General Information on Exhaust/Intake backfire

[Phares]

My pickup has a pretty nasty exhaust backfire that just started since I repaired the EGR pipe, did a valve job, and rebuilt/adjusted the carb. I also set the timing. I am trying to gather intel on the generic causes/effects of system backfires. I am hoping the more intelligent theory smart characters in the group might provide input here for everyone's benefit. So thanks in advance. Information on my issue can be found in my truck thread.

 

[krzyabncanuck]

When is it back firing ? Is it just after letting off the gas after giving it lots of gas ? Or all the time ?

 

[Phares]

All off throttle applications (Shifting Gears, letting off gas) and when turning off the engine.

 

[mtweller]

Edit - I reread the OP.

Check for vacuum leak first and foremost.

R

 

[Phares]

I replaced the fuel filter and all vacuum hoses while I had the carb out. I'm really thinking the problem is in the emissions (EGR) stuff.

 

[Phares]

How does a catalytic converter work? And why would that cause backfire?

 

[kulangot]

Sounds like a lean condition backfire to me…could be caused by a vacuum leak.

Your running lean, adjust the mixture.

Look at the plugs - they will tell you what is going on.

Search google images for pic of lean vs rich plugs.

 

[krzyabncanuck]

Did you use an AFR gauge to properly adjust your carb ?

 

[mtweller]

What's your wideband O2 sensor reading

R

 

[kulangot]

go pick up a used 5 gas analyzer and dial it in with that

 

[kulangot]

http://www.ebay.com/itm/Gunson-Gast..._Automotive_Tools&hash=item4159e73608&vxp=mtr

 

[Phares]

Spent about a half hour dialing in the idle speed and mixture. No more back fire. I had to readjust the timing once I got the idle set and the engine up to temp. Thanks for everyone's help. Especially Kul. He got me on the right track.