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Backfiring through carb at idle>>> Fixed

LilRedBronco

Sr. Member
Joined
Feb 25, 2008
Messages
428
Loc.
Belmont, California
ETA: Fixed it for now. So others may learn from my mistakes... I went down and yanked off the valve covers and with the help of wife who is detail orientated I marked off each cylinder on the distributor . I then when cylinder by cylinder lining up the rotor with the markings she made and I "relashed" each valve. So now it doesn't backfire anymore. So crisi over for now. I also found that the intake bolt on the left side under the carb missed my tightening sequence. Gotta learn to not be so rushed. Thanks for the help...

My burly mechanic:

BestHadapict.jpg




I am almost fed up with this truck!!. Anyhow it was running really well then suddenly just as I was about to do the last mile before home started back firing REALLY bad through carb while parked and at idle.

Now it just pops and pops. Vacuum seems good new lines all snug.

Could there be some bad gas? If I didn't set valve lash perfectly would it screw up the cam? I only drove about 5 miles maybe a little more.

Jesus I just want a weekend of relaxing between 14 hour work days. About had it.

302
New heads
New Intake
New carb
 
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broncnaz

Bronco Guru
Joined
May 22, 2003
Messages
24,341
If the valves are to tight they could be holding open a little when the cylinder fires or its just your igintion timing is to advanced and its lighting the cylinder off before the valves are closed.
While tight valves could screw up the cam more than likely you would be hearing ticking noises that grow louder as the cam wears.

Right now my bet is on timing maybe the dist is a little loose and to advanced.
 

Viperwolf1

Contributor
electron whisperer
Joined
Aug 23, 2007
Messages
24,316
Timing would have to be advanced more than 90 degrees to catch the intake valve open. They close between 60-80 degrees ABDC.

I broke an exhaust rocker arm once and it would pop through the carb every time the intake valve opened.
 

broncnaz

Bronco Guru
Joined
May 22, 2003
Messages
24,341
I was hoping to clam his nerves a little by not mentioning broke parts as the possible cause. As I need to be his neighbor first. Last time he was about ready to off load the bronco for pocket change.
 

Viperwolf1

Contributor
electron whisperer
Joined
Aug 23, 2007
Messages
24,316
I see. The good news about a broke rocker arm is that's about as cheap and easy a fix as broke gets. Does that help?

Seriously though it could be something real simple like a backed off rocker nut.
 

recoiljunky

Bronco Guru
Joined
Jul 19, 2007
Messages
1,478
Loc.
Dothan, AL. USA
Mine is doing the same thing and it's either a wiped exuast lobe on the cam or the lifter is bad. Mine pops on every cycle of the engine. I've pulled the driver side valve cover and verified that the number 5 exaust valve is barely moving even though it's adjusted properly.

Sorry, but misery loves company.
 
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OP
LilRedBronco

LilRedBronco

Sr. Member
Joined
Feb 25, 2008
Messages
428
Loc.
Belmont, California
Sorry for the delay was down in the parking lot first thing trying to figure this out. I took the valve covers off everything is intact none of the rocker arms are broken..

It sounds as though there is a leak on the drivers side bank. As if number 6 cylinder isn't sealing as the piston comes up.

broncnaz I appreciate the consideration. If I were on a project to get this rolling down the road I'd have more patience but after spending all of this cash to replace all this stuff the possibility that I thrashed a cam lobe in 5 or so miles of driving (in traffic). Then realizing that I may have to pull the engine and put a new cam in has just been a bit much... Long story short I really have the worst luck and the last 3 yrs have been 1 big crap fest. I was driving down the road yesterday thinking wow I did it then this...

It has been so bad this last 3 yrs that my first thought was I cursed myself by thinking I had done something right. Just another thing in a multi yr stretch of ugliness. REAL ugliness. Including the day I went for a run in Utah 100+ degree heat with a bad chest cold (the Marine in me says running fixes everything) and having a heart attack at 41 and almost dying on side of the road. VERY lonely feeling as the lights turn off...

Anyhow, I set the valve lash by letting the rotor move on to the cylinder rolling, the push rod to ensure it was free then tightening until I felt it start to "grab" a bit from friction then giving it a 1/4 more turn.

The problem with this method is my fingers are too big to get in there good to roll the push rod. So I am afraid that my method suffers both from getting a great purchase on the rod and lack of experience.

So if I had tightened these like this how precise is the feel? If I wasn't precise enough is it likely that I did major damage.

It seems very subjective to me and therefore it could mean it is not as precise as I think it needs to be or it is as precise as I fear it is and I screwed up a cam or ripped up a valve so it isn't seating right.

If it doesn't require extreme precision I might be ok and I may just need to redo it all. I am going to loosen them all up start from scratch and reset like I did before. I gotta get it out of the store parking lot.

Recoiljunky I feel your pain and I hope you get yours squared away ASAP. I think I need to take the 338 Lapua to the range and blow off a hundred rounds at the 1K range. Do you know of any targets that have an Early Bronco silhouette? ;D

Thanks again folks and I appreciate the insight. Wish me luck
 

DirtDonk

Contributor
Bronco Guru
Joined
Nov 3, 2003
Messages
47,345
Two things come to mind here LilRed.
That loose intake bolt, what side was it on again? Same side as is having the issue by any chance? An incorrectly torqued intake manifold is just a leak waiting to happen. So maybe that's what's up here?
Hope so, since it's better than number two.

Even though it's only been five miles, in all this did you remember to break in the cam correctly? Or is this a roller cam? I can't remember. But if it's a flat-tappet cam, the initial adjustment and break-in are pretty critical.
Just ask recoiljunky... ;D

Paul
 
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LilRedBronco

LilRedBronco

Sr. Member
Joined
Feb 25, 2008
Messages
428
Loc.
Belmont, California
I didn't change the cam and to be honest I am embarrassed to admit I have no idea. I am guessing it is not a roller cam it is whatever type came with the 302 in 1972.

Anyhow, I am 99% sure I got it fixed. I cruised around Belmont today and just going up the hill to the house is a good stress test and it passed. TONS of torque no hesitation and smooth acceleration and gear changes. I did tighten up the intake bolt and yes it was n the side giving me issues.

I was WAY I mean WAY off on the valve lash. As I said I went back found the advice broncnaz had given me before on setting the lash and long story short I misread a step. I thought I was supposed run the rotor past for example number 7 bumping the rotor to number 8 and then setting the lash on #7 cyl.

No idea how I got that out of what he described. Yet another in a long line of goofing up.

I don't want to curse it but I do feel really good about the work I did (finally) I am "pleased as punch" as grandma used to say. No idea what the hell it means but that is how pleased I am ;D So going to drive it in to work tomorrow with fingers crossed and hopefully it will again be my DD vehicle.

Thanks again Gents I certainly owe you beers and then even more beers or other adult beverage of your choosing. Or a Shirley Temple if that rocks your boat too.
 

DirtDonk

Contributor
Bronco Guru
Joined
Nov 3, 2003
Messages
47,345
Sounds great. How did the drive go today?????
Hopin' it went without a hitch in your punch. Whatever that means.

Shirley Temples are great. But I recently discovered Arnold Palmers and now have a new addiction.

Paul
 

barronj

Bronco Guru
Joined
Jun 6, 2009
Messages
1,859
I see. The good news about a broke rocker arm is that's about as cheap and easy a fix as broke gets. Does that help?

Seriously though it could be something real simple like a backed off rocker nut.

Reviving this old thread with a "thank you". I was doing some tinkering, trying to get it to lean out & stop stinking like gas so bad, so I'd made some changes to the idle mixture screws, take it for a test drive, and it starts popping like crazy, limps home. Turn the screws way out to richen it up, the idle picks way up, but the pop is still there, rythmically. I come inside, do a search, and after reading this, I figure I need to pull a valve cover... Sure enough, the exhause valve on #4 has a broken STUD. Damn cheap ass chinese... wait a tic, these are AFR's... why did that happen?
 
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