Popping/backfire through exhaust on deceleration - Vintage Mustang Forums
 3Likes
Reply
 
Thread Tools
post #1 of 29 (permalink) Old 08-16-2013, 10:59 PM Thread Starter
Senior Member
 
buening's Avatar
 
Join Date: Sep 2000
Location: Stinky Town, IL
Posts: 8,657
Popping/backfire through exhaust on deceleration

I finally got the car back on the road after a yearlong hiatus. Prior to parking it a year ago, the motor had a real bad bog at WOT and one of the intake banks feeding the plugs looked lean compared to the other bank. So I pulled the carb, disassembled it and blew carb cleaner and compressed air through all orifices. I also noticed the transfer slots in the carb were completely covered up, so I adjusted the curb idle screw so that I had the correct exposed amount of transfer slots (per Holley manual). Put all back together, removed Bosch Platinum plugs and went with some Autolite 45s, and then I took it out for a spin today. The car ran like a raped ape with absolutely NO BOG .

I noticed after the car warmed up and on the way home, the car would pop and backfire though the exhaust on deceleration. It was worse going down the hill, but also did is randomly on level ground. I've read some people claim it is due to a rich condition and fuel is getting into the exhaust and detonating. Is it as simple as adjusting the idle mixture screws?? I need to check but they are roughly 1 full turn out.

Below is my specs and settings


351w stock bore/crank/rods/pistons/heads
Duraspark dizzy, MSD superconductor 8.5mm plug wires, Autolite 45s @ 0.045" gap
Crane Hi-6 with LX-92 high energy coil
Timing set to 34 with 20 mechanical and 14 initial, all in by 3000
Aluminum dual plane intake
Holley Street Avenger 670cfm with stock 65/68 jets and idle set to 700rpm
Hooker Comp full length headers feeding into Magnaflow 2.5" stainless dual exhaust
Stock mechanical fuel pump
T56 trans

If you need other settings let me know.

70 Mach 1 - 351w/T-56
70 SportsRoof - 302/C4
05 F150 FX4
06 BMW 530xi

T56 Conversion
DIY Ron Morris-like Motor Mounts
DIY Engine Crossmember
DIY Camber Eccentric Eliminator Plates
buening is offline  
Sponsored Links
Advertisement
 
post #2 of 29 (permalink) Old 08-16-2013, 11:12 PM
Senior Member
 
Woodchuck's Avatar
 
Join Date: Mar 2001
Location: Peoples Republic of Vermont
Posts: 33,055
Send a message via Yahoo to Woodchuck
Could also be too lean and, lacking any vacuum advance, the mixture is still burning when the exhaust valve opens.

Bart

What, me worry?
- Alfred E. Neuman

Woodchuck is offline  
post #3 of 29 (permalink) Old 08-16-2013, 11:33 PM
barnett468
Guest
 
Posts: n/a
hello

your jetting is lean, all the sa carbs are set up lean, i would run 68 min 69 max in front and 70 on rear.

what are your cam specs?

is your vacuum advance connected to ported or unported vacuum? your dist should be connected to ported vacuum not to unported vacuum in your case. if it's connected to unported vacuum then under decel it might advance so much it might pop.

forget the holley book for now, you must have no vacuum at your ported vacuum on the carb otherwise you will have advance when you connect the dist to it and your car might also pop on decel, lol.

before setting your idle set your gas level to 3/16 below inspection window otherwise it might pop on decel or stall or have temporary erratic idle immediately after quick stops.

14 deg of initial advance might be a touch high, maybe not.

Last edited by barnett468; 08-16-2013 at 11:37 PM.
 
post #4 of 29 (permalink) Old 08-17-2013, 08:11 AM
Senior Member
 
Jays66gt's Avatar
 
Join Date: May 2011
Location: Southern New Hampshire
Posts: 1,104
Had a similar issue with my stock 225HP A code. I was running an Edelbrock 1406, 600 cfm carburetor on top of a 1" spacer. Would pop and occasionally stall when stopping. Was a bear to get running again too. I mean an bear!

My solution after much trial and tribulation was to try an Autolite 4100. It does not solve your problem, but man oh man what a difference it made for me.

I was also told that I was running way too much carburetor for the stock engine. 600 cfm versus the 480 cfm seems to make a huge difference. It could have simply been the wrong carburetor for my application.

My gas mileage has dropped from 18-19 to 16-17. But she runs so much smoother!

1966 GT coupe, A code 289, 4 speed.
Jays66gt is offline  
post #5 of 29 (permalink) Old 08-17-2013, 09:57 AM
Senior Member
 
Join Date: Jun 2011
Location: Colorado Springs
Posts: 9,300
It's pretty hard to get a carburetor too lean. Most people run them too rich. But if your car was in bad shape before you started the engine, a rebuild kit may be necessary.

Improperly adjusted valves can also create the symptoms you describe. So, if your firing order is correct and your ignition timing is correct, I would recommend you check your rocker arms. If they are positive stop, they may not be tightened down. If they are adjustable, adjust them. It won't cost you any money and you can check that off the troubleshooting list.
Klutch is offline  
post #6 of 29 (permalink) Old 08-19-2013, 11:09 AM Thread Starter
Senior Member
 
buening's Avatar
 
Join Date: Sep 2000
Location: Stinky Town, IL
Posts: 8,657
So the things that can cause it is timing, valve adjustment, or a too lean or too rich carb? Looks like I have a laundry list of items to check this evening.

To answer above questions, the cam is an XE262H, pretty sure vac advance is to the full manifold port, gas level in both bowls are right at the bottom of the sight window, motor has 30k on a rebuild and ran fine before with exception to the bog (which I cured with a carb teardown), and it has positive stop rockers.

The popping did not occur when I drove it last year. It had a big bog, likely due to partial carb blockage or transfer slot adjustment, so it didn't run as well as it does now. I have a feeling I need to revisit my mixture screw adjustment, as I didn't do it per a vacuum gauge.

70 Mach 1 - 351w/T-56
70 SportsRoof - 302/C4
05 F150 FX4
06 BMW 530xi

T56 Conversion
DIY Ron Morris-like Motor Mounts
DIY Engine Crossmember
DIY Camber Eccentric Eliminator Plates
buening is offline  
post #7 of 29 (permalink) Old 08-19-2013, 11:15 AM
Senior Member
 
67coupe393C's Avatar
 
Join Date: Mar 2013
Location: Oregon
Posts: 1,600
Quote:
Originally Posted by buening View Post
I finally got the car back on the road after a yearlong hiatus. Prior to parking it a year ago, the motor had a real bad bog at WOT and one of the intake banks feeding the plugs looked lean compared to the other bank. So I pulled the carb, disassembled it and blew carb cleaner and compressed air through all orifices. I also noticed the transfer slots in the carb were completely covered up, so I adjusted the curb idle screw so that I had the correct exposed amount of transfer slots (per Holley manual). Put all back together, removed Bosch Platinum plugs and went with some Autolite 45s, and then I took it out for a spin today. The car ran like a raped ape with absolutely NO BOG .

I noticed after the car warmed up and on the way home, the car would pop and backfire though the exhaust on deceleration. It was worse going down the hill, but also did is randomly on level ground. I've read some people claim it is due to a rich condition and fuel is getting into the exhaust and detonating. Is it as simple as adjusting the idle mixture screws?? I need to check but they are roughly 1 full turn out.

Below is my specs and settings


351w stock bore/crank/rods/pistons/heads
Duraspark dizzy, MSD superconductor 8.5mm plug wires, Autolite 45s @ 0.045" gap
Crane Hi-6 with LX-92 high energy coil
Timing set to 34 with 20 mechanical and 14 initial, all in by 3000
Aluminum dual plane intake
Holley Street Avenger 670cfm with stock 65/68 jets and idle set to 700rpm
Hooker Comp full length headers feeding into Magnaflow 2.5" stainless dual exhaust
Stock mechanical fuel pump
T56 trans

If you need other settings let me know.
backfire on deceleration can be caused by a exhaust leak, it can be a small one that you can not even hear and still do it.


Dan Nelson
67 coupe, 393 Cleveland
Toploader, 4:11, detroit no spin

http://i880.photobucket.com/albums/a...ps30f73058.jpg


http://s880.photobucket.com/user/62h...y393c/library/?
67coupe393C is offline  
post #8 of 29 (permalink) Old 08-19-2013, 11:36 AM
Senior Member
 
Join Date: Jul 2000
Location: New Jersey
Posts: 2,597
probably too lean/richen the fuel and try it.
Tonys65Pony is offline  
post #9 of 29 (permalink) Old 08-19-2013, 12:01 PM Thread Starter
Senior Member
 
buening's Avatar
 
Join Date: Sep 2000
Location: Stinky Town, IL
Posts: 8,657
Quote:
Originally Posted by 67coupe393C View Post
backfire on deceleration can be caused by a exhaust leak, it can be a small one that you can not even hear and still do it.
This may sound weird, but I can definitely tell it is coming out of the tailpipe and not near the headers or mufflers. The sound appears to be behind me....but I guess could be deceiving. I'll check the exhaust over again though just in case

70 Mach 1 - 351w/T-56
70 SportsRoof - 302/C4
05 F150 FX4
06 BMW 530xi

T56 Conversion
DIY Ron Morris-like Motor Mounts
DIY Engine Crossmember
DIY Camber Eccentric Eliminator Plates
buening is offline  
post #10 of 29 (permalink) Old 08-19-2013, 12:06 PM Thread Starter
Senior Member
 
buening's Avatar
 
Join Date: Sep 2000
Location: Stinky Town, IL
Posts: 8,657
Quote:
Originally Posted by Tonys65Pony View Post
probably too lean/richen the fuel and try it.
I'm going to go through the idle mixture screw process again using the vac gauge. I likely dialed them back too much and caused a lean condition. Considering it only pops at off idle and infrequently, its gotta be in the idle or transfer circuit. If it was too lean per jetting, I'd thing it would pop/backfire at light, half, or full throttle as well.

70 Mach 1 - 351w/T-56
70 SportsRoof - 302/C4
05 F150 FX4
06 BMW 530xi

T56 Conversion
DIY Ron Morris-like Motor Mounts
DIY Engine Crossmember
DIY Camber Eccentric Eliminator Plates
buening is offline  
post #11 of 29 (permalink) Old 08-19-2013, 01:01 PM
Senior Member
 
Blues Power's Avatar
 
Join Date: Mar 2010
Location: LI NY
Posts: 4,778
Garage
I would get that once in a while coming off the freeway after a long cruise at a steady RPM.

it wasent a continuous issue so I didn't worry too much about it.

had Tim do my dizzy last year and the prob seems to have gone away for good.
hasent happened in over a year now

I can only attribute to either the new working dizzy internals that are all up to specs or when i put the dizzy back in i timed in a few degrees differently from what I had.

prob a little of both if you ask me

1970 Mach 1 San Jose built Dec 23 1969. Marti 1 of 7. Purchased in 1987. Original family owner of the power train
351C 2v FMX.


1993 GT 13,000 miles, Built 2-12-93 Bought 8/3/93 Auto, 3:27 Axle, cloth, sunroof, AC. Unmolested all original car. Purchased new 8-3-93. still has the factory windshield fluid.
Blues Power is offline  
post #12 of 29 (permalink) Old 08-19-2013, 01:50 PM
Senior Member
 
Woodchuck's Avatar
 
Join Date: Mar 2001
Location: Peoples Republic of Vermont
Posts: 33,055
Send a message via Yahoo to Woodchuck
Quote:
Originally Posted by barnett468 View Post
hello

your jetting is lean, all the sa carbs are set up lean, i would run 68 min 69 max in front and 70 on rear.

what are your cam specs?

is your vacuum advance connected to ported or unported vacuum? your dist should be connected to ported vacuum not to unported vacuum in your case. if it's connected to unported vacuum then under decel it might advance so much it might pop.

forget the holley book for now, you must have no vacuum at your ported vacuum on the carb otherwise you will have advance when you connect the dist to it and your car might also pop on decel, lol.

before setting your idle set your gas level to 3/16 below inspection window otherwise it might pop on decel or stall or have temporary erratic idle immediately after quick stops.

14 deg of initial advance might be a touch high, maybe not.
We both agree that it's probably lean... From there, keeping in mind that a lean mixture takes LONGER to burn than a rich mixture, you want that vacuum advance working when the throttle plates are closed, so you want it connected to MANIFOLD vacuum and you also want base timing set as high as it will tolerate. If that doesn't work, you can increase the side of the idle jets and/or idle channel restrictors. Also, check for plugged idle discharges in the secondaries. They are there and even though you may not have 4 corner idle screws there is a fixed idle circuit in the secondaries. It's there to keep fuel in the secondary bowl from becoming stale.

Bart

What, me worry?
- Alfred E. Neuman

Woodchuck is offline  
post #13 of 29 (permalink) Old 08-19-2013, 03:33 PM Thread Starter
Senior Member
 
buening's Avatar
 
Join Date: Sep 2000
Location: Stinky Town, IL
Posts: 8,657
bartl, I tore down the carb before installing it and blew all the passages out, including the main body and all passages in the metering blocks.

I checked the idle mixture screws at lunch and they are both 1 1/4 turn out. Within reason, but may be a tad on the lean side. I haven't touched them with the vac gauge yet though, see below.

I admit, I have a LOT to learn about holley carbs. When I first got it, I used the mixture screws to get max vacuum with a vac gauge, and then used the curb idle adjuster screw to turn down the idle. What I ended up doing was completely covering the transfer slots (curb idle screw controls this), and thus causing my bog issue. So this time, I adjusted the curb idle screw until the exposed transfer slots were square (0.020" sounds familiar). Then I installed the carb and then turned the idle mixture screws in until the idle was 750rpm or so. It appears 1 1/4 turns was where I ended up at. I have read this is the correct procedure to follow, but not everything you read is correct. I got rid of the bog, but now have the popping which I assume is because the idle mixture screws need to come out a bit. If I turn them out to get max vacuum, my idle will go up and thus turning the curb idle screw will mess with my transfer slots. I don't want to chase my tail again. I need some advice If this sounds about right, then I can look into getting more timing in the car. I won't get into the ported vs manifold vac argument.

70 Mach 1 - 351w/T-56
70 SportsRoof - 302/C4
05 F150 FX4
06 BMW 530xi

T56 Conversion
DIY Ron Morris-like Motor Mounts
DIY Engine Crossmember
DIY Camber Eccentric Eliminator Plates

Last edited by buening; 08-19-2013 at 05:07 PM.
buening is offline  
post #14 of 29 (permalink) Old 08-19-2013, 07:29 PM
Senior Member
 
67coupe393C's Avatar
 
Join Date: Mar 2013
Location: Oregon
Posts: 1,600
Quote:
Originally Posted by buening View Post
This may sound weird, but I can definitely tell it is coming out of the tailpipe and not near the headers or mufflers. The sound appears to be behind me....but I guess could be deceiving. I'll check the exhaust over again though just in case
if its an exhaust leak up by the headers it can draw air in on deceleration and it will backfire out at the tail pipe, not at the leak.


Dan Nelson
67 coupe, 393 Cleveland
Toploader, 4:11, detroit no spin

http://i880.photobucket.com/albums/a...ps30f73058.jpg


http://s880.photobucket.com/user/62h...y393c/library/?
67coupe393C is offline  
post #15 of 29 (permalink) Old 08-19-2013, 07:51 PM
Senior Member
 
1fastcar's Avatar
 
Join Date: Jun 2009
Location: Louisville, KY
Posts: 269
Well I once had pulled the carb and went through it like You did after sitting a while. took it out and ran great for twenty minutes and it started sputtering and popping. I looked at various things. finally I removed the carb again and reopened it up. Seems some sediment must have been trapped in the fuel lines or pump and reclogged the carb even with a new fuel filter. replaced the stock pump, new line from chassis to pump and from pump to carb and cleared problem up. also new gas sucks with ethynol and dissolves gaskets , rubber in fuel systems. Might not be the same problem but possible.

My1970 mach1 blue Owned 44 yrs was used for Coker Tire ad. photo by Byrd
1961 T-bird convertible, possible 1961 Elvis movie , "Follow that Dream " red florida car
1991 Mustang GT convertible
previously owned rides
1971 mach1 429SCJ Grabber Yellow
1969 Cougar XR7 428 CJ
1965 Mustang GT Fastback
1965 Mustang fastback
1968 Torino notchback
1958 Thunderbird
1976 Mustang cobra II 302 4spd
1979 Mustang Cobra Turbo 4
1980 Mustang 4
1970 Dodge Challenger R/t 440
1968 Firebird 400
1fastcar is offline  
Sponsored Links
Advertisement
 
Reply

Quick Reply
Message:
Options

Register Now



In order to be able to post messages on the Vintage Mustang Forums forums, you must first register.
Please enter your desired user name, your email address and other required details in the form below.

User Name:
Password
Please enter a password for your user account. Note that passwords are case-sensitive.

Password:


Confirm Password:
Email Address
Please enter a valid email address for yourself.

Email Address:
OR

Log-in










Thread Tools
Show Printable Version Show Printable Version
Email this Page Email this Page



Posting Rules  
You may post new threads
You may post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is On
Trackbacks are On
Pingbacks are On
Refbacks are Off

 
For the best viewing experience please update your browser to Google Chrome