Backfire

[71_Duster]

Specs: 440 with Edelbrock RPM heads, RPM air gap intake, AVS2 800 cfm carb, Firecore distributor and wires custom curved to the engine build . Comp xtreme energy 244/252 solid flat tappet cam. TTI headers. Mopar electronic distributor with newer box (The one E-berg sells/recommends) Timed with a known good snap-on advance light with 14 deg initial. I can't remember what it was all in at right now.

Engine was rebuilt last year and broken in on the dyno with the carb that's on it. Ran fine no issues. This winter I pulled the transmission for a freshen-up, change the governor and 1 - 2 shift spring and a new stall. Also changed the mufflers to a straight through type. Didn't touch the engine, carb etc at all. I put the trans back in and the first drive was fine.

Took it for a cruise a week ago nothing abnormal but on moderate accel I am now getting a backfire from the exhaust. Cruise fine, idles fine no problems on the highway just a quick stab gets a backfire. I played around with the settings for the accel pump linkage with no change.

Checked plugs, wires etc. Not sure where to start. Ideas? The old converter way to tight for the engine and the governor was shifting too soon. Do you think the trans and increased revs unmasked a problem?

 

[RemCharger]

Could be a million things.

First thing I'd wonder is if the carb has a lean condition because it wants a rebuild.

 

[HEMI-ITIS]

If the carb is not going lean I would try more timing.

 

[71_Duster]

Could be a million things.

First thing I'd wonder is if the carb has a lean condition because it wants a rebuild.

Carb only has 1 summer on it.

 

[RemCharger]

Back to the 999,999 thousand things lol.

 

[Geoff 2]

Could be wiped cam lobe...& many other things.
My first thought though is electrical. Is the ign box getting the full 12v?
If you have another set of plugs & plug leads, I would swap them to see if that helps.

Also, 14* init is waaaaaaaaaay too low for that combo. Try this simple test: engine idling, in gear, loosen dist clamp & slowly turn the dist to advance timing. Idle speed will increase & idle will be smoother, both because the engine 'likes' it....

 

[capt46]

Just had similar experience on my challenger,
Idles great rev and backfire, drive and accelerate again shuddering and backfire, check alternator mine was very weak noticed gauge was center and when using blinkers dropped, changed alternator runs excellent all troubles gone.

 

[71_Duster]

Update: Timing was at 24 deg initial, I can't seem to find my paper which had the distributor info for what the base was supposed to be at but it sounded high so I back it to 20 deg. I also lightened up the tension on the secondary air door by 1/8 turn and the backfire is gone.

I wasn't expecting anything to happen so I wish I had changed 1 thing at a time so I know what the cause was.

 

[matchek]

I know this was 13 years ago, lol, but when you hit the accelerator hard, is that when it backfired?

 

[HEMI-ITIS]

I know this was 13 years ago, lol, but when you hit the accelerator hard, is that when it backfired?

 

[HEMI-ITIS]

Do you have the same problem?