 1/16 to 1/8 turn, or hot and running, make the rocker stop clicking, and go 1/16 turn max. Use poly locks or double rocker nuts.

The local speed shop sold dozens of those cams years ago. Usually, after we tuned on them and crutched them for a while, the customer had the 292 pulled out and replaced with a nice custom cam about 12 degrees smaller at 0.050". The car often picked up 2 tenths in the 1/8, ran better in town, got double the fuel mileage, and about ten times the plug life.

If it is an automatic car, it needs a good 3000-3500 stall converter, and at least a 3.73 gear.

[CrateCamaro]

It had a delco mechanical fuel pump to start...3psi at 1000 rpm....changed to a holley mechanical and it now has 6psi at 1000 rpm. Tried moving the timing around....30 total...all the way to 40 total. I havent ruled out that the balancer hasnt moved....that is on my to do list but it looks about right comparing to other balancer of the same size. it has a Magnaflow dual exaust and it pretty new so im gonna say that isnt a problem and there is plenty of exaust comming from the pipes.

Alan - I have done a leakdown....less than 10% on a cold engine which I though was exceptional for a 14 year old piece. Compression was 150-160 PSI across the board. Timing tab and balancer are all 70`s GM. Im gonna go ahead and throw a chain and gears at it while its apart but a simple eye comparison on the upper cam gear showed it was identical to a GM piece I had laying around in the shop. The lower gear has 3 keyways on it and its on the `Straight Up` slot. Im going to go ahead and check and see if it has a sticking valve but you would think it would tare a lobe off the cam with a sticking valve or even knock a pushrod out. Tough to say I know but I will definitly check. Im gonna assume with an 041 head and a flattop 4 eyebrow piston it has about 9 - 9.5:1 compression. I know that a 292H isnt a great camshaft but it should at least run decent without shooting a 4 foot flame out the carb. Keep the ideas comming guys!!!! Thanks again

PS...one thing i did notice was the valve spring pressure was really low. Didnt check it yet but just by pushing the valve open by hand it feels very weak.


PSS...its a stick car. M21 transmission. I could care less about the performance I just need the loaf to run again.

[pmrphil]

Try removing one plug wire at a time to see if it's only one cylinder that's giving you grief, it might help get you going in the right direction.