mysterious backfiring problem

[monster man]

hello all, I have a very strange problem that I can't seem to get rid of.

It's on a Chevy 396 that's in my '78. A couple weeks ago I was getting ready to smog it, changed the plugs and oil, and fired it up. It backfired through the carb, and then started running poorly and flooding. Took the carb off, the float was stuck, fixed that, still the problem. Changed the: plug wires, plugs (again), cap/coil (HEI) rotor, vacuum advance cannister, PCV, any suspect hoses, swapped carbs. Can't seem to knock the problem Also checked time with the marks on the balancer and it doesn't look like the chain or distribnutor jumped any teeth.

've been trying the past couple weeks to get it running right and nothing seems to completely cure it. As of right now, it starts right up, runs smooth at idle, and will rev to 4,000 RPM strong and steady. The trouble comes when accelerating. it will get up to speed if you give it light throttle. Granny low and first gear accelerates no sputtering or backfiring, 2nd gear and third gear if you try to accelerate too quickly it pops and shutters through the carb, and the truck bucks. Mash your foot down it will sputter a bit and then go strong. Forgot to mention, it's a SM465 trans. That's about the best description I got. In a certain RPM range (1000-1500 with the 38" boggers on, 1500-2000 with the stock tires on), if it is loaded it pops. Barely any visible smoke since I put thicker oil in. The motor has about 200,000 miles on it, and has always burned oil but just keeps going and going. It ran decent before this happened.

My best guess is a stuck lifter, bent pushrod, or valve, or broken or weak valve spring on one of the intake valves.
Really don't want to pull the valve covers because that means moving the AC and everything else.

any ideas? thanks!

 

[1985_K5_Silverado]

My best guess is a stuck lifter, bent pushrod, or valve, or broken or weak valve spring on one of the intake valves.
Really don't want to pull the valve covers because that means moving the AC and everything else.

any ideas? thanks!

Have you hooked up a vacuum gauge?

 

[diesel4me]

Holley carb???

If it has a Holley carb,it might have blown its power valve when it backfired..if not,it could still be carb related--float level too low,bad accelerator pump,etc..Vacuum leaks can cause backfiring too,but would usually cause poor idling also..

Valve springs are a definate possibility on a BBC with that kind of mileage--its fairly common to see failed valve springs on BBC motors..it may be a valve with a burnt spot in it leaking only under high loads,when compression is greatest..weak spark can cause misfiring under a load too,but it'll run normal under light loads..

HEI modules can act goofy too..they sometimes lose their dwell advance curcuit,and the vehicle will run but be "doggy" and could backfire...a failing pick up coil in the distributor could be the culprit too..or even one plug wire that has a burnt spot or is chafed thru,and grounding out momemtarily..I've seen spark plugs crack for no apparent reason too,or lose some porcelain off the firing tip..some crack as they are being installed!..

I know you said you changed plug wires and plugs,so that probably isn't your problem...(unless you put in platinum plugs,they have caused backfiring in many chevy motors I put them in!--I stick with AC plugs now,no fancy ones!!..they just dont work right in chevy motors!..

Make sure the coil and pick up coil "match" each other...if you put the "wrong" coil or pickup in,it'll run,but the polarity of the coil will be reversed,which lowers the intensity of the spark by nearly half!..GM identified the module,pick up coil,and coil with a spot of white or yellow paint on the cap,also by the color of the wires going to the coil and pick up coil..might also check the advance weights and springs in the distributor,and see that they move freely and aren't too wobbly on the pins..

 

[monster man]

didn't mention- I did hook up a vacuum gauge to tune both carbs that I've tried (both rochesters). This motor pulls a nice steady vacuum, no oscillations, and it pulls more vacuum than the 350 in my truck! Also forgot to mention I checked compression, and from what I could gather (I had to use the rubber adapter because I didn't have thr ight thread in fitting), it pulls equal compression on all cylinders- about 110-120 psi. Not good, but at least equal.

the cap/rotor/coil was a matched unit from Summit, so I don't suspect that. Is there other pieces in the ignition that are easily replaced individually?

might also check the advance weights and springs in the distributor,and see that they move freely and aren't too wobbly on the pins..

That's something I have neglected. The mechanical springs look and feel pretty worn, but since it ran good before this I figured they weren't the cause. I'll throw some new ones in there to check.

On another note, you mention weak spark. Before this backfire happened, I'm not sure at what point around then, the battery stopped charging as much as it used to. It would always read 14+ volts when driving, now it only reads 12. Could a blown alternator regulator be causing weak enough spark under load to backfire?

thanks, guys

 

[Thunder]

Is there other pieces in the ignition that are easily replaced individually?

Have you replaced your HEI Distributor Module? Sounds like either your module or magnetic pickup coil are bad. Module is easy to replace. Its the black box in the disributor. To replace the pickup coil you have to pull the dist and take off the drive gear. Or try a new or rebuilt Dist.

 

[monster man]

Have you replaced your HEI Distributor Module? Sounds like either your module or magnetic pickup coil are bad. Module is easy to replace. Its the black box in the disributor. To replace the pickup coil you have to pull the dist and take off the drive gear. Or try a new or rebuilt Dist.

nope- I'm an HEI dummy, didn't know there was a separate module, figured it was justa coil-cap deal :-) Will do that, along with replace the altenrator regulator to make sure my juice delivery is full on. Thanks. Then hope it's not the pickup

 

[diesel4me]

low voltage..

If your alternator stopped charging,then your running off the battery alone..it would take awhile,but low voltage to the HEI could cause your symptoms..but you'd notice slow cranking,and other woes, far before that would occour..

I'd guess its the pickup coil or module,since the valves seem to be working ok according to the vacuum gauge and compression test..over-advanced timing can also cause backfiring thru the intake..try using manifold vacuum to the vacuum advance instead of ported vacuum..some motors run best with lots of advance at idle...

A flat exhaust valve lobe on the cam can cause backfiring too--it causes the exhaust to pile up in the cylinder(since the valve does not open enough to let it all escape)..then the next time the intake valve opens,the built up exhaust pressure will rush by the intake valve and cough up thru the carb..but you'd have trouble getting it up to 4-5000 rpms without misfiring if it had a flat cam lobe..

Might be worthwhile to see if any exhaust pipes are squashed flat,restricting the flow of exhaust..same goes for muffler,the heat riser and catalitic converters,if any..

 

[chevy wrangler]

that use 2 happen 2 my truck after i rebuilt the rochester so i just bought a holly and that dosent happen no more just my timming os off a bit

 

[3 on the tree]

Is it a bear to start when its cold???? Could be your accel pump has gone south. The backfiring on accelerating is the most common symptom of a bad accel pump. When you hit the gas, the mixture goes lean cause of the throttle blades opening up quickly. The accel pump squirts gas to correct the lean condition till vacuum pulls more fuel thru the carb.

 

[monster man]

I thought about the accelerator pump first off- but swapped the carb, same problem, and then inspected the original and put in a different accelerator pump. I'd like to think it's not that......

I keep thinking maybe the cam is bad, but it's not a constant backfire or sputter, and it'll rev up high fine.

WOuld a blown out exhaust manifold to pipe gasket cause this? I know those are blown out in a couple spots, cheap paper.

When it flooded after the first backfire, I had to crank and crank to get it started, and then I had to crank it with the starter to move it back into its parking space. That put a load on the battery for sure, and since then it sometimes will turn over slow. However, it always seems to idle smooth once started, and will start up cold easily. In fact, it runs a little better cold, maybe I should lean out the idle mixture a bit more.

gonna start with the module and advance springs, then alternator regulator, then manifold gaskets

 

[ecameron87]

I used to have a backfiring problem and it turned out to be my ignition switch. Whenever the keys would move while going down the highway, power would be lost momentarily to the HEI unit and it would backfire. I replaced the switch and now all is good. Hook up a meter to the power going into the HEI and see what happens when it backfires.

 

[73k5blazer]

You could also compression check the cylinders. I had a 350 motor do this a few years back, after replacing every distrubtor part, rebuilding the carb,someone suggessted a compressions check, and sure enough 1 cyl. was off, just a bit though. Pulled the heads, and had a small chip in one valve. Motor would idle perfect, ran smooth at speeds up to 60mph, any heavy accelleration or 70mph+ driving it would spit and backfire and buck.

 

[monster man]

You could also compression check the cylinders. I had a 350 motor do this a few years back, after replacing every distrubtor part, rebuilding the carb,someone suggessted a compressions check, and sure enough 1 cyl. was off, just a bit though. Pulled the heads, and had a small chip in one valve. Motor would idle perfect, ran smooth at speeds up to 60mph, any heavy accelleration or 70mph+ driving it would spit and backfire and buck.

oh man- that could be it. I ran a compression test, but only had the rubber adapter, so it wasn't very accurate. I need to get the screw in adapter and get a real reading on each cylinder. I couldn't tell, but each one appeared to be within a few pounds of the other. But, those sound like my exact symptoms

 

[mike n]

Unplug your vacuum advance unit and see if the problem disappears.
If it does, replace the pickup coil.