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

[RemCharger]

Forget the Holly calibration crap and learn how to do it yourself if you don't already know how to.
I am not sure if that carb has a power valve or not. If it does I would start there.
This day and age I don't know why anyone would want to mess with the carb crap.

It has step up rods for enrichment.

When they finally build something that works as good or better than a carb.. might be worth looking into.

 

[mopar 3 B]

A mate put one of these on a 400 Pontiac & it always bogged when the secs opened. Rang Holley tech [ expensive ] & they had no idea. Carb came off, QJ went back on, no more bogging....
Is this bogging on the primaries...or when the secs open?
If just on the primaries, bigger acc pump shot may help. In my opinion, not using an air valve dashpot [ saving $$$? ] like the TQ & QJ, is the main reason why these bog when the secs open.

TQ has way bigger secondaries. More air, more fuel plus you don't have to deal with oversized primary feed. Your 383 is a quicker rev engine that a 440. So forget everything you learned about it when building a 383.
Big and heavy internals are not good. Heads have to breath which maybe the first problem with the intake choice. Plus when the 383 wants fuel it wants it now not latter.
I forgot is this an automatic or manual car? You may even have the wrong torque converter installed or wrong gears in the rear. All can cause bogging.

 

[mopar 3 B]

Sorry was not trying to direct the above to any one individually. Just the way my mind works at times.
Been building my own 383's for 50 years and have learned a thing or two.
Will I build a 440? Only because that's what everyone else wants to see.

 

[Sam69sat]

 

[Geoff 2]

Post #42 is full of erroneous & incorrect info.
Firstly, the TQ secs are NOT way bigger than the SD secs. They may actually be smaller in combined area because the section between the secondaries on the SD has been removed, allowing for a lot more area.
Nothing to do with engine size, gearing, engine size or torque c'ter. All of these parameters are part of the ign & carb tuning process.

 

[440 PHIXX]

I assume you have a new fuel filter. I was a having an intermittent stumble in my 383. A new fuel filter seems to have cured it.

 

[dvw]

Stumble on tip in can usually be related to to things. Not enough intial timing. Or lean fuel mixture. When I say lean fuel mixture we are not talking jetting. These carbs have a pretty responsive looking booster. Should be easy to get it right. Either the secondaries are opening too quickly or the squirter timing is late/lack of squirter volume. I'm assuming this carb is fairly new and clean with all factory adjustments. It is basically the same as a Thermoquad. Zip tie the secondary air door so it can't open. Does this fix the bog? If so tighten the air door spring. If no? it probbaly needs a larger orifice squirter. Here is some info on these carbs. Though a far different application.
https://documents.holley.com/newchevy.pdf
Doug

 

[Geoff 2]

Not sure if these carbs have a removable link between the pri & sec like the AFB/TQ. If so, a good idea to disconnect the link & work on the primaries. If no bog with primaries only, then obviously the problem lies with the secs.

 

[AR67GTX]

To the OP, have you simply tightened the air-door?

Excellent point - yes it does have a spring tensioned secondary air valve. Tighten it up a couple of turns and see if the bog goes away. If so, back the spring off a half turn until the bog starts to reappear, the tighten it 1/4 to 1/2 more turn and tighten it down.

 

[coloradodave]

As always, Doug(dvw), the voice of reason...Very refreshing!

 

[oldkimmer]

Excellent point - yes it does have a spring tensioned secondary air valve. Tighten it up a couple of turns and see if the bog goes away. If so, back the spring off a half turn until the bog starts to reappear, the tighten it 1/4 to 1/2 more turn and tighten it down.

I would only go about a 1/2 turn at a time as u wouldn’t want to break that spring. Sometimes a little does a lot. Kim

 

[HAHNAGON]

To the OP, have you simply tightened the air-door?

Yes. I have. I have tightened the secondary air door(counterclockwise) about 4 1/2" turns and I'm afraid to go much more because I've seen some people break those springs. Tightening it did help a little bit but did not solve it. I'm going to try another 1/2-1 turn probably.

 

[AR67GTX]

Rather than tightening it further, try zip tiring it closed as DVW suggested and if it still bogs then it’s not the secondary circuit. Just for info, I’m rebuilding a 1970 AVS and the rebuild instruction call for adjusting the air flap by tightening the spring until it barely closes the flap, and then tighten it 2 more turns. You sound to be well past this point.

 

[HAHNAGON]

Rather than tightening it further, try zip tiring it closed as DVW suggested and if it still bogs then it’s not the secondary circuit. Just for info, I’m rebuilding a 1970 AVS and the rebuild instruction call for adjusting the air flap by tightening the spring until it barely closes the flap, and then tighten it 2 more turns. You sound to be well past this point.

ok. I agree. Thanks for reminding me. I will try that.

 

[HAHNAGON]

Couldn't get the stumble to go away. Removed Street Demon and installed Edelbrock. Problem solved. Oh well, I really wanted the street demon to work but its just not going to work 100% correctly. I can finally chirp the tires. Big difference.

 

[Geoff 2]

It is called 'Demon' for a reason......
You now have a good carb on the engine.

 

[HAHNAGON]

It is called 'Demon' for a reason......
You now have a good carb on the engine.

Haha.......yeah. You are right. I knew better, but someone sold me on it and I thought I would give it a try.

 

[1STMP]

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.

View attachment 1577736

Maybe try a different rate
accelerator pump cam?

 

[HAHNAGON]

Maybe try a different rate
accelerator pump cam?

They don't have those on the street demon.

 

[69_A12_6BBL]

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.

View attachment 1577736

Check ACCELERATOR Pump, and what is your TOTAL timing?