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BIG BLOCK ENGINE STUMBLES ONLY UNDER QUICK/HARD ACCELERATION???

HAHNAGON

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Location
MOUNT VERNON, IN 47620
I am currently having an issue with a stumble or backfire only when in drive and when I try to accelerate quickly and stab the gas pedal. Engine stumbles, backfires and sometimes dies. My choke actually slammed shut once and I had to remove air cleaner pry it open on side of the road. Ok.....so it is a rebuilt 383 bb with very mild cam, aluminum edelbrock victor intake(because I already had this intake lying around), New Holley street demon 750cfm carb from Holley, new fuel pump, sending unit, tank, new oe style distributor and other distributors, etc. Restoration was finished this fall and everything is new. Current Distributor is a new factory style magnetic pickup with a Miller Ignition CD ignition box(like MSD 6al) and ronfrancis express wiring kit, super conductor wires, champion plugs gapped arouond .040, mopar performance coil(basically msd 2 with mopar sticker). At first I blamed timing because timing was advancing too much so I did disassemble distributor and weld some material in the advance slots to limit that a bit?? I also put 2 light springs in distributor, instead of one light and one heavy. Timing is currently set at 12 initial, 36-38 mechanical and I have plugged off the vacuum advance for now. I have tried adding timing and retarding timing. Engine runs very strong when in park. I can rev it to 6k all day and not have any issues but when I stab the gas under a load in drive it backfires or stumbles. I have also tuned with a vacuum gauge for max vacuum. Adjusted idle mixture with vacuum gauge more than once and reset to TDC on #1 and set timing again and again multiple times in hopes I missed something. I have also tried adjusting the accelerator pump linkage to notches 1, 2 and 3. Plugs look good and maybe a little on the rich side but my first guess is its not getting enough fuel but i find it hard to believe that a very mild 383 isn't getting enough fuel from a new 750 holley carb. Any recomendations? Discharge nozzle? Jet change? Or something else? Thank you.

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Chokes do not slam shut once open if working correctly. What is the fuel level set at? Was the cam degreed.
 
Recommendations? I would run an NGK XR5 plug gapped at .035 to start with. And go from there.
 
Chokes do not slam shut once open if working correctly. What is the fuel level set at? Was the cam degreed.
Yeah, I hadn't had that happen before. I didn't think so. I was thinking about removing it and then take it out for another drive because that shouldn't happen. Carb came new from holley and says "it is calibrated from the factory and should NOT need adjustment"...? but I definitely had to adjust idle mixture and I have tried adjusting the choke.
 
Victor intake has no heat crossover. Not atomizing fuel correctly, wrong intake for a mild engine.
 
I'm not familiar with Street Demons but what about the initial fuelshot?
Yeah, I read a different "discharge nozzle" might help? I increased the accelerator pump rate at the linkage but it didn't help much, if any. I just find it hard to believe it needs more fuel. Especially with the plugs closer to black than gray or brown. I've had 440s with big cams, heads, etc and 750 carbs worked fine on those.
 
Victor intake has no heat crossover. Not atomizing fuel correctly, wrong intake for a mild engine.
Yeah, I know. I had it lying around and didn't want to buy another one because I was planning on pulling the 383 and swapping to a 392 this summer.
 
2 light springs will let the advance come in quicker than your original setup, and 38 degrees timing is a bit much imo. These 2 things have nothing to do with your stumble, but I would think your are hearing a bit of detonation?
 
Yeah, I hadn't had that happen before. I didn't think so. I was thinking about removing it and then take it out for another drive because that shouldn't happen. Carb came new from holley and says "it is calibrated from the factory and should NOT need adjustment"...? but I definitely had to adjust idle mixture and I have tried adjusting the choke.
So with no heat crossover and cooler ambient's, hesitation and carb backfires will be pretty common. Factory set does not mean anything. You have to check everything and fuel level can be critical. I assume you have power feeding the electric choke?
 
You restored the whole car.
Why not get the right intake?
Yes. As mentioned above....I have a 392 crate engine kit that I am going to upgrade to, hopefully this summer. I went ahead and did a mild build on the 383 so I could drive it, move it around easier and dial in the new adjustable suspension. I figured I could always sell the 383 when I'm done with it and not lose any money.
 
Sounds like an accelerator pump problem but not sure exactly where. I see this Holley is a quasi-Carter AVS. I’m not sure how the accelerator pump bowl is fed, through a small check valve port from the bowl or a “V” notch in the side of the pump well wall or something else. I once had trouble with a Edelbrock AVS type with the V-notch in the AP well that the pump height setting had to be increased to get the pump skirt above the bottom of the notch to let gas in. Once I did that the stumble was gone. If it’s the former with the small check valve port then I would probably assume since it’s new it’s probably OK. I would try a bigger shooter nozzle and see if that helps. Keep the accelerator pump link on the #1 hole until you get it working right and then switch it back to #2 and see if it still works OK. Every time you change positions on that link, the pump height needs to be checked and adjusted to maintain its setting.

In case you don’t have this forreference.

https://documents.holley.com/lit701rev10.pdf
 
Once the intake is up to operating temperature, it will not be an issue.
If it was, every car at the track would be backfiring/ stumbling/ dying right at the line.
Drag racing wouldn't be fun to watch anymore.
 
So with no heat crossover and cooler ambient's, hesitation and carb backfires will be pretty common. Factory set does not mean anything. You have to check everything and fuel level can be critical. I assume you have power feeding the electric choke?
Ok. Yes. 12vlt ignition signal(from fuse panel)to choke(not from coil) and ground.
 
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