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Thread: 496 BBC Hard Start & Low End Stumble

  1. #1

    Default 496 BBC Hard Start & Low End Stumble

    I own a 1966 Impala Convertible with a 496 stroker. I’ve owned the car 20 years, and the engine was built about 15 years ago. This year I decided to upgrade to an EFI. A local “speed shop” told me the MSD Atomic EFI system was the way to go. They installed it, added a new tank with an in-tank pump and a return line. To cut to the chase, it’s been a disappointment. The engine struggles to start. It cranks just fine but struggles to fire off. Most times it takes me three or four tries to get it to run. When it finally starts it runs strong at WOT, but stumbles anytime I’m off, then on the gas again. I’ve had it back to the shop four times. Each time they say it runs great and fires right up for them. They are a new shop in the area, and aren’t building a great reputation.

    I’m looking for advice in troubleshooting the starting issue, and the low-end stumble. I’ll post my numbers in a follow-up message. I just want to get the ball rolling here. Now for the numbers. Let me know if you see anything out of whack.

    ATOMIC EFI DASH
    TESTING DATE - 8/21/2019
    ECT - 172
    IAT - 139
    TPS - 0
    MAP - 16.3
    FUEL PRESS - 42
    BAT VOLTS - 13.4
    A/F RATIO - 13.5
    INJ DUTY - 4
    IGN TIMING - 0
    IAC POS - 28
    THROTTLE - CLOSED
    CLOSED LOOP - ON
    LEARNING - ON
    FAN 1 - ACTIVE
    FAN 2 - ACTIVE
    DECEL FUEL - OFF
    IDLE COND - ON
    FLOOR CLEAR - OFF
    REV LIMIT - OFF
    NITROUS - OFF
    DTC COUNT - 0
    CAMSHAFT TYPE - PERF
    FUEL PUMP TYPE - NON PWM W/ REG
    IDLE RPM TARGET - 825
    REV LIMIT - 5500
    TIMING CNTL - DISABLED
    FAN 1 - 185
    FAN 2 - 190

    A/F TARGETS
    IDLE - 14
    CRUISE - 14
    WOT - 12.8
    NITROUS - OFF
    BOOST - 10
    PUMP SQUIRT - 18
    PWR VLV ENRICH - 15

    496 Engine Details:
    454 BBC bored 0.060 over with 4.250" stroker crank
    Compression 9:1
    480 HP
    580 TQ
    EFI Distributor

    Camshaft Specification Table
    Part Number 11-246-3
    Engine 1965-1996 Chevrolet 396ci-454ci 8cyl.
    Grind Number CB XE274H-10
    Intake Valve Adjustment 0
    Exhaust Valve Adjustment 0
    Intake Gross Valve Lift 0.552
    Exhaust Gross Valve Lift 0.555
    Intake Duration At 0.006 Tappet Lift 274
    Exhaust Duration At 0.006 Tappet Lift 286
    Valve Timing At 0.006
    Intake Open Intake 31
    Intake Close Intake 63
    Exhaust Open 77
    Exhaust Close 29
    These Specs Are For The Cam Installed At 106 Intake CL
    Intake Duration At 0.05 230
    Exhaust Duration At 0.05 236
    Intake Lobe Lift 0.325
    Exhaust Lobe Lift 0.327
    Intake Lobe Separation 110

    Turbo 350 Transmission

    Any help/suggestions are much appreciated. Thanks, Jeff
    Last edited by Jeff 1966 Impala; 08-22-2019 at 10:40 PM.

  2. #2

    Default

    Changing Cam setting to “mild” helped reduce the stumble, and it runs smoother overall. The starting issue is still there. Took me five tries yesterday. Suggestions would be appreciated. Jeff

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