It’s a 451 and am running a Holley 950 on it. I’ve tried a number of things but am having trouble with it back firing through the carb when I mash the throttle. This carb ran great on a buddies BBC with similiar cubes and a big solid cam.

For starters, make sure the secondary squirter is not plugged but those need help regardless.........

ITS LEAN...under load.

I like #45 squirters in everything!

What model # "950"?

What's the COMPLETE carb tune?

CR?

Cam specs?

Gear, tire, stall speed?

Launch RPM?

It's a 0-82951.

The engine has 440-1 heads, CR is 11.0 to 1. Pump Gas. Mostly a street car, 4.56 gear, 275/60/15 MT tires, 3800 stall.

I haven't really driven the car much. I am just trying to get the carb to respond right then I can fine tune it, but I feel like I am too far off.

You still haven't given any details of the cam or the carb's current tune. That doesn't make it any easier to try and help.

The 0-82951 is the "Street HP", right? Check the actual venturi diameter and see if it's 1.38" or 1.56" I've seen conflicting specs, depending upon when the carb was made I guess. The tune would likely vary quite a bit depending upon which size it is.

I have no first-hand experience w/ the "Street HP" stuff. I'd hope that they're "flexible" enough to be tuned to work w/ your combination.

You still haven't given any details of the cam or the carb's current tune. That doesn't make it any easier to try and help.

The 0-82951 is the "Street HP", right? Check the actual venturi diameter and see if it's 1.38" or 1.56" I've seen conflicting specs, depending upon when the carb was made I guess. The tune would likely vary quite a bit depending upon which size it is.

I have no first-hand experience w/ the "Street HP" stuff. I'd hope that they're "flexible" enough to be tuned to work w/ your combination.

Hmmm...
-> 440-ish cubes with "big" heads that work better on 500+"
-> moderate CR
-> 260+ at .050" street roller
-> limited stall speed
-> street-tuned(?) carb w/ 1.56 venturi & downleg boosters
...

Starting to look to me like a mismatch of parts, of which the carb is likely one of the "What's wrong with this picture?" items, along w/ the converter being too tight. The carb may be workable, but I'll take a s.w.a.g. that the way it's set up right now is... uh... "wrong".

FWIW, the backfire upon WOT acceleration (IIRC what you posted originally correctly) would point me to verifying fuel bowl levels are high enough & accelerator pump passages & pumps are clear and working properly, and then making sure none of the air bleeds are plugged up (dirt or whatever).

My (which are usually only worth about $.0125) is to tear it down completely and get (buy/borrower) the proper tools to measure all the circuits & orifices so that you KNOW what the current tune is. Post 'em, and get some feedback on what makes sense to change from there.

This is still a pretty basic carb... it can't be THAT effed up.

You have to to in order to have a base tune and to know what needs to be changed to bring it around to your cars needs...........no other way........

We need to talk, too much to type.......... with one finger

-> 440-ish cubes with "big" heads that work better on 500+"
-> moderate CR
-> 260+ at .050" street roller
-> limited stall speed
-> street-tuned(?) carb w/ 1.56 venturi & downleg boosters

Those are my thoughts as well.

1.56 venturi downleg carbs can be a real challenge to "tune the bog out of".

IMO, I think a main body swap to something with a smaller venturi would ultimately yield better results(I'd probably use a 1.400 x 1.750)....... But if I were trying to work around that combo, I would expect to have to run fairly large squirters, and probably a 50cc pump/cam on the secondary side, and I would absolutely ditch the 2.5 pv.
You want to get a head start on the main circuit enrichment when going to wot...... It would get a 6.5 if I were messing with it.

There is a lot of plenum and runner volume in that combo, and not a lot of cubes generating signal.......... It's going to need a lot of help(pump shot) to give the motor something to work with until the main circuit is activated.

Not what you asked, but there is some performance left on the table with that cam, in that combo.

whats the fuel pressure set to...

Kid,

The fuel pressure is 6.5-7 PSI. I run an electric pump with a return style fuel system with -8 lines.

http://www.compcams.com/Company/CC/cam-specs/Details.aspx?csid=739&sb=1

Installed straight up with 1.6 ratio rockers to bring the lift up.
Cam Detail....

Did you degree the cam or did you install it with the dots "straight up" ? If not degreed you should do that now
I would advance it so the intake lobes are from 107 to 105 max lift after top dead center
I've ran a very similar cam (260@.050 int., 266@.050 ex. ground on a 108 LC installed two degrees advanced on the intake lobes)) Comp solid roller with 1.6 ratio rockers, no bog even with a six pack set up after I had to sort it out.
I had another bracket motor with a Holley 80496 950 HP on a TM7 intake with ported 906 heads and a bigger Crane solid roller and it never bogged ever, that cam was advanced 4 degrees

Thumper,

What all is involved in this and what specialty tools are needed?

Still waiting for a call...........lower the pressure to 5.5 max IF you run .120 needle n seats which you should.......

I had thought about installing it advanced and regret not doing so at this point. The cam was degreed.