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Thread: Stumble and backfire when cold

  1. #1
    Join Date
    May 2018
    Posts
    6

    Default Stumble and backfire when cold

    Jeep CJ7 AMC 304 V8
    Edelbrock Performer Intake
    MSD 6AL ignition, MSD Distributor for AMC
    Headers, Flowmaster exhaust
    Aeromotive fuel pump in tank, with return and regulator.

    Before the engine reaches temperature when trying to climb a hill to get out of my neighborhood the Jeep stumbles, loses power and then back fires through the intake. It feels very weak.

    If I wait until the next day, and then manually set the mixture to a richer setting of 11.5, it runs fine and will pull the hill with no issues. Not a hint of a sputter and no backfire. Of course then I have to manually back the settings to normal.

    Atomic firmware has been updated to 1.2.12

    Settings:
    Street Stock cam
    Non-PWM with Regulator
    Air Pump No
    Idle RPM target 825
    Rev Limit 5000
    Timing Control Disabled

    A/F Settings
    Idle 13.3
    Cruise 13.0
    WOT 13.0
    Nitrous Off
    Pump Squirt 20%
    Power Valve 24%

    Fuel pressure 43 psi
    IAC at idle when at temp 16
    Engine coolant temp on hand held matches dash gage.
    No vacuum leaks that I can find.

    Any ideas? Thanks!

  2. #2
    Join Date
    May 2018
    Posts
    6

    Default

    I was told that with fuel injection systems, there is a separate, richer mapping that is used when the engine is cold. Is that true with the Atomic? Manually enriching through the hand-held definitely takes care of the issue, but it's a pain to keep doing this.

    Thanks

  3. #3
    Join Date
    May 2018
    Posts
    6

    Default

    It's been a couple of weeks, and I can see that there have been many views of this thread, but not seeing any other responses and I'm curious as to why. Am I missing something? Is the issue that I'm having the way the Atomic is supposed to perform? Have I missed something else?

    Besides this one issue I've been quite happy with the MSD products that I've installed on my Jeep. However, this one item really hampers the overall driveability of the vehicle.

    Thanks

  4. #4

    Default

    Why so rich in idle and cruise? I run these numbers a bit higher. Stoich would be ideal, but at idle I go just a bit richer. 14.5 IIRC.

    Did you try playing with your pump squirt and PV enrich to see if that helped out? Does your jeep have an auto or a manual? If auto, is that IAC count in drive or neutral? If in neutral, you'll probably find you have a bit more IAC in drive and want to bring that down a bit.

    I ended up having to go up a fair bit on both pump squirt and PV enrich, but my car has a single plane intake.

  5. #5
    Join Date
    May 2018
    Posts
    6

    Default

    Thanks for the reply. It doesn't run well at any leaner settings. Low on power, stumbles and backfires when leaner.

    I've adjusted the pump squirt and PV settings and seem to have found a sweet spot. Once it's up to temperature it runs nicely. The only problem is when the engine is cold, and taking the mixture richer takes care of the problem. I'm wondering if it's got something to do with the computer.

    It's a standard shift, and the IAC settings are at operating temperature with the Jeep in neutral.

  6. #6

    Default

    Have you watched the handheld (or preferably, had a helper do that) to see what's going on when you try to climb the hills in these modes?

  7. #7
    Join Date
    May 2018
    Posts
    6

    Default

    Yes, I had my son watch it on the handheld as we drove and the A/F mixture leaned out as the power dropped and it backfired.

  8. #8

    Default

    Are you holding steady throttle or accelerating?

    I apologize if you already know this, but for the sake of tuning I'd like to say it. It's important to calibrate the idle, cruise and WOT targets on their own, separate from the transition settings. So set idle and cruise with the throttle at steady state. Technically WOT too - that's for when the throttle is steady wide open. Once you've got those relatively dialed in, play with the part throttle adjustments.

    What I did was raise pump squirt and power valve alternately to get a feel for what the settings do and how they impacted drivability. Definitely get your son out there and get part throttle tuned on the flats first, so that you're hitting very slight rich spikes rather than lean spikes when transitioning from steady throttle (I did this at 15, 25, 45 mph). Once you've done that, try the hills again and see how it goes. Be mindful of what your right foot is doing when you see spikes in the AFR. I ended up raising PV and PS a lot more than I thought I'd have to, I'm running a decently aggressive 400 small block with nice heads and a mild cam (but much more aggressive than anything you'd put in a 4x4).

    Hopefully I can help you get this fixed over the wire. Good luck!!!

  9. #9
    Join Date
    May 2018
    Posts
    6

    Default

    Thanks for the feedback. I certainly don't claim to be an expert. :-) Hopefully we'll get a chance to work with it over the weekend and I'll let you know how it goes.

    Thanks!

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