Please bear with me as I am not a carb guru.
I have a 70 SS 402 , bored .030 over, 290 heads, Crane 133942 cam,.515,.510 lift,Performer RPM Intake,Holley 770 Avenger carb .Accel blueprinted distributor W/points.I've done a search on bogging problems but don't know where to start.
The engine starts,runs, idles great,but if I floor it it will bog to the point it will stall unless I back off the throttle. Only when I mash the throttle not if I depress it smoothly. This happens both when I am in neutral or from a standing stop and sometimes I'll hear a slight pop like a small backfire. From a rolling stop it goes like it was stung by a bee, and will keep pulling through every gear until around 5000 rpm when it seems to stutter a little but then gets through it.
The carb has the factory settings,except I adjusted the choke a little to drop the idle.The electric choke opens all the way when it heats up. It has the silver spring in it for the secondaries.Holley blue pump with regulator and pressure around 7.Fuel bowl level can be seen through the site plugs.
Where do I start?

I would try a set of bigger jets. When you mash the gas it creates a lean condition in the motor. The carb is not providing enough fuel to the motor to let it take off like you want. I had the same problem with my Chevelle 402 .030. I have an Edelbrock, and I changed the springs and metering rods to the next one up. It works great now.

good luck

Ken,the accelerator pump is responsible for covering up this lean condition that Steve mentioned. First check to see if the pump arm moves at exactly the same moment that the throttle lever moves,and it must be exactly. Then with the engine off look into the carb. at the accelerator pump nozzles and move the throttle just a little like an 1/4" and see if you get a squirt of gas.Then close the throttle and then open it to WOT and watch the nozzles,you should have an immediate response and a strong stream of fuel,if this checks out I would suggest a larger pump nozzle this will give you more fuel quicker and this may be what you need. If you dont want to pull your nozzle off before you order, someone on this board or the tech line at Holley can tell you what is stock. Some people drill them out with pin drills until the bog goes away and then order the proper size. Just my .02 cents. Also with the throttle completly closed there will be a small amount of clearance between the pump arm and lever from the throttle shaft,it needs to be correct,but some one else will have to give you the spec's. Good luck.

Good point Mike.

Ken, I would try Mike's way first. If that doesn't work, spend a couple of bucks and pick up some bigger jets. If it still doesn't work you'll just have to buy an Edelbrock. :)

Where is your timing set?

I believe that carb comes with 76's in the rear. Way too lean. I'd go straight to 80's or 82's. Also, you should NOT have any slack or play at the pumper arm at idle. At WOT, you should be able to move the small pivot arm on the pumper housing another .015" -.020". This is to prevent too much stress on the pump diaphragm. So many people get this confused, as Holley's website instructions are not clear on this setting. If you have any initial play, it will certainly stumble.

If I'm reading you right, it sounds like you're going lean on the secondaries. If it's the squirter, it should happen all the time, and it would be immediate. You could try changing the squirter, you could move the cam to the number 2 position, but I'd check the rear jetting first. The silver spring should work fine for the secondaries, but if after the jet change it still bogs, try the next stiffest spring.

Hope this helps, Steve

Steve,thanks for catching and correcting that,I don't know why I missed it I reread the post two times before posting it.

Thanks for all the suggestions guys
I checked the accelerator pump arm and it has no slack at all at idle. As M. Maner suggested, as soon as I open the throttle it moves and squirts gas. I closed it ,then opened to WOT and a steady stream of gas flowed into the carb. At Wot there is a small amount of movement,maybe .015 max.flex.
The initial timing was set at 12. I admit I have never checked total timing or when it all comes in by.
The stock primary jets are 72 and the secondaries are 75 according to the carb spec sheet.Stock discharge nozzle is .025.
I just checked the secondary spring and it is the silver one. However it was not placed over the center post of the cap as the instructions state. It was just sitting in the vac diaphragm housing.

Before you do anything else try bumping timing to 16* or 18*. It will make a big difference. When you said it "popped" thats kind of a dead giveaway of too-little timing. Does that distributor have vaccum advance?

Thanks vince g I'll give that a try first. Yes it has vacuum advance that is adjustable . I set it with a 3/32 hex wrench per the instuctions, 4 turns clockwise from the stop which should be approximately 7 degrees dist. (14 degrees engine) at about 12 " vacuum as a starting point.It said to adjust it further if surging or pinging was noticed at cruise rpm. It was not so I adjusted it no further.

The cam you have will likely need some more initial timing than 12*. Unplug the vac advance (plug the line so no vaccum leak) and set timing 16-18* at idle. Then rev engine and check total timing should be around 38*. When done, reinstall vac. avdance hose. BTW, your timing at idle w/vac adance hooked-up will be around 38* - thats what mine is.

I have a 454 crate motor w/a 4spd. I have the same carb you do - no popping or anything like that. That carb should run good as is unless it was tinkered with. So check out the timing 1st b/c if thats not right no matter waht you do it will never run the way it should.

Report back to us.
Vince

Ken; What they mean by "adjust it further if surging or pinging is noticed" is to back off the timing. You can add a couple degrees at a time, and if it pings, then back off the timing. If that carb does not have a power valve in the rear to supplement those tiny 75's, I'd highly recomment switching them to 80's. I think you will be pleasantly surprised. If you add too much total timing and you're running lean at WOT, it could spell serious detonation. Low speed ping, or lack of, cannot give you any indication of high load detonation.

Steve

I had the same bog on my modified Holley 3310 and the problem was the diaphram spring. Mine only bogged when you jumped on it, take it up slow and it was fine I would buy a quick change kit for about $12.00. It makes spring changes a 2 minute job for testing.

Update;
I reset the initial timing,bumping it up to 16 with the vacuum advance line removed and blocked off(blocked at dist.vac can too). It starts instantly,even better than before. The rpm at idle had to be lowered back down a little. A couple blips of the throttle and it revved up quick, but it still has a slight bog, no where near what it had. Kinda sounds like a Cigarette boat when it idles at 600 in the garage!( Wife yelling, dog barking) I'll have time tomorrow to take it for a ride and see how it runs.I readjusted the vacuum advance to the 4 turn starting point and I'll be able to adjust it when I take it out tomorrow.The Avenger 770 carb has no secondary power valve.I I can get the 80 and 82 jets at the local speed shop.I didn't want to change too many things without seeing the results of these changes.

Sounds good Ken, but try 18* before you do anything else. Couple of guys here are running even more initial (like 19-20*). I'm 17-18*, and your cam is more radical in that setup than what I have.

Vince

Definitely mkae sure you are running plenty of timing. I bolted on the 870 I have that is destined for the 502 motor just to move the car around. Surprisingly, even with 78 primaries, it was not that rich. 74 -75's in the front would probably work a little better. However, even with all that extra fuel available off idle inclding a .040 nozzle, it still has a hesitation from idle. I am inclined to want to dive into the timing issue myself. I am inclined to think, the change in timing from full manifold vac. to mechanical advance is severe enough to cause a misfire and hence the flat spot - not fuel related. I have also heard guys running upwards of 40* total with closed chmaber heads - with 20 built in that would require about 20 initial. I know it seems severe, but it just may be that these motors need all that timing, especially when we add more cam.

BTW Ken just something else that may help. My setup w/17-18* initial will show 38* @ idle once the vac adavance is hooked-up. This may help you adjust that vac can. I have an MSD HEI FWIW.

Ken, you have the same engine I do, but with a little hotter cam. A 25 shooter is way too small IMO. I run in the high 30's, like a 37 or something. If the car runs great once it gets moving, your jetting is a secondary issue (no pun intended) to the discharge nozzle for the off-idle stumbling.

Still, do the timing first. Test it, then if/when it still stumbles jump up in shooter sizes significantly. like to a 35 or so. Test it again, the stumble should be significantly reduced if not eliminated. From there you can fine-tune it.

I'd leave the jetting alone for now.
A bigger squirter and 18-20* base should do it too.
Do you have a higher than stock stall convertor?

I was able to take it for a run tonight and the timing change has made a big difference. I thought it accelerated fast before! The whole front of the car lifts and when I shift it lifts right up again and keeps pulling. It still had a slight hesitation but nothing like before.This is definitely headed in the right direction. I'll try a few more degrees and see if it likes it.
I had adjusted the vacuum advance( Accel blue printed dist) and I had no surging or pinging at cruise rpm or on the highway. The engine does idle rougher, before it had a very smooth idle. I failed to mention it is a 4 speed car.The carb comes standard with a.025 pump discharge nozzle. The Avenger 870 has a .040 and the 570 and 670 have .031?. I'll pick up a larger discharge nozzle and the 80 or 82 jets, but first I'll try a little more initial.
Thanks again for all the suggestions, I am going to try each one.First a little more timing, then a larger nozzle and the bigger secondary jets, cause I don't want any detonation at WOT as mentioned here. The carb does have the quick change spring setup.

Ken, I have a 4spd (454) car too. I called Holley and they said that carb was calibrated for a mild big-block, so no drastic changes should be needed. Remember, different carbs use different jetting (just b/c one particular carb uses x jetting doesn't necessarily mean others will need it). I tried a .31 squirter and I think it likes it but I'm not sure (it didn't make a big difference), so I put the stock one back in. No bogs w/this 454. Overall, the 770 carb works good for me.

Your idle should of gotten smoother w/more timing so something else is up. Are you running ported vaccum? If so, try full manifold vaccum (you will then need to lower your idle). You'll get another pleasant surprise.

Whats your timing @ idle w/the vac advance hooked up?

Had the same problem today with my BB...monkeyed with the carb for two hours thinking it was lean off idle.[bog. pop, stutter]

I had set the timing as my FIRST default...35 total had always been fine. At the end, after I had pulled all my hair out, I bumped the timing and the bog, pop, and stutter went away like magic. Should really have know better...35 worked last year but not this year.

Ken, I Basicly Have The Same Set Up You Have But I Have The 375hp Solid Cam In My Motor,but I'm Also Stumbling Off Idle To Acc With A Four Speed,i've Done Some Of The Things The Other Guys Have Posted For You And It Helped Out So Far,i Have All Msd Ign System In My Car And That Made A Big Difference In Running And Starting, I'm Wondering If Changing The Power Valve From 6.5 To 8.5 Would Take Care Of This Problem, i currently have 12 in of vac at idle, bill

I just fixed a really bad bogging problem as well. It turns out the mechanical advance in my distibutor was way off. I took my car to a place that has a dyno and machine that measures air/fuel mixture by placing a sensor in the exhaust. Turns out my distibutor springs were too light so at low rpms my timing was too advanced, but at WOT there was not enough advance. They recurved my distributor and that alone gave me a 51 hp increase and completely eliminated the bog.

Thanks for all the info guys. I haven't been able to get back to it to run it again yet,12hour days and way too much rain. I will post results of changes, as the timing has made a big difference. I want to try the manifold versus ported vacuum next as Vince suggested to see if that helps more.