[Tim Corcoran]

My engine has a popping sounds like backfiring out the carburetor during full throttle at around 4500 RPM. It feels like it's still making power and wants to rev more but the popping has me wondering what is causing it. In the past if the Q-Jet runs out of gas then it just noses over and stops making power this is different. Could this be caused by a loose timing chain? When setting the timing when it gets to about the same RPM the timing starts jumping around so bad I can't tell where the total timing is at. Any ideas?

[Schurkey]

Carbon knock? Look at the rear view mirror when it's popping--see black smoke? That's the carbon getting blown out of the combustion chamber.

Decades ago, I had a Chevy 350 that would knock on heavy throttle the first time I nailed it after street driving for a week or two. Once the carbon was gone, it was fine...until I drove it gently for awhile.

You could also try the "drizzle water down the primary venturis" trick to get rid of the carbon, too.

[77 TRASHCAN]

When my TA got to 70,000 it would start popping past 2/3 throttle. Number 7 exhaust lobe was going away on original cam. I babied that engine, took extremely good care of it. No reason for that to happen! Took the 400 out in 92. Sitting on a stand in garage, today!

[firechicken]

Yes sounds like a flat exhaust lobe...

[Cliff R]

Weak or broken valve spring will cause it.

I used to drag race a high revving SBC based on the 3" stroke. I shifted at 7000rpms and went thru just a little over 7000 on a 1/4 mile run. Way back then I'd get about 60-80 good runs out of the springs then I'd here a very slight "popping" right up near the top end of the track. I'd replace the springs and it would be fine on the next outing.......

[Gary H]

Originally Posted by Tim Corcoran

My engine has a popping sounds like backfiring out the carburetor during full throttle at around 4500 RPM. It feels like it's still making power and wants to rev more but the popping has me wondering what is causing it. In the past if the Q-Jet runs out of gas then it just noses over and stops making power this is different. Could this be caused by a loose timing chain? When setting the timing when it gets to about the same RPM the timing starts jumping around so bad I can't tell where the total timing is at. Any ideas?

First, I would pull the spark plugs and see what they look like. They should all look very similar color wise. If they are all very dark then you have either poor ignition, or too much gas, or both, causing them to foul. If one or more looks very different from the rest then that cylinder's got an issue the needs to addressed. If the plugs look good then I'd lean toward a valve spring issue.

[ErikW]

Classic indication of a valve related problem. Cam lobe, lifter, valve spring etc.

[firechicken]

Yup pull the valve covers and check the valve train first

[dmac]

Could it be too much advance from the distributor causing it to fire while valve is still open? Maybe a loose mech advance spring. The timing jumping around says to me to look at that before tearing into engine.

[PontiacJim1959]

Originally Posted by dmac

Could it be too much advance from the distributor causing it to fire while valve is still open? Maybe a loose mech advance spring. The timing jumping around says to me to look at that before tearing into engine.

Yes, do the simple stuff first. You did not state if points, HEI, electronic conversion, or aftermarket distributor. Checking the action/parts in the distributor would be my first "go-to." Could be as simple as a hair line crack in the cap causing arcing within the distributor. Look for any traces of a burn spot or burn line that may tip you off. A good cap will have a nice round rotor button shape where the rotor contacts it. Over time, it'll wear flat and indicate time to replace. I prefer brass contacts in the cap over the cheaper aluminum which corrode much faster. If points, pitting and/or bad condensor. These is a small wire that I am thinking is a ground (black wire) that can go bad/wear through. Move/grab everything to make sure noting is real sloppy or loose. Check all the wires/ground in the distributor to make sure something did not rub through.

With the cap off and rotor still on, you can check at the same time if the timing chain has gone south, ie sloppy. You do not need to find TDC or the timing mark on the crank balancer for this test. Rotate the crank CW until you see the rotor move - this is taking up the slop in the chain/gears. Make a mark on the balancer that lines up with the "0" on the scale. Now rotate the balancer CCW (with the mark following down along the timing scale) and watch for the rotor to move and stop. Look at the mark/balancer and see how many degrees the mark moved from "0" to where it is now. Should be about 3-5 degrees distance if it is a good chain. If it is more, then the chain is probably toast OR possibly a worn distributor gear - but you will know this when you manually rotate the rotor with cap off to see how much play you have. You should be able to verify your degrees by rotating the engine back CW and watching the rotor. When the rotor moves, stop, and the mark should be right back lined up with the "0" on the scale.

This is easier to do, rotate, with all the plugs out and no compression, but it can be done with plugs in, but you want a cold engine.

Have you done a vacuum gauge test to make sure you do not have a burned valve. Another basic and easy test with the engine running. If a burned valve, the vacuum will not remain steady and it will fluctuate.

What is being suggested here with regards to the valve train is not pulling the engine apart. You will be removing the valve covers and inspecting for the obvious. Then, again, with plugs pulled to make this easier, have a buddy crank over the engine and watch all the rocker arms. It may be very obvious if you see one that is not quite lifting as high as the others. Could be a bad cam lobe, bad lifter, or bent pushrod. If all look good, then I might consider it to be a weak valve spring.

Still could be a carb issue, but if you feel it is not, then I would look into the other items above.

Just some thoughts.

[geeteeohguy]

Most likely cause is a burnt intake valve, or intake valve issue (spring, cam, rocker, etc). A vacuum gauge will help, along with a compression test and leakdown test of the cylinders. It could be fuel or ignition related, but most of the time, if the car runs ok but pops through the intake at RPM, it is a valvetrain issue.

[Tim Corcoran]

I checked the valve springs and I have right around 114 on the seat and 295 at .500 lift which is a tad over my max lift. So now I am stumped as to the popping. Everything is fine under normal driving but at full throttle pulls real hard until you get to around 4500-5000 RPM is starts this popping. Could this be caused by a bad hyd lifter?

[PontiacJim1959]

Originally Posted by Tim Corcoran

My engine has a popping sounds like backfiring out the carburetor during full throttle at around 4500 RPM. It feels like it's still making power and wants to rev more but the popping has me wondering what is causing it. In the past if the Q-Jet runs out of gas then it just noses over and stops making power this is different. Could this be caused by a loose timing chain? When setting the timing when it gets to about the same RPM the timing starts jumping around so bad I can't tell where the total timing is at. Any ideas?

OK, so you know 1-thing that it is not. Now go and do all your other checks that have already been suggested.

Is a lifter pumping up? Too heavy an oil weight can do this not allowing the lifter to pump down fast enough.

Did you "Zero Lash" all the lifters with the engine warm and running?

No coil bind and plenty of clearances at full lift between retainer and valve guide, valve coils, and any other area that could be hitting. No dimples or marks in the tops of the valve cover due to rocker arms hitting?

Assume plenty of clearance between valve and top of piston? May be hitting at the higher RPM's, but not at the lower RPM's - possibly due to sloppy chain changing valve openings.

Last full tune-up when you replaced all parts? No plug wires crossed/touching or grounding out on headers?

Have you tested the free play in the timing chain?

Recall a silly fix that was simply disconnected the tach wire because somehow the tach was causing the problem in the ignition system.

Have you checked fuel pressure when you are WOT and popping starts? Could easily be a lean condition, even though you are not feeling that nose over sensation. Maybe a vacuum secondary flap fluttering rather than open as intended? Choke loose and fluttering.

The list goes on.............

[steve25]

Springs have a fuss point where even though they allow the peak upper rpm you are looking for they loose control of there own mass at one or more lower rpm points other then at peak rpm and this can easily make for your issue since it sounds like your valves are floating below peak rpm.

When this condition takes place neither the top of the spring is in contact with the retainer, and the bottom of the spring is not in contact with the head!

Beehive or Conical type springs have a far higher harmonic / resonate fuss point and do not suffer this condition.

By the way, 114 psi on the seat is not enough for a 2.11" Intake valve with a 30 degree seat in conjunction with some aftermarket performance Cam lobes despite what the Cam card might say in regards to needed seat pressure.

If you are running a spring set up with only a outer coil and a flat wire damper I would bet that if you change over to a true dual spring set up with even just 120 psi on the seat( after break in ) that your issue goes away.

[Scott65]

For grins, check voltage to ignition.

[steve25]

The voltage is easy enough to check, but if it cleans up at a higher rpm when the firing rate makes for a greater load on the electrical and ingition system then I can't see that being a cause for his issue.

[LKLusza]

Wow Tim, your description of the engine sound brought back memories from a long time ago. Among the other things people are recommending, I'd also check the fuel pump out.

Years ago, I ran a Camaro with a 350, TH400 and 4:56 gears. During one weekend at the drag strip, the thing started to not get past the top of 2nd gear without nosing over and making the exact sound you describe. Sounded like something between popcorn and a whole pack of firecrackers going off. In my case, the culprit was a mechanical fuel pump that was starting to give up the ghost. It would pump enough volume to sustain cruising on the street, but when going WOT it used up all the fuel in the bowl by the top of 2nd gear . As soon as I let off, it started to act normally again. New pump - problem gone.

[HWYSTR455]

If you have a spare coil, try that, have seen coil related symptoms that you describe. Hadn't seen that mentioned yet.

.

[Scott65]

Originally Posted by steve25

The voltage is easy enough to check, but if it cleans up at a higher rpm when the firing rate makes for a greater load on the electrical and ingition system then I can't see that being a cause for his issue.

I didn't see anything said about it cleaning up... Its a simple test for a condition that will for sure cause the symptoms he's seeing.

[Jim Moshier]

I had an engine do that, with my dual points and my big yellow Accel Super coil and after about 2 weeks it started popping pulling some rpm's and I would change out my plugs.....and all is well again. So change out those plugs and see if that fixes it. If it does yep it's ignition gremings