I recently had some of the same problems with a Holley 600 DP and went through all the steps you have and never did get the thing to work to suit me. Mine didn't backfire but had a bad bog right off idle when I jumped on it hard, otherwise thing worked fine.

Finally fixed mine Replaced it with a Proform 750 DP and now my little 350 screams.:thumbsup:

Good luck with yours.

 

Try alittle more initial timing and use a hollow squirter screw.

 

The brass nozzles aren't all that necessary. On squirters and cams: smaller nozzles deliver stream over more time, bigger ones over shorter time. Cams will directly affect how quickly the shot comes in, put a bunch together, pass a finishing nail through the #1 hole on em and use a bolt to hold all the cams to the same pivot you will see the differences from right to left. Check that your stock black cam isn't actually the dark green one. They look similar.

Fix a stumble by first feeling real good about the rest of the operating range. Good cruising mix (primary jets right), proper idle mix, and strong mid RPM acceleration (secondaries coming in at right time, proper Power Valve size). If no wideband then plug colors and what you feel will tell you. What I've learned is that those fundamentals need to be right or it will seem like no shooter combo seems to work. 31 squirter on a 350, that's a good size already. If you're trying at 36 and still no dice you might want to think about the other aspects of the carb as stated.

To give you a reference, my 383 settled on really liking a 28 shooter, 75 jet and the red cam on my Holley 770. The red cam smooths out the shot and lets it linger- I had a "rich" bog which is the more unusual kind. You might try the pink cam, it's more aggressive.

This stuff requires tremendous patience and it's time consuming. I always removed the carb rather than attempt shooter changes on the car. I couldn't risk fumbling one of those little washers or something down the manifold as my luck would have it.

 

This stuff requires tremendous patience and it's time consuming. I always removed the carb rather than attempt shooter changes on the car. I couldn't risk fumbling one of those little washers or something down the manifold as my luck would have it.

Man that's the truth. Every time I changed my shooter I was SO paranoid that I'd drop something down the carb. You'd think I was doing brain surgery how careful my movements are.

Good advice from Steptoe - "Return the accel pump back to factory and sort the backfire issue BEFORE going any further."

 

Like Everett , suspect maybe not enough advance at the rpms which u are dropping the hammer
So what rpms does it bog?
by upping the initial a few degrees and if it goes away , is a good test, but not a soln
putting up the initial will also increase your total, and if that has been dialed in then u will loose power at the higher rpms..
Soln in this case is dropping a lighter spring in..
your mixture screws will not change anything... the work on the idle circuit, once the car is moving then u are on the power/ cruise circuits.

lever set to 0.015" OK. (Feeler gauge is kinda tight at WOT)

15 to 18 so should be loose...to tight and one can get a bog...

And lean is a backfire thru the exhaust not the carb.
If u have flames out of the carb,,, u must also have something to ignite them...and to ignite them u need a flame source, and the only flame source is in the combustion chambers....which means u have a valve open when firing on a cylinder...and that could be anything from a valve leaking to a couple leads around the wrong way or timing way the hell off
And I believe that is where your issue stems from....
messing with accel pumps squirters etc u will end up chasing your self around in circles an still not fix..
Return the accel pump back top factory and sort the backfire issue BEFORE going any further

 

I was SO paranoid that I'd drop something down the carb

Thats good.. paranoid enough close the dle speed screw, wire the throttle butterflies closed AND drop a rag down each bore?
If not.. not paranoid enough..

 

Really? So all those pops and spits and fireballs in the wintertime aren't from running with no choke in 10 degree weather? Lean backfires most certainly do come out of the carb. My eyebrows taught me that a long time ago.

Hmmm...wife tales just dont go away....in spite of basic automotive practice engineering
end of the day its your engine m8 you have the last call
Dump a data logger on.. NOT a bloody AFR meter... because it is too slow and will not registrar the info to play back later... THAT will tell u exactly what is happening rather than the hit in miss process u are currently banging your head against

 

I'm not sure what wives tales you're referring to. If you start up a car in cold weather without a choke you often get a backfire out of the carb. It's caused by being too lean, not from having plug wires crossed or the valves magically out of adjustment or the timing moving by itself and then moving back when the motor warms up. If you back off the accelerator pump adjustment you'll get a lean sag under acceleration. Back it off too much and it'll pop out of the carb. Fix the pump adjustment and it goes away. What causes it? Being too lean. I don't know the theory about why, the results are more important.

A quote from an old Q-Jet book...."When you are tuning, watch the results. If the results disagree with your theory - believe the results and invent a new theory."

 

Wow! Good theories about what to take a look at. I'll check the timing and for vac leaks, float level, etc.

I'd think if the timing was off enough for a valve to be open during combustion, the engine would barely run straight - it would basically be open on stroke 1 or 4 and don't see how the engine could even run under an open valve condition.

I'll take a good look at timing and let you know where I'm at with initial and total.

About the accel pump lever, thinking about it, the 0.015" feeler gauge is tight at WOT. I tried adjusting the lever screw with the spring, but the lever always seems tight with the feeler gage. It's like the lever wants to spring back really hard at WOT.

Thanks, guys!