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Thread: Stumble and back fire through throttle body

  1. #1

    Default Stumble and back fire through throttle body

    I had the Atomic EFI as well as the MSD Ignition installed on my 1975 Ford F-250 429. The problem is, it runs OK some of the time, but wants to stumble and backfire through the throttle body at no particular time. My mechanic has spoken to several MSD techs and seems to get a different answer. Now we're told to drive the truck and let it "learn" the system. How long does this thing need to "learn" the system?

  2. #2

    Default Stumble and back fire through throttle body

    JimB



    Just as Cayenne06, it sounds like a possible timing issue. Try an initial timing setting of 15 degrees @ 1000rpm.



    X


  3. #3

    Default Stumble and back fire through throttle body

    What's the best way to do this? Sould I wait until the engine has come up to temp, or can I reset these numbers when it's started and cold?

  4. #4

    Default Stumble and back fire through throttle body

    Is the system set up to allow the Atomic to control timing?

    If so, the timing can be changed at any time using the handheld unit.

  5. #5

    Default Stumble and back fire through throttle body

    Yes it is. I'll go and try that. Thanks

  6. #6

    Default Stumble and back fire through throttle body

    I did as you suggested, but it didn't seem to help. The idle seems too fast and iac went from 0 to 36. That doesn't seem right.

  7. #7

    Default Stumble and back fire through throttle body

    what is your idle vacuum (and cam specs) sounds like you could have a vac leak... or valve problem (bouncy needle on vac gauge). May even run a line and put the vac gauge under you face while driving to see if one of the thermal vac switches is acting up intermittantly. The 70's were rough for that with all the hoses on the smog stuff...

  8. #8

    Default Stumble and back fire through throttle body

    No smog, no vac. I was told when I bought the truck it had a mild cam. My mechanic told me he thought it was an RV cam. With that, we set the cam setting as "mild".
    I've had the MSD ignition installed. I've come to the conclusion that I just need to drive it and see if the system learns the settings.

  9. #9

    Default Stumble and back fire through throttle body

    Okay, no smog eliminates a lot of variables.

    What I was asking is how much vac does the engine make - an RV cam (and I kind of dislike using such terms as they aren't really detailed - in an RV with a 400" SBC the cam is the pretty close to the same as a 350 Passenger High Perf - doesn't really give an idea of what is really happening - I understand actual profiles better and actual idle Vac tells a bit more) will make about 16-17" vac at 750 RPM - the more aggressive the profile the lower the vac at idle (for example my 383 makes 12 at 750, with the 7450 Holley I had to idle up to 950 to smooth out idle lope and get 16" for the power brakes - so I also run a 1200 stall converter - gonna get her lit up on the Atomic this weekend).



    Typically when the symptoms appear at no particular time - you have more than one issue to address - the point where multiple issues intersect at worse condition is where the symptom shows up - so you end up chasing a moving target.



    Would hate to see your system learn parameters for a vac leak, flat lobe, etc. - it will do it - but you leave power and economy on the table and leave hungry! Spitting up the throttle is a sign of lean, high back pressure, under advanced timing, or under scavenging (flat lobe) the cylinders. All of which show up to an extent on the vacuum gauge!


  10. #10

    Default Stumble and back fire through throttle body

    OK, thanks. I'll pass this on to the people who installed my system.

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