Holley Street Avenger on 408 / 416

Anyone install a Street avenger on their small block stroker?

I installed a 870 cfm Street Avenger on my 408 and it stumbles horribly. Want to know what modifications others have done to the out-of-the-box carb before I start experimenting with accel pump nozzles and jetting.

408 4spd 3.23

Gonna need a lot more info than that. When is the engine stumbling? Idle, acceleration, WOT? What are the specs on the engine- compression, cam, intake, timing, heads/flow? An 870 could be a lot of carb for a mild 408, all the way up to not enough carb for a hot 408. Auto or manual transmission? Was the engine running right before this carb?

I tried both a 670 and 770 street avenger on the 340 in my Duster. Couldn't get rid of an cruise-to-hard acceleration stumble with either one, and I played around with jetting, pump nozzles and cams, secondary springs, the works. All tuning was done using a wideband A/F gauge, I can see exactly what my A/F ratios look like. Finally went to a 750 DP with mechanical secondaries, no problems after some minor jetting changes. The vacuum secondaries on the Street Avengers weren't fast enough for the transitions I was getting with my 4 speed. But my 340 is pretty far from stock- .060" over, KB 243 hyper pistons, Lunati 60404 cam, duration 276/ 284, Lift .513/ .533. Compression ~9.8:1, 308 heads ported and flowing 264cfm @.500, Eddy Air Gap, Doug's headers, Holley 750 Ultra DP. It runs at 20* advanced at idle, 34* all in, and pulls 9-10" of vacuum at idle.

I have two brand new 670s i am using. They seem to be jetted pretty lean out of the box. The one I had working well but it was pretty rich ran between 10-1 and 12-1 a/f ratio. Tried leaning it out for better mileage and just couldn't get rid of the off idle stumble even with some huge squirters. I think the 670 might be too much carb for the 302 i have it on.

Then I put another one on a 351 engine with giant ports. I got the jets setup pretty decent now but idle and off idle are just horrible no matter what I do to the damn thing.

So I haven't had much luck with these street avengers.

if it does good at WOT id leave the jets as they are and mess around with the squirter and idle circuit first.

SirDan said:

I have two brand new 670s i am using. They seem to be jetted pretty lean out of the box. The one I had working well but it was pretty rich ran between 10-1 and 12-1 a/f ratio. Tried leaning it out for better mileage and just couldn't get rid of the off idle stumble even with some huge squirters. I think the 670 might be too much carb for the 302 i have it on.

Then I put another one on a 351 engine with giant ports. I got the jets setup pretty decent now but idle and off idle are just horrible no matter what I do to the damn thing.

So I haven't had much luck with these street avengers.

if it does good at WOT id leave the jets as they are and mess around with the squirter and idle circuit first.

Click to expand...

Was the 351 running anything like the engine in the video below with off idle stumble?
[ame="https://www.youtube.com/watch?v=oPlHI1SoAbM&feature=youtu.be"]Holley Street Avenger Stumble / Hesitation - YouTube[/ame]

I have an aluminum 570 version slated for my built slanty,hope that's not what
I'll be dealing with,not that it won't be cured.I just have better things to do than chase
away driveability issues outside normal adjustments/changes.
My only experience w/one was a customer w/a chevy 350,comp cams,world prod.
heads etc. he had stumble/flat spot, then over rich on the top end. The float levels out of
the box were way off, pri. side too low, and sec. side too high, after setting them correctly
it ran perfectly,backed up by my wideband FAST A/F setup which also showed the lean cond.
on initial accel. and the rich cond when the sec.s opened(10.8:1 !!).
Hope thats helpful to you, is there anything else that changed when you did the carb
install? Sure no vac. leaks etc.?

I don't think the street avengers are bad carbs at all. One thing to look out for on them is how the lever arm contacts the pump cam, some of them need some adjustment as they can be a little loose from the factory, which can lead to a lean spot on acceleration because there's a small delay on the accelerator pump shot. Doesn't seem like that's a big issue from the video. With my wideband I was able to get both the primaries and secondaries tuned in great. I was just never able to get the transition between the two quite right, and I tried every secondary spring, pump cam, and nozzle I could think of. But, after talking to a few people it seems like the issue on my car was the vacuum secondaries. With the engine I have, and the 4 speed, the mechanical secondaries are really the trick. I think that if I had an auto in my car it wouldn't have been a problem. And on that note, when my GT gets back on the road it will have the 670 on it. But its a little milder as far as build goes, and will keep the auto trans.

Back at the original OP, is that your car in the video? Seems to me like its dumping too much fuel for the amount of throttle you're giving it. The secondaries aren't even opening because its a no-load condition there at idle, so its all on the pump cam and nozzles. An 870 has a #40 nozzle, the 770 has a #25. That's a BIG difference. I never went any bigger than a 35 on mine when I was tuning the 770 (it was overly rich at that point). You have more cubes, but your 408 doesn't sound as high strung as my 340 (which is actually 350 cubes @ .060") .

Also, what is your vacuum at idle? The 870 also comes with a 4.5 power valve, which may be too low if you're pulling decent vacuum at idle.

As 72BLUNBLU said in post #2 explain the stumble more in detail (does it hesitate when you just give it a little gas or does it hesitate when you floor it, etc) and a complete run down of your engine build sure would help. If your going to ask a technical question please have the courtesy to reply to questions asked to you

Nevertheless here's a quick run down of things that can cause a stumble.

Lean jetting

Too small of a squirter or a lazy pump cam for the accelerator pump

Too loose a spring on the vacuum secondary's

Improperly adjusted floats (generally set too low which I've seen from the factory)

Every engine is different and can require specific tuning

A buddy of mine bought a 770 avenger brand new and out of the box it was a mess. Floats set way too low. Electric choke set so tight it wouldn't come off all the way until the engine was fully warmed up and by then it had the plugs fowled. Looked like a 5th grader set it up. After I went through it and adjusted everything properly and installed a stiffer spring on the secondary's it ran great