Backfire thru exhaust at idle

[Griff]

Good evening,
I'm a new member here and would like to post my first thread, if I may.

I'm having a recent problem of a backfire thru exhaust on my newly rebuilt 440. This is a 72 - 440 in my 69 Roadrunner. I have read some other threads, but I was wondering if I could start my own discussion on this topic.

Thanks in advance guys.

 

[pnora]

Explain the backfire?

 

[CarolinaCharger]

I would first confirm that none of the spark plugs are crossed wired.

 

[ruffcut]

I'm gonna guess plug wires in the wrong order. ruffcut

 

[Griff]

Expla.ining the backfire...it is was a pretty good exhaust bang out of what seemed like the passenger side. I running 2" headers into 3" exhaust straight back with glass packs about rear 3/4 back dumping a balony cut tip at axel

 

[eldubb440]

just one backfire?

 

[WileERobby]

Ok, first, welcome to the site. Now... is this an isolated incident, or is it something that just started occurring ? Is this engine freshly started after a rebuild ? Need more info

 

[Outsider]

Adjustable rocker arms with lash too tight, holding a valve or more open?

 

[Griff]

Elaborating more...
I built the motor myself, broke in perfectly for 25 minutes with the high zinc Valvoline 20-50 break in oil (still in engine). I have run it since the break in approx. 2 months ago every weekend for about 10-15 min. with no problems. Idle, high rev, burping it all went fine. Very strong running in my opinion. Then, about 3 weeks ago, after I hooked up my old school SW gages under dash and connected cig lighter (which I don't think had an effect) but anyways, the next start up the following weekend, she started banging at idle after warming up.
I took all plugs out, they seemed pretty wet. I cleaned all with sand paper, replaced and it ran great even at idle for about 13 min. then the popping started again.
More background, is I done everything on this car, every nut and bolt. Welded my own exhaust, built entire motor, and all assembly as well.

 

[Griff]

I really appreciate the comments, and chat and I will try and answer as quickly as possible this evening. Got my beer and popcorn and will settle in for the chat.

- Stock stamped rocker arms

 

[pnora]

So it does it just sitting at idle you do not have to be driving it.

 

[WileERobby]

Ok... first tell me, what is your vacuum gauge reading ?

 

[Griff]

Engine is freshly built. I have been running about 3 hours total before this problem.

I went back in an pulled all plugs again, and again they were wet. I sanded, checked gap, cleaned the new NGK plugs and put back in. Ran again today, runs super strong, lightning quick throttle response and again about the 13 min. mark started to bang again, not quite as load or strong a few times at idle. Crack throttle open and it roars and no bang.

As well, I'm sure alot will give me grief, but I have a fairly brand new Edelbrock 500 Carb. I also, just to check, took it apart after first banging session, carb cleaned it all out, took metering rods, floats, all apart and it looked good and new.

 

[Griff]

Haven't put a vacuum gage on it yet, actually need to buy one tomorrow

I think if it bounces around and won't hold steady then I'll need to look into springs and valves.

I'm sure a check list in order of easiest to more invasive tasks will be needed.

 

[eldubb440]

what type ignition?

90% of carb problems are electrical

 

[QuickBpBp]

Electric fuel pump???

 

[Griff]

Never been on road, just finishing build...Metro Detroit here

 

[Griff]

Stock fuel pump

 

[Griff]

Orange box, new and do have it grounded

 

[pnora]

Wet plugs could be to much fuel pressure or weak spark.