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Old 09-19-2020, 01:22 PM
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Default Popping through carb

I have been having some trouble with my 455. It’s a new engine with no miles on it. Cam breakin seemed to go fine. However, the engine pops through the carb when you jab the throttle. It doesn’t pop at idle. It also doesn’t pop when you hold the rpm’s steady. I have checked the exhaust for leaks thinking that I may be hearing an exhaust leak but that is fine. I’ve swapped intake, carb, distributor, and even the heads. I’ve inspected the lifters and cam while I had the intake off and there is no noticeable or unusual looking wear. I adjusted the valves before I ran the engine and I even re-adjusted them with the engine running and valve covers off and they seem just fine. The valves are quiet. I thought I found my issue this morning but it didn’t seem to fix it. The bottom of my new cap was melted so I just replaced the whole distributor, cap and coil but it still makes the popping noise when you jab the throttle. It doesn’t make a difference with the popping noise when you adjust the timing. The engine is a 455, 6x heads, Lunati 704 cam, stock rockers, and performer intake. Does anyone have any ideas? My next step is to re-check the cam and crank timing gears.
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Last edited by John Milner; 09-19-2020 at 01:31 PM.
  #2  
Old 09-19-2020, 02:46 PM
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For the cam phasing, might be quicker to get an idea if you can measure the intake and exhaust tappet rise at TDC.

At 106 intake centerline the Voodoo 704 intake tappet should have .085

From Harold B.
Quote:
The intake lifter should be about .030" higher than the exhaust, with an OK range being .025" to .040". If the intake is anywhere in this range, your engine will run good. If the exhaust valve is the same height or higher than the intake, the cam needs to be advanced, either by a bushing, or even by advancing a whole tooth. It just depends how far off it is. And this does not mean the cam was ground wrong, but that all the clearances involved have stacked up the wrong way. As long as you get the intake lobe in the 'sweet zone', the engine will work as promised..

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Old 09-19-2020, 05:20 PM
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Sounds like a bad valve..

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Old 09-19-2020, 06:52 PM
PontiacJim1959 PontiacJim1959 is offline
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I don't think it is a valve needing adjustment based on your description.

What it sounds like to me is that you are not getting a good shot of gas from the accelerator pump. When you stomp the gas, you are opening up the carb and you are pulling in more air than gas and getting the backfire.

You did not say what carb you are using.

Is the gas squirting from the nozzles in the primary carb bores when you look down the carb (engine off)? You should see 2 good streams of gas shoot into the carb when the accelerator pump is pushed down (as in opening the carb wide open).

You could have a bad accelerator pump which is not doing its job.

With the larger cam, I would want more gas entering the primaries when you stomp it. Adjustments depend on the carb you are using.

Your idle may also be too low with that cam. Try raising it up a little and see if that may help.

Have you checked for a vacuum leak?

What is your timing set at? Timing can be a factor.

What trans, torque converter, and rear-end gear? Cam may not be matched to the drivetrain and you are causing a "bog" just enough to allow the engine to hickup and cause the backfire.

So many things it can be. So in my opinion, could be the carb set-up. I may be a valve that is too tight or improperly adjusted, but your symptoms sound like carb issues.

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Old 09-19-2020, 10:00 PM
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Thanks for the replies. I thought it was a bad valve at first too but ruled that out with the head swap. Both sets had been valve jobbed and were in good shape. I started out with a 800 78 t/a Quadrajet. I thought maybe there was a carb problem so then I took a 750 edelbrock off of one of my drivers since I knew that carb worked fine. Still no change. I did check for vacuum leaks. I have the rear port on the carb plugged temporary. The only things I’m running off of the carb are the vacuum advance and pcv valve. I have not driven the car down the road yet. It has only ran in my shop for roughly 45 minutes. One thought going through my mind is the stock 1.5 rockers with the .525 lift cam and maybe there wasn’t enough clearance. But I don’t have any bent pushrods and have not heard any clicking. Really stumped on this one since it sounds great idling and holding the rpm steady.

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Old 09-19-2020, 11:17 PM
PontiacJim1959 PontiacJim1959 is offline
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Quote:
Originally Posted by John Milner View Post
Thanks for the replies. I thought it was a bad valve at first too but ruled that out with the head swap. Both sets had been valve jobbed and were in good shape. I started out with a 800 78 t/a Quadrajet. I thought maybe there was a carb problem so then I took a 750 edelbrock off of one of my drivers since I knew that carb worked fine. Still no change. I did check for vacuum leaks. I have the rear port on the carb plugged temporary. The only things I’m running off of the carb are the vacuum advance and pcv valve. I have not driven the car down the road yet. It has only ran in my shop for roughly 45 minutes. One thought going through my mind is the stock 1.5 rockers with the .525 lift cam and maybe there wasn’t enough clearance. But I don’t have any bent pushrods and have not heard any clicking. Really stumped on this one since it sounds great idling and holding the rpm steady.
What is your timing? Initial & total? Is the vacuum advance direct to engine vacuum or ported?

Just for fun, wire the choke somewhat closed to richen the carb and try that and see if it pops.

How did you set your valves? Are you running an adjustable valvetrain with poly locks?

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Old 09-20-2020, 05:51 AM
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The fact is lean misfires which can make a motor pop back thu the Carb will not cause burns in the cap and or rotor.

Are you triple sure your firing order is right and your wires are good?

If the Dizzy advance system is hanging up then the rotor can be pretty far off from where it should be but yet allow the motor to run, yet fire off other cylinders when they should not be creating a pop back.

I have seen the firing order be off in such a way that you would think that he motor was ok except when it was under load.

I would also pull out off of the spark plugs, jam the Carb wide open and do a compression test counting only the gauge reading on the 5th hit.

If you have a blown head gasket or one not sealing inbetween cylinders that can make for a pop back condition also.

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Old 09-20-2020, 10:44 AM
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Thanks for the replies. I have tried both ported and manifold vacuum for the advance. I have bbc studs and nuts on my heads and adjusted the valves by following the procedure from the picture I attached. When I noticed the popping issue, I then adjusted them with the engine running, thinking I maybe had one too tight.. I backed them off until I could hear a click, then tighten till it got quiet and added about another 1/4 turn. I feel like the valves are adjusted right on. On my timing, I just adjusted until the engine sounded the best since I haven’t had the chance to actually drive the car. I got the timing advanced enough to where the engine would kick back on the starter when trying to start and backed it off slightly. It starts fine and idles fine. Sounds good revved up to 3,000 rpm. I really feel like it should be close enough to run ok until I can get it on the street. It did not make a difference with the popping no matter how I advanced or retarded the timing. This is with two different distributors. The parts in one of my distributors are new. Wires are new stock replacements. That’s certainly not to say they are good though. I’ve never seen a new cap and rotor go bad so quickly. The contact points all were chalky white looking and as you can see the new cap was burned. This was roughly 45 minutes of run time. The second distributor had used parts in it. It sounds just the same as before. I’ve double and triple checked my firing order with both distributors.
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  #9  
Old 09-20-2020, 10:56 AM
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Did you try a different condenser in the distributor?
The points are trying to tell you something.

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  #10  
Old 09-20-2020, 12:45 PM
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Reverse polarity

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  #11  
Old 09-20-2020, 08:44 PM
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I’ve had two separate vehicles act this way and both turned out to be crappy spark plug wires. One set being brand new but after changinging the plugs and coils too I eventually realized the wires were crap. A new set of Taylor’s race wires fixed both vehicles. Both would idle good and rev slowly good until it warmed up and you couldn’t give it any kind of throttle past snail speed. It’s definitely worth swapping them out and seeing. Everything seems to be junk these days.

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Old 09-20-2020, 10:42 PM
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My vote is the coil in the distributor cap is missing the ground link. Some coils use a formed link and others used a ground wire, but from the mess with the cap, I'm betting that the ground is missing.

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  #13  
Old 09-20-2020, 11:36 PM
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The burned cap does have the ground link in there. I put a new coil in and made sure to install the link. The condenser and module were new as well. On my second distributor I tried, I did install a good used cap/coil off of a Chevy hei in it. I didn’t think about a Chevy coil maybe being reverse polarity from a Pontiac. I haven’t swapped plugs and wires yet. Maybe I’ll try that tomorrow just to see.

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Old 09-21-2020, 04:36 AM
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Pontiac HEI uses a coil with white and red leads.

Info from Schurkey:
There are two color-coded (coil-in-cap) ignition coils, and three color-coded pickup coils. (Note that external ignition coils are not color coded.) One coil-in-cap ignition coil uses red and yellow wires; the other
uses red and white wires. They are functionally identical except the magnetic (not electrical) polarity is
opposite. The yellow/red ignition coil is neither a higher- or lower-performance version of the white/red
ignition coil. They have the SAME electrical specs, differing only in the direction (not strength) of the
magnetic field they radiate.
One pickup coil uses either a yellow tyrap or a yellow plastic connector body on the wires that attach to
the module. Another pickup coil uses either a blue tyrap or black plastic connector body, and the third
pickup coil uses a clear tyrap or a clear plastic connector body. (Some aftermarket manufacturers will
use no connector body instead of a clear one.) The yellow pickup coil has opposite magnetic and electrical polarity from the blue/black, and clear pickup coils. The wires leading from the blue/black; and clear
pickup coils to the module must be crossed to correct the electrical polarity, leaving the magnetic polarity
“backwards”. The metal connectors are sized differently; and the molded plastic connectors are positioned in a way that prevents incorrect connection.
Delco discovered that magnetic interference from the starter/battery cable could trigger “false” sparks at
low RPM—especially during cranking. Sure, you can use mis-matched components—but if you have misfires, backfires, or other ignition problems especially at cranking speed—you’ve been warned. Pickup coils
are selected based on engine family. Ignition coils are then selected based on which pickup coil was selected.
So, for the “traditional” V-8 engines, Chevrolet, Cadillac, and Olds Toronado use the yellow-coded pickup
coil, and because they use the yellow pickup coil, they use the yellow and red ignition coil. Put another
way, the yellow coils are a matched set. Buick, and Oldsmobile except Toronado, use the blue/black
pickup coil, and therefore use the white and red ignition coil. Pontiac uses the clear coded pickup coil. It
is wound the same way as the blue/black coil, but has longer leads to physically fit in the Pontiac version
of the HEI distributor. Therefore it, too, uses the white and red ignition coil. Many parts catalogs indicate
the wrong pickup and ignition coils for Olds Toronados.

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Old 09-21-2020, 11:35 AM
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Looks like my 1st coil that I used with the cap that melted had the red and white leads. The second one is for sure a Chevy coil.

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Old 09-21-2020, 11:49 AM
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That is a very very good write-up on the HEI parts by Schurkey.
THANKS SCHURKEY AND KENTH

Tom V.

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  #17  
Old 09-21-2020, 12:59 PM
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What happens when you slowly accelerate? If you can "sneak up" on high RPMs, then it's probably your accelerator pump and you are getting a "lean pop". If you are breaking up and misfiring as soon as soon as RPMs increase, it's electrical.

Make sure your engine is properly grounded as well. I've seen weird electrical issues like this because the engine wasn't properly grounded, and that showed when higher electrical demands (high rpm) were needing the additional ground.

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Old 09-21-2020, 01:25 PM
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Quote:
Originally Posted by Navy Horn 16 View Post
What happens when you slowly accelerate? If you can "sneak up" on high RPMs, then it's probably your accelerator pump and you are getting a "lean pop". If you are breaking up and misfiring as soon as soon as RPMs increase, it's electrical.

Make sure your engine is properly grounded as well. I've seen weird electrical issues like this because the engine wasn't properly grounded, and that showed when higher electrical demands (high rpm) were needing the additional ground.
It seems to break up as soon as the rpm increases. The carb that is on it now, I run on one of my daily drivers so I know it works good. It did the same thing with the quadrajet though. I’ll recheck my ground. I thought one lifter plunger felt odd when I was adjusting the valves the first time. One didn’t seem to want to compress like the others did. But the valves are very quiet it seems like.

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Old 09-21-2020, 01:32 PM
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Quote:
Originally Posted by John Milner View Post
It seems to break up as soon as the rpm increases. The carb that is on it now, I run on one of my daily drivers so I know it works good. It did the same thing with the quadrajet though. I’ll recheck my ground. I thought one lifter plunger felt odd when I was adjusting the valves the first time. One didn’t seem to want to compress like the others did. But the valves are very quiet it seems like.
The volume of fuel and air requirements for your daily driver are likely nowhere near the same for your 455. If you are putting the same volume of fuel in, the 455 is going to pull a lot more air...and give you a lean pop at RPM.

When I went from a 350 pontiac to a 400, the 600 Edelbrock that I had was no longer up to the challenge and I was shooting fire through car on acceleration. A Q-Jet with it's giant secondaries cured that issue. Double check you timing to make sure you are being conservative (12 initial is always a good starting point).

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Old 09-21-2020, 10:07 PM
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I did a little more testing tonight. It now has the right coil. Still no change on the popping situation after the swap. I then ran it and started pulling plug wires. I pulled number two plug wire and it did not pop anymore when I revved it up. I checked the valve adjustment again on that cylinder and it was right on. I had trouble getting the number two exhaust gasket to seal off to my ram air exhaust manifold. It leaked past the shim gasket during breakin. I finally got it to seal off with a header gasket. The manifolds are originals and were surfaced flat before I installed the engine so they should be good. Just to test, I loosened the whole manifold and left the ports wide open and ran the engine for a few seconds. It was very loud but I could not see it or hear a pop through the carb when I revved it up a time or two. I guess I’ll pull the manifold and check to see if that port might be blocked somehow? I don’t see how it could possibly be blocked but the engine sounded much happier with it disconnected.

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