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Thread: Stumble and back fire through throttle body

  1. #21

    Default Stumble and back fire through throttle body

    Make sure you take off all your fittings that you bought for the unit, and take off any carb cable attachments, you might not get them back !! That happened to me when I sent my TB back they just swapped it out for new one, then I had to find the parts again that I put on it....jsut a heads up

  2. #22

    Default Stumble and back fire through throttle body

    We already thought of that, thanks.

  3. #23

    Default Stumble and back fire through throttle body

    My EFI unit was returned yesterday. MSD found nothing wrong with the unit. I see there is an update for the hand held. I will, of course install it. One thing I was told by a MSD tech is that we were supposed to do the throttle blade adjustments with the engine at temp and the vehicle idling in gear. Because we didn't adjust the throttle blades this way is why, he said, was the reason my truck stalls and backfires threw the throttle body at any given time. I didn't see this in the installation instructions. You would think an adjustment as important as this would have been spelled out in the instructions. I will have my mechanic reinstall the unit and reprogram the computer this weekend. Stay tuned for on the seemingly never ending story of the MSD Atomic EFI vs. the Ford F250.

  4. #24

    Default Stumble and back fire through throttle body

    We reinstalled the throttle body, module and hand held along with updating the software. With a few adjustments my truck runs the best it ever has. With that said, I believe MSD did, in fact, replace something. I say that because the timing was different as well as the IAC numbers. What ever they did, fixed all the problems. A big thanks to the guys at MSD.

  5. #25

    Default Stumble and back fire through throttle body

    I took the truck out for a little spin today. It didn't run as well as it did yesterday. Still wanted to cough through the throttle body, but not as bad as before. Any suggestions?

  6. #26

    Default Stumble and back fire through throttle body

    Too little adv or too lean will cause intake backfiring.



    Increase pump shot and decrease power valve about 5% each and see what happens. If adding fuel changes it to a real pop instead of just a rumble set both back and add 2 degrees timing across the board - either by going up in total adv two - or down 100 on both idle adv RPM and total adv RPM...

    J Wm Bishop, ASADE

    Racing without turns is such a drag...

  7. #27

    Default Stumble and back fire through throttle body

    What are the timing settings?

  8. #28

    Default Stumble and back fire through throttle body

    A/F targets:

    Idle 14.0

    Cruise 14.0

    WOT 12.8

    Boost 11.9



    Timing:

    Idle RPM 725

    Total RPM 2500

    Idle Advance 10.0

    Total Advance 34.0

    Vacuum Advance 10.0



    Pump Squirt 25%

    Power Boost 19%

  9. #29

    Default Stumble and back fire through throttle body

    With what I just posted, what changes should I make?

  10. #30

    Default Stumble and back fire through throttle body

    Are you sure your not hearing/feeling ping? You are timed very aggressive out of the gate and leaving it unadvanced where you need it most!



    Here's what my old notes say on the 429 (heart shaped combustion chamber).



    Total Adv RPM 3000-3200 (<9.0:1 comp ratio - 3000, 11.0:1 - 3200)

    @ RPM 34-38 deg (9.5:1 comp ratio is 38, 11.0:1 is 34)

    Idle 10-14@750 (270 cam duration - 10, 280 - 12, 290+ - 14)

    curve start 100-200 RPM above actual idle speed.



    Using that and not knowing your comp ratio and cam specs I would start conservatively on 89 octane with:



    Timing:

    Idle RPM 1000

    Total RPM 3200

    Idle Advance 10.0

    Total Advance 32.0

    Vacuum Advance 8.0



    Give it a chance to learn - just a quick half throttle drive around the block will help. Then do all tuning step by step to make sure one thing out of whack is not effecting another!



    Step one (max advance) - increase total advance 2 degrees at a time until it pings at any RPM during WOT, then back off two.

    Step two (curve advance) - decrease idle adv RPM 50 at a time until it pings on off idle stab - then add 50 back. (repeat step one if desired to narrow the curve)

    Step Three - initial timing - increase idle advance two degrees at a time until it lugs over when hot starting (or idle vacuum drops off which ever occurs first) then back off two.





    Then you can play with AFr to get the targets where it works best for ya - just keep in mind over lean can make the same feel as over advanced.



    When tuning AFr do one or two parameters per day for best learning around the targets (giving about 25 miles per change to learn). Start with WOT - get a plug check and make sure you are NOT too lean (sb around 12.8-12.2), then set idle to leanest smooth (sb around 14.8-15.2), then set cruise, then PV and then pump shot.

    J Wm Bishop, ASADE

    Racing without turns is such a drag...

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