Exhaust Backfire

[jb427]

Looking for help on chasing an intermittent exhaust backfire at idle (haven't driven the car yet as this is a new install). I have a BBC with HP ECU. I'm virtually certain it is not an intake leak. Timing is 30° @ idle (both on timing light & ECU). Air/Fuel Ratio appears to be hitting targets (14.0 at idle). This is very intermittent. Any suggestions appreciated. Thanks.

[Danny Cabral]

Not enough information. What does your ignition system consist of? You'll have to record & review a short datalog (Overlay it onto your Global File) of this occurrence to determine what's wrong.

Originally Posted by Danny Cabral Quick Datalogger Information: Always provide data, not just symptoms. Is the AFR & ignition timing optimized? Overlay the datalog onto your Global File. Ensure the Target A/F Ratio table is properly programmed for your engine. Ensure the ECU is "Learning" (Learn Table LINK). Scroll through the datalog to the problem area. Then look exactly where this happens on the Base Fuel & Timing Tables. Scrutinize: RPM, MAP, MAT, TPS, CTS, IAC, Target AFR, AFR Left, CL Comp, Ignition Timing, Duty Cycle, Fuel Flow, Fuel Pressure & Battery. Using the Holley EFI software (LINK), look at where the live cursor moves to during this occurrence, and manually tune that area of the Base Fuel Table (enrich or lean) & Base Timing Table (optimize ignition timing). I find the most helpful datalog function, is overlaying the datalog on your Global File. I'm in the habit of using the datalog Overlay feature every time I review a datalog. Click on "Datalog" (on the top Toolbar), "Activate Overlay", then "Open Data Log". You can literally "playback" a recorded event, and watch it as it happened on any screen. Minimize (shrink) the datalog window, and move it to the bottom of any Global File screen. Then click & scroll anywhere on the datalog, and watch it playback on your Global File (EFI software). http://documents.holley.com/techlibr...9r10543rev.pdf (Holley EFI Datalogger Instructions) https://forums.holley.com/showthread...s-amp-Datalogs (Datalog & System Log Information - Read "NOTES") https://www.youtube.com/watch?v=iqHO...C6FEA0BB99AF17 (How To Record & Email A Datalog) https://www.youtube.com/watch?v=q7OZUXLRS1Q (How To Record A System Log & Diagnose A No-Start Condition) https://forums.holley.com/showthread...7531#post77531 (Closed Loop Datalog Tuning - Posts #2, #4 & #6)

[Kirsh]

If you use an MSD magnetic pickup, check polarity.

[Gaz64]

Have you checked rotor phasing with a modified cap with a hole drilled in it, and a timing light? Gary

[Danny Cabral]

To ensure the distributor's rotor-phasing is correct:
http://documents.msdperformance.com/8644_tb.pdf (MSD Rotor-Phasing Document)
MSD has a good video on why it's important:
http://www.youtube.com/watch?v=aWMlNwGW0tM (MSD Tech Rotor-Phasing Video)

For naturally aspirated engines, the rotor is usually phased halfway between minimum & maximum amount of timing advance used.
For forced induction engines, the rotor is usually phased at maximum boost retard, because that's when cylinder pressure is the greatest.