# Popping Sound during acceleration



## vera_jr (Aug 17, 2012)

The engine is a 455, but I just started hearing a bad popping sound. The sound isn't apparent during idle. If I hit the accelerator hard / quick, it pops loud. It also starts popping at high RPMs, mainly over 4K. 
I have tried to see if it was a cylinder, so I accelerated rapidly while removing a spark plug wire, one cylinder at a time. The sound didn't change.
It sounds like a back fire around the intake manifold, or Carb. 
Any suggestions??


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## rickm (Feb 8, 2012)

check ignition coil, points n condenser, valvetrain.


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## ALKYGTO (Mar 29, 2010)

Could be valvesprings, do they have a lot of miles on them? stock? big cam? Need more details.


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## geeteeohguy (Feb 2, 2008)

Check the basics, as stated: timing, vacuum leaks, etc. Is it running a little rougher than before at all times? Could be a burnt valve (not uncommon). A compression test would be a good call if it's running a bit rough.


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## PontiacJim (Dec 29, 2012)

The previous checks are all good ones. Personally, if it runs good at idle and up to around 4,000 RPM, I would make sure it is not a fuel delivery problem first which could indicate a lean out at the carb. Last fuel filter change? How old is the fuel pump? Bad carb gasket or hose allowing air in and causing a lean out condition? Rubber fuel line collapsing as fuel demands create more suction that can collapse a weak hose internally? How full is your gas tank? Low fuel levels can cause fuel starvation on hard accelerations. Fill the gas tank and see if any changes.

Next will be the ignition checks which have been mentioned. Is your timing correct? Is your mechanical and vacuum advance working properly? Condition of cap, points, rotor, condensor? Enough coil output voltage at higher speeds? Cracked cap causing cross firing within the cap? Bad ground? Crossed plug wires causing misfiring? Check the ignition system while running, in the dark, to see if any arcing. 

If all checks out, then as posted, could be a weak valve spring allowing a valve to hang open or even a burnt valve (which usually will "pop" at lower RPM's as well - my limited experience on this one). Then it may be time for a compression check.

Probably other checks that others can recommend as well. These are just a few I would do, but more info would help everybody help you. Did you alter/change/modify anything before this started? Did it just happen out of the blue? Did you pull a 300' 6,500 RPM burn out for your U-tube video that you wanna share with us all? Miss a shift perhaps? In all seriousness , just a little more info like mileage on your engine, carb type, etc. that may help, and let us know what you have done or tried using our tips and checks so we might be able to keep steering you in a direction to cure your ills.


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## vera_jr (Aug 17, 2012)

I have checked all the electrical recently because of an ignition problem. Turned out, the "on" wire was broken off the ignition switch. But at that point, I checked all the points, coil, plugs, etc. It all looked good. I have done a short burnout recently, but nothing close to 300'. maybe 30'. I have not had the engine pulled, cleaned, or anything like that since I have had it. I bought it just over a year ago, and it sat for a couple years prior. I had friend tell me the fuel could have a major problem. I use 83 octane, but it may contain too much ethanol. I need to go fuel it up at an ethanol free station. 
If it is a carb issue with it leaning out because of gas, hopefully fresh fuel will fix the problem. 
I will try the simple stuff first, then let you guys know the results. 
Thanks for all the input.


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## vera_jr (Aug 17, 2012)

with the slow disappearance of ethanol free gas stations, what is the next solution to fuel the car properly


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## chuckha62 (Apr 5, 2010)

This is generally not an issue with a Pontiac due to the low position of the exhaust manifolds/headers, but make sure you don't have plug wire resting on a header tube. I've seen this happen before and create a pop.


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## PontiacJim (Dec 29, 2012)

vera_jr said:


> with the slow disappearance of ethanol free gas stations, what is the next solution to fuel the car properly


Good old 89 octane should be OK, but it depends on your compression ratio. What year engine, stock/modified, high compression heads, etc.?

Make sure that your carb choke is fully opened when warmed up as well, and not fluttering (loose) or partially closed. What type carb? Its possible that if you have a Q-jet, the secondaries are opening up and could be using what fuel they need, but the supply of fuel is not being filled fast enough to keep up with the engines demand and it goes lean at the RPM you state. Again, this falls into a fuel supply problem.

If you know all the electrical items are indeed good, then fuel delivery/carb is the next focus.


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## vera_jr (Aug 17, 2012)

PontiacJim said:


> Good old 89 octane should be OK, but it depends on your compression ratio. What year engine, stock/modified, high compression heads, etc.?
> 
> Make sure that your carb choke is fully opened when warmed up as well, and not fluttering (loose) or partially closed. What type carb? Its possible that if you have a Q-jet, the secondaries are opening up and could be using what fuel they need, but the supply of fuel is not being filled fast enough to keep up with the engines demand and it goes lean at the RPM you state. Again, this falls into a fuel supply problem.
> 
> If you know all the electrical items are indeed good, then fuel delivery/carb is the next focus.


It is an Edelbrock 1605 I believe. 600 CFM. Not an OEM Q-Jet. Is there an adjustment that I can make to the carb to see if I can get the problem to go away?


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## vera_jr (Aug 17, 2012)

Could a vacuum leak produce the backfire in the carb? 
I am still very suspicious of the valves. if a valve is stuck, will it produce the backfire in the intake? There is a tapping as if the rocker is loose. Novice thought here, but if the rocker is loose, could it cause the valve to not open properly, which would cause the engine to backfire like that?


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## BearGFR (Aug 25, 2008)

Yes and yes. First order of business it sounds like is to make sure all your valves are adjusted correctly. We can't tell you what "correct" is for your engine though without knowing the details of how the valve train is built and what parts were used. A loud "pecking" sound in the valve train could be anything from a loose rocker to a bent pushrod to a bad lifter to a flattened cam lobe --- and more. Also possible you've got more than one problem working here.

Bear


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## chuckha62 (Apr 5, 2010)

If an exhaust valve is not opening fully (whether the valve is too loose or the lobe is flat), the cylinder will not fully exhaust it's charge. Therefore, when the intake opens on the next cycle, the remaining charge will exhaust back through the intake and pop through the carb.


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## geeteeohguy (Feb 2, 2008)

Again, the OP needs to do a basic compression test to baseline the health and mechanical condition of the engine. Anything else right now is chasing ghosts.....


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## 69Goatee (Aug 14, 2009)

How are the motor mounts? I have seen where worn out motor mounts can let the engine rotate too far and the fan hits the shroud under heavy load. If it is an automatic, open the hood and watch, or have someone else watch the engine when you power-brake it. The drivers side should rise a few inches, but anything over about two inches or so is too much. If it is a manual you can do the same test but it is a little harder as you need three legs or a really good park brake. In either case look on the inside of the shroud for spots where the fan has made contact. A worn out or broken trans mount can do the same thing causing the rear of the the trans to lift or shift from side to side.


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## geeteeohguy (Feb 2, 2008)

Goatee, he's chasing a backfire/engine tune issue here.


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## 69Goatee (Aug 14, 2009)

I make my ends meet as a mechanic and have seen many weird things. I am just throwing out something I have seen before where a strange noise was hard to pinpoint from the drivers seat.


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## vera_jr (Aug 17, 2012)

I removed the valve covers and checked the rockers. there were a couple slightly loose, but not enough to warrant major adjustment. I tightened up the loose ones, and now they are fine. The popping still persists. I started looking at the carb again. This is where the backfire is occurring. When I move the throttle cable on the side of the carb to rev it up, I notice that the plate at the top of the carb is not moving. Should this be moving to open and close the air flow? Could this have anything to do with the popping? Yes? No?


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## PontiacJim (Dec 29, 2012)

Did you mean the 1405? It is a square bore 600CFM (what we call an AFB in the "old" days). First, it will not fit the factory intake unless you used an adapter. Not always a good idea, but I have done it. Depends on the adapter style. If its open, you could have problems as the factory intake is a dual plane and the open type adapter may cause a flow problem.

My opinion is the 600CFM is too small by far on a 455CI. It is really made for smaller cubes. Factory Q-jet is 750CFM and the later 800CFM. If you put it on out of the box, my guess is it is set up too lean, which will give you that popping sound at your higher revs. You have to have the correct jets and components in the carb to match the engine. Edelbrock has a good website that may be of help.

Here is a complete 40 page guide I just found on the AFB. Choosing the correct CFM for your engine based on size and RPM, adjustments, tuning, exploded views, etc. This is what you need to read and look at. Very specific. Just type the words "Federal Mogul Carter AFB, pdf" in your web search engine and it should pull up. It will be listed as - Carter AFB Selection and Tuning Guidelines -Mopar, then click on it to open it up. 

U-tube does have a video on the Edelbrock 1405 in setting it up and looking for problems. Check this out.

The 455CI needs air. I feel the 600CFM (if that is what you have) is too small and is where your problem might be. Matching intake for this carb is also a must. You could install bigger jets and metering rods to get it to run richer, but I will bet your gas mileage will suffer greatly. I would move up to the Edelbrock 750CFM or 800CFM depending on your cam.

Give us more details on the intake, what do you have on the 455CI?


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## vera_jr (Aug 17, 2012)

the intake is an edelbrock intake. When I got the car, these parts were on it. I haven't had an issue until recently. I don't know if the cam has been modified. I know the engine, even for a 455 has been modified. It has the 6x-4 heads.


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## BearGFR (Aug 25, 2008)

vera_jr said:


> ... there were a couple slightly loose, but not enough to warrant major adjustment. I tightened up the loose ones, and now they are fine...


You realize that you're dealing with thousandths of an inch here, right? The concepts of "slightly loose" and "not enough to warrant major adjustment" don't really apply.

Bear


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## PontiacJim (Dec 29, 2012)

vera_jr said:


> I removed the valve covers and checked the rockers. there were a couple slightly loose, but not enough to warrant major adjustment. I tightened up the loose ones, and now they are fine. The popping still persists. I started looking at the carb again. This is where the backfire is occurring. When I move the throttle cable on the side of the carb to rev it up, I notice that the plate at the top of the carb is not moving. Should this be moving to open and close the air flow? Could this have anything to do with the popping? Yes? No?


Look at the PDF I suggested. It shows the exploded view of the carb and the parts. The plate at the top of the carb, are you referring to the choke? or the secondary air valve?


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## vera_jr (Aug 17, 2012)

It is the Choke that I am looking at.


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## Instg8ter (Sep 28, 2010)

once the car is warmed up the choke should stay full open and not be effected by the throttle.


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## PontiacJim (Dec 29, 2012)

Instg8ter said:


> once the car is warmed up the choke should stay full open and not be effected by the throttle.


This is the answer to your choke question. 

If you can't find the problem with all the checks that have been set out here on the forum, then it may be time to put the car in a shop and let them place it on a diagnostic machine that will monitor the electrical items to ensure they are putting out all the voltage throughout the RPM range and getting enough voltage through the resistor wire going to the coil. They can also confirm your battery is good and the charging system is putting out enough voltage - have had both of these cause problems when it was all said and done, low output voltage can cause problems at higher revs. They can verify your timing and the advance in the distributor through the RPM range and make sure it is working properly and not stuck, the weight springs are weak, the vacuum lines are hooked correctly, and the vacuum diaphragm on the distributor is doing its job. They can run a vacuum gauge on the engine to make sure you don't have a valve bouncing on the valve seat due to a weak valve spring which you won't know unless you take them off and bring to a machine shop to test the pressures. Want to make sure you did not burn a valve, vacuum test will show this as well. Could even have a bad gasket somewhere or something cracked. Then they can monitor your fuel pressure to make sure it is not a fuel delivery problem. Could be a bad filter or even a rubber line that is collapsing internally once the carb draws more gas needed at higher RPM's. 

You can replace a lot of parts not needing replacing and spend cash to do it only to find out what it is. The engine needs 2 things, gas and spark. If you know it is not the ignition components and you know it is not the gas delivery system, then it is either the carb itself or a mechanical problem with the engine. If you don't have the equipment to perform these checks, you could go out and buy them, but this is an added cost and if you only use the equipment once and it does not show a problem, its money gone -and more frustrations.....been there, done that.

So, not trying to ditch you or be a smart a**, but this is what even I have to do when I can't figure it out with my knowledge and equipment. I don't have all the knowledge nor all the equipment. Sometimes even I swallow my pride, throw in the towel, and bite the bullet and shell out the money for someone else to figure out the problem. Sometimes in the long run it saves me time,money, and a lot of frustration going this route. I just hate it when the fix was so simple it was in my face and it would have cost me a fraction of the mechanics bill to fix.....if only I had known! HaHaHa. But, its all about driving my car and enjoying it....and not doing any further damage by catching some things early.


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## vera_jr (Aug 17, 2012)

Great advice PontiacJim. If after a couple cheap, easy check we don't figure it out, I will definitely take it in.
Compression in the cylinders. I pulled these numbers today.
182, 180, 188, 182, 180, 170, 166, 178
Any significance here? I also just replaced the spark plugs, the old ones were very old / rusted on the outside, so I just replaced them. It is still doing the same thing.


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## Cory (Mar 2, 2014)

Those numbers look even. 
You mentioned you found a few loose rockers, the factory rocker studs are shouldered, meaning if initially torqued to specs, then in theory shouldn't be loose. A flat cam will not necessarily show up on a compression test, and can certainly cause popping when accel.

I'd make sure you check to see that all your valves are opening fully. Use a dial-meter, heck even a tape measure will work. You just want to see the valves opening up the same amount.


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