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#1
12-19-2012, 09:41 AM
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Popping/backfire?
So I rebuilt a qjet for my 455 per Cliffs book. Installed an RPM at the same time. No changes to the dist from when the Holley was on there. I am getting a popping in the exhaust when revving or driving. Do you suppose that I am sucking in air from someplace and that would be causing it? It starts up and idles fine. Any other ideas?
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#2
12-19-2012, 10:54 AM
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Although its a possibility.....this says otherwise! Guess there is something else going on!
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#3
12-19-2012, 11:10 AM
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Did a hot cranking compression test since I was pulling the plugs anyway. 180-185 across the board. Sound normal to you? 30 over 455, 60919 cam, Rhodes lifters, HS 1.65 rockers. It is a fairly new build.
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#4
12-19-2012, 02:14 PM
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Compression is great. Looks like you are running a little rich; getting some unspent fuel in the exhaust that then ignites causing that popping sound you hear. Lean it up a step at a time until pop goes away.
Jim
__________________
65 Catalina sedan. Allen Thomas Performance 495. KRE Heads at 310cfm ported by SD Performance, ProSystems Dominator carb on ported Victor intake, P-Dude custom grind hydraulic roller, MSD ignition, 3.50 Moser/Ford rear. F-Glass front bumper by son Rob, rear by the old man and joint effort for trunk lid. 3950# w/driver. Best of 9.5761/139 on 175 shot, 6.01 /114 in 1/8. |
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#5
12-19-2012, 03:51 PM
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Ditto, what Jim said; we had the same situation on Curtis' car when the mixture was too rich, popping in the exhaust.
__________________
69 Bird w/1970 400 block(409 cubes), #64 heads, hyd. roller, Q-jet by Jeff E., original interior, ps, pdb, th350, and 3.73 gears. Pump gas, street driven muscle. 3800 lbs. race weight. Best, 11.39 @118, my son's car. 79 T/A w/463, Scat crank, Eagle rods, Icon pistons, Lunati solid roller, 262/270, KRE 325 heads, Northwind intake, QF950 carb, full interior, ps, pdb, th350, and 3.73 gears. Pump gas, 3650 lbs. race weight. 10.68 @ 126 so far... no tuning yet. |
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#6
12-19-2012, 07:11 PM
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Yeah its rich looking at those plugs. Problem is when I turn the a/f screws in, it wants to die and/or runs like crap. Makes no sense. Maybe I better pull it apart and see if something is leaking. I tried going in on the APT screw a bit with no change. Suppose I need to go ahead and replace those plugs? Maybe they are gonners and wont clear up? They are only a couple weeks old, but if they are fouled....
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#7
12-19-2012, 09:52 PM
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Did those plugs foul up from normal stop and go city driving? Maybe the idle screws are set okay and the problem is in the primary jets/rods being too rich at low speed cruise. Just thinking out loud of what might be going on here. Have you tried closing the apt all the way down to see how that affects part throttle? How does it run at greater throttle openings? It may take some tinkering to get it right but will be worth it in the long run.
__________________
69 Bird w/1970 400 block(409 cubes), #64 heads, hyd. roller, Q-jet by Jeff E., original interior, ps, pdb, th350, and 3.73 gears. Pump gas, street driven muscle. 3800 lbs. race weight. Best, 11.39 @118, my son's car. 79 T/A w/463, Scat crank, Eagle rods, Icon pistons, Lunati solid roller, 262/270, KRE 325 heads, Northwind intake, QF950 carb, full interior, ps, pdb, th350, and 3.73 gears. Pump gas, 3650 lbs. race weight. 10.68 @ 126 so far... no tuning yet. |
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#8
12-19-2012, 10:58 PM
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I really havent had a chance to drive it. It runs so crappy I cant really take it anywhere. So its mostly from idling. It really wasnt that bad before, but now its super bad.
It has been sitting mostly because after I rebuilt it, the secondaries hang up just a tiny bit and nearly got me killed. It wont do it from blipping the throttle at idle, but does it when you get into the secondaries under load. But that im confident I can find and fix. This was a kit I got from Cliff and its is nearly the same setup his car was, so im confident he sent me the correct rods and jets. But there is a pretty good chance I didnt do something right during the rebuild. This is the first time I have ever had my hands on one Now it just runs like hell and doesnt seem to want to clear up at any throttle. I think im going to replace the plugs (again) and see where its at, and go from there. If I dont get it soon, I may just haul it over to P-Dude and see if he will have a look at it. I have been jacking with this thing off and on for like 2 years with no joy. Im at my limit. It was never quite right with the Holley either, but never this bad. Fresh rebuild, new (after the rebuild) 60919 cam and rhodes lifters, new HEI, new RPM intake, and now the rebuilt q-jet. Unless you know someone good down here in Daytona Beach? |
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#9
12-19-2012, 11:49 PM
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I'm going with a bad sparkplug wire if I'm seeing the plugs correctly. Plugs are all rich, but one (2nd pic, 3rd plug) looks crusted up like it wasn't firing very often (if at all). This would put an unfired mixture out of that cylinder into the exhaust where it would be fired off by other cylinders' hot exhaust gasses.
__________________
Mick Batson 1967 original owner Tyro Blue/black top 4-speed HO GTO with all the original parts stored safely away -- 1965 2+2 survivor AC auto -- 1965 Catalina Safari Wagon. |
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#10
12-20-2012, 01:47 AM
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Thats very possible. And after looking at the pics that one does look a little worse. Its number 5 by the way. But the damn things are new! I bought new ones to rule it out. Easy enough to ohm them out though. I got a good deal on these wires, but they have the straight plug ends and I hate them. The number 7 wire is a pain to get on because of the steering column. And it rubs. Like I said Ive been chasing problems for a long time.
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#11
12-20-2012, 05:28 AM
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Turning what you call the a/f (actually they are idle mixture) screws in will not lean out a rich mixture under power you will need to go to smaller jets in the primary and possibly secondary (under full power) fuel circuits. Or if the jets are close in the primary circuit you can lean it out with larger diameter metering rods.
Hopefully Cliff will chime in here I am not a quadrajet person. |
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#12
12-20-2012, 07:43 AM
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Quote:
__________________
 1968 Firebird 10.817/122.33 3600lbs trw 455, iron d-port heads/SD intake/q-jet, 9" tires 1.440 60ft1962 Catalina 4100lbs trw 455, iron d-ports,perf RPM, 800holley, 9'' tires,(12.157@108.91)1.578 60' Oct 2010!!    
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#13
12-20-2012, 08:21 AM
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Yeah so far mine hasnt done much more than idle. Im interested to see what a new set of plugs do. I didnt change anything and it wasnt this bad a couple weeks ago. Obviously its not right, but I really think the plugs are not letting me tune it out. Unfortunately I am transitioning jobs and working pretty much 2 full time jobs at the moment. Only one job over the weekend though, so hopefully I can get the plugs in it Sat or Sun. Gunna be down in the 40's-50's this weekend though...brrrrr!
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#14
12-20-2012, 08:58 AM
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Might be pulling fuel from the secondary nozzles at idle and low speeds. Verify that the throttle plates are full closed/seated. If they stuck or "hung-up", most likely they need some attention, hitting the gasket, intake, etc.....Cliff
__________________
If you can read this, thank a teacher. If you can read this in English, thank a Veteran! https://cliffshighperformance.com/ 73 Ventura, SOLD 455, 3740lbs, 11.30's at 120mph, 1977 Pontiac Q-jet, HO intake, HEI, 10" converter, 3.42 gears, DOT's, 7.20's at 96mph and still WAY under the roll bar rule. Best ET to date 7.18 at 97MPH (1/8th mile), |
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#15
12-20-2012, 09:13 AM
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Yeah I actually have a spring pulling the secondarys shut right now. But I took the carb off once to see if they were catching on the gasket. Didnt seem to be hitting anything and seemed to operate just fine. Put it back on, went out and stomped on it, and thy hung up again. Never went any further with it. Just stuck a spring on it for the time being. Just so I knew they were getting closed up well. One less variable.
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#16
12-20-2012, 09:49 AM
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What Cliff said... I know from my own experience that the throttle plates can fail to close all the way if they are not 'centered' in the openings; just a little off one way or the other and they don't close all the way. It took me several times loosening the screws and moving the plates slightly to get them dead center and closing properly. Also, I would put in new plugs, if those can't be cleaned properly, before anything else; you can't do a tune up with fouled plugs.
__________________
69 Bird w/1970 400 block(409 cubes), #64 heads, hyd. roller, Q-jet by Jeff E., original interior, ps, pdb, th350, and 3.73 gears. Pump gas, street driven muscle. 3800 lbs. race weight. Best, 11.39 @118, my son's car. 79 T/A w/463, Scat crank, Eagle rods, Icon pistons, Lunati solid roller, 262/270, KRE 325 heads, Northwind intake, QF950 carb, full interior, ps, pdb, th350, and 3.73 gears. Pump gas, 3650 lbs. race weight. 10.68 @ 126 so far... no tuning yet. |
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#17
12-20-2012, 09:56 AM
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Yeah I am sure I can get that worked out. And I grabbed some new plugs on the way to work this morning. 7:30am-10:00pm work day today. Maybe I will try to throw them in tonight. Although with 2' of pipe and turndowns.....im betting the neighbors wouldnt like me running it!
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#18
12-20-2012, 10:55 AM
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popping
Have you checked to see if the power piston is pulling down correctly at idle? I think you can probe down through the vent to check to see if the piston is up when not running, but will move up and down against the spring. When running at idle, the piston should be down, pulling the rods down into the jets. I'm no expert, but just my thoughts. Billk
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#19
12-20-2012, 11:05 AM
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Second picture 3rd plug is carbon fouled the worse, and second picture last plug is the only decent one (not fouled). You are way too rich on the other 7 cylinders. The reason the one plug is ok is probably a vacuum leak at that cylinder. I've seen cleaner plugs on cars with stuck chokes.
__________________
Jeff 
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#20
12-20-2012, 03:55 PM
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Quote:
And that last plug on the second pic is fouled too. Maybe just not quite as bad as the others. It was still completely black. Fresh set of plugs in it and it fired right up and was smooth as ever. Just have to figure out why it got so bad. Turned the idle mix screws in til it died, then turned them back out til I got the most rpm. Left er there. Got a belt squeeling too that will surely drive me insane. http://www.youtube.com/watch?v=id7vak63uU8 |
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