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Can't stop dieseling. 4150DP on 383 Blueprint.

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  • Can't stop dieseling. 4150DP on 383 Blueprint.

    I'm really struggling to get things tuned properly.

    Carb is 4150 DP 700 cfm 0-4778C. Electric choke.
    Engine is a 383 stroker from Blueprint.
    Cam is 230/236.
    700R4 transmission.
    I have Progressive Ignition distributor.
    Tuning with a wideband O2 sensor.

    Generally idles at 10 inHg vacuum, but have managed to get it as high as 13 inHg.

    My main issue is it's dieseling. The engine likes to idle around 950 RPM (which is what BP suggest), but at this high idle it diesels. If I drop the RPM it diesels less, but is too low for the automatic transmission. It doesn't run as smooth and then drops a lot more RPM when in drive.

    It diesels less with more initial timing. I'm now around 18 (BP say 20 has been good). I've also tried all the way down to 12.

    I'm now running a fast idle solenoid so it completely closes the throttle. But even this will diesel.

    I don't have any vacuum leaks, but I do run a very short PVC line into the manifold in the middle of the valley.

    The secondaries are fully closed (well screw backed off and they look closed). Where do I start, what haven't I thought of?

  • #2
    Maybe ignition related? https://www.msdperformance.com/suppo...s/#EngineRunOn (MSD Troubleshooting - Engine Run-On)
    May God's grace bless you in the Lord Jesus Christ.
    '92 Ford Mustang GT: 385"/6.3L SBF, Dart SHP 8.2 block, forged steel crank & rods, forged pistons, Trick Flow T/W 11R 205 CNC comp ported heads, 12:1 comp ratio, 232-244 duration/.623" lift/114 LSA camshaft, TFS R-Series FTI comp ported intake, BBK 80mm T/B, Holley Dominator MPFI & DIS, Holley 36-1 crank trigger, MSD 1x cam sync, PA PMGR starter, PA 200A 3G alternator, Optima Red battery, 100HP progressive dry direct-port NOS, R134a A/C, Spal dual 12" fans, Frostbite 3-core aluminum radiator, Pypes dual 2.5" exhaust, off-road X-pipe, shorty headers, Earl's -6AN fuel system plumbing, Walbro 255 LPH in-tank pump & Pro-M -6AN hanger, S&W subframe connectors, BMR upper & lower torque box reinforcements, LenTech Strip Terminator wide-ratio AOD, 3000 RPM C6 converter, B&M Hammer shifter, Stifflers transmission crossmember & driveshaft safety loop, FPP aluminum driveshaft, FPP 3.31 gears, Cobra Trac-Lok differential, Moser 31 spline axles, '04 Cobra 4-disc brakes, '93 Cobra booster & M/C, 5-lug Bullitt wheels & 245/45R17 M/T Street Comp tires.

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    • #3
      I've put a multimeter on the coil and it goes to zero almost instantly. But maybe the multimeter doesn't show it quick enough. The engine is not so much running on, but usually running backwards. I'll look more into this though. I have Accel 0416S-4 spark plugs. Maybe need to go colder?

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      • #4
        What's your cranking compression and what octane fuel are you using?

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        • #5
          They say 10.0:1. I did a full tank of 91 with octane booster. So I should be OK there.

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          • #6
            What's your idle speed with the solenoid disconnected, 950 and then 500?
            If your carb is truly completely shut, the engine should not idle.
            Pull the carb, check the throttle plate to bore fitment, since they may need to be recentred. Gary
            Regards, Gary

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            • #7
              I would check the anti-siphon parts.
              1. Could be under the shooters.
              2. Funky red valve thing on pump plate.
              On behalf of every manufacturer, delivery drivers, supply chain representatives, and workers of the world, "We're Sorry."

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              • #8
                I would advise you to put a multimeter on the coil, and see what it shows when the engine is running under load and when running at cold speed. So if you have an idle speed with the solenoid off, 950, and then 500, you should remove the wall work carburetor, check the throttle valve to fit the hole. So at least we can conclude that it works wrong.

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