1974 Nova Backfire HELP!!
 

Im working on a 1974 Nova with the following engine:

350/041 heads/292H Comp Cap/HEI Ignition/Air Gap intake/650 Holley/Headers/Magnaflow Exaust

Car has backfired probably about 200 times in the last 10 years. Sometimes it runs good, other times it runs like a bag. Very Lazy.

Came to my shop a week ago, changed the power valve and it hasnt run since. I dont think its ever run good so thats why im in search of an answer. Changed the carb first with a good 750 and it will sit and run ok but as soon as you rev it, it pops and sprays fuel out the carb and then a 4 foot backfire. Pulled the intake checked all the lifters...very good...checked the pushrods, re sealed up the intake with new gaskets and set the rockers at 1/2 turn preload. Same backfire. Reset the rockers at .008,( thinking the lifters were pumped up) same back fire. Checked the timing numerous times. 34-36 degrees total.
Changed the distributer with two other good running HEI units...same backfire. Changed the plugs, wires, cap, rotor, mod,coil, wiring inside the distrib, hotwired the HEI right to the battery....same backfire. Changed the fuel pump...new line from the carb to the pump. Same. Car only has one vacuume line from the booster to the back of the carb....changed that as well with no luck. Just pulled the front of the engine down to see of the cam timing was off....its perfectly lined up. Chain is a bit loose but not enough to cause this problem. Compression test was ok...150-160 across the board and a leakdown shows no more than 10% on a stone cold engine.
This thing is fouling the plugs like crazy too.

Im reaching out for help here boys....ive been poped in the ear about 60 times in the last week and I cant take it anymore. Please help!!

Re: 1974 Nova Backfire HELP!!
 

Does it foul all the plugs the same?
What fuel pump and regulator? How much pressure?
Have you tried moving the timing around, to see if you get any change?

just some thoughts
good luck

Ken

Re: 1974 Nova Backfire HELP!!
 

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.

Re: 1974 Nova Backfire HELP!!
 

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.

Re: 1974 Nova Backfire HELP!!
 

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.

Re: 1974 Nova Backfire HELP!!
 

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.

Re: 1974 Nova Backfire HELP!!
 

If you have fire in the intake, you have only one of two problems:
1) Timing of spark is incorrect (firing order, ignition timing, cam timing)
2) An intake valve that is not sealing.

I would check your valve springs if you think they are weak. As Alan stated, have you checked it with a vacuum gauge? If you have a valve hang open (sticking guide or weak spring), it will show on a vacuum gauge.

You can have a valve hanging or weak spring and still have good leak down and compression.

Re: 1974 Nova Backfire HELP!!
 

Tried all 8 wires off one at at time and it didnt change a thing.....still popping. Im gonna have to throw a new chain and gears on it anyways and then Ill definitly have a look at the valve springs. Who knows it might be hanging a valve just enough to allow some fuel back up into the intake. Thanks alot fellas!!!!

Re: 1974 Nova Backfire HELP!!
 

I would check the valve springs and installed height. They should be at least 135@1.75 on the seat and 295@ 1.25 open. Do you have a single spring with damper? This cam requires a double spring and a 1.44 seat diameter. What rockers do you have? If they are stock, you will not have the required slot length to accept the lift. This cam has characteristics of creating all sorts of issues and won't co-operate with low static compression. 12 years of backfiring...hope we can help.