Help! Engine dies at idle when braking hard

[garry viohl]

The engines struggles and almost dies when braking hard in neutral for the last 10 to 20 feet of stopping. Here is what I have and done so far.

428 fe
Edelbrock rpm heads cam and manifold
Holley 800 double pumper
15 degress timing at idle
7 to 8 inches of vacuum at idle

I have tried adjusting floats
jet extensions on front bowl
vent whistles on front and back bowls
increase idle
dash pot
loop vent tubes on top of carb together
installed vacuum canister and tried them on carb pcv port and manifold port
called holley tech dept

Nothing has worked

Any ideas? Thanks.

[cdnus]

1: get rid of primary jet extensions

2: same thing with vent loop--not necessary

3: adjust floats to window level.

4: try leaning idle A/F a little @ a time.

Hope this helps, I know these gremlins can drive one crazy sometimes.

Craig

[ERA Chas]

cdnus x 2- he's right on. What worked for me was cold air to the carbs (carb in your case) all the time. see:

Never had the problem again.

[jopper]

my opinion is this can only be caused by fuel slooshing. Have u tried lowering the fuel slightly below the sight plug ?

on the secondary bowl

[Xavier]

With your engine running, your hand wrapped wround the carb to gun the engine while you are watching the fuel sites on the passenger side, seee what happens when you gun it. Does the fuel site come back to the same level each time? Thats a good thing. Now make certain that the fuel level is sitting a little over half, again about 60% from the bottom.

Thats what did it for me, in addition to what CDnus said - with the exception about it being level. All of the tuners I cam across who did carb tune up professionally said 60%, and that fixed the issue. Found it in my manual as well after I did it...

[FWB]

my observation.....800 holley on that build is too big.
7-8 in vac....there is an issue there,with the rpm cam should have at least 10 in. intake is prolly leaking. are these power brakes? if so the vacuum leak could cause the stalling when you rob all the remaining vacuum to stop. if no power brakes its the carb. its just too big.

[cdnus]

Carb is definetely NOT too large for the application, just needs to be set up properley.
I have found that anything more than about 1/3 fuel level above the sight hole will produce loading up & engine almost or stalling out under hard braking in my case. You can get away with it a little higher in some cases but most times it will flood under hard braking.
I suggested that as a starting point to eliminate that possibility. Easy to do & undo if no improvement.
A A/F meter goes a long way in sorting out these problems.

Craig

P.S.

I have a 920 on my 408---very streetable
My buddy has a 1000 on his 427W---also very streetable, watched it dynoed last night @ 452 RWHP

It can be done.

[Mdman352]

Jet extensions and bowl extensions are what I am getting ready to try and eliminate the issue with my car. a friend had the same issue and that is what worked for him.
David

[vector1]

my bg was doing the same thing, wasn't when i first got it, then i got to playing with it and it would stall on braking and cornering, now it is pretty much gone although i don't know how i fixed it since i usually would try a couple things at once.

if you have the 4 corner idle setup, open up the secondaries and primaries same amount, uncovering a small portion of the t slot, this way when you brake and you uncover the front bleeds you still run on the secondary idle circuit, kind of a carb in reverse. it seems when mine was dying, the afr would go lean so this might help you. definitely recommend a wide band setup if you can fit it in, around $200 and money well spent. like the oil pressure at idle thing, if you get it set up to run in the fashion you prefer, you will probably have to let the idle fall where it may. you can effect idle with vacuum advance also, depending wether you hook up to manifold vacuum or port vacuum. i have to run without on the street car or the idle is up around 1500rpm, for long road trips i hook it up and gain about 2 mpg i believe. right now the crate motor idles at 1000 and pretty much withstands any braking, little bobbly on turns though, but i think this can be taken out with higher idle. anyway, don't get fixated on a certain idle rpm. i've read one guy prefers the fuel level to be below the top emulsion tube air bleed so you might look at this. when you make adjustments to the holley based carbs it seems they are touchier then they look.

i would set both butterflies as far open as you dare at the recommended 20-40 thou showing on the t slot, or a square, hook everything up, fire it up and get it hot and see where idle is at,you can always close the butterflies at this point, just don't go any further open, or you might have to hook up the vacuum to get the idle where you want it. set the engine to a low idle around 1000, easier to set mixture here, once you get it warmed up set your idle mixture screws all the same to get the best idle, then put the butterflies back to where they belong, adjusting both, and you should be set on your idle and transition circuit. the only thing to adjust after this is the fuel level and main jets. adjust the fuel level one flat at a time on the adjustment nut. like the man above said you're just guessing without an o2 sensor, or have a pretty good ear/feel for how an engine should run.

imo, the carb is not too big, hook up a vacuum gauge and see what the vacuum is at wot, don't know what the recommendation is, but less is better. usually little over 2 cfm per ci. can be made to work.

[Excaliber]

Shelby had the same problem with the competition cars, I'm surprised no one has mentioned his solution.

Three words:

Center Pivot Float

[garry viohl]

More info

I set the idle to 800
Its a mech advance distrib
Its has lemans bowls now, but did the same things with original cathedral bowls
Its a two corner idle carb, 1 and 1/2 turns out, thats the most vacuum there, and runs the best.
Only stalls during hard braking at idle for last 20 feet in neutral
Runs great under hard cornering or launching

Still puzzled....

[ERA Chas]

Don't know why you're still puzzled.
Your carb is eating hot stagnant air under that condition.Colder, denser air is the solution not a lot of setting changes.

[garry viohl]

I have a turkey pan installed, plus plastic carb spacer. I dont think hot air is the problem.

[ERA Chas]

Unless they're sealed to the scoop opening those pans are an appearance item.
Good luck with your problem but you can chase your own tail by changing settings.

[Xavier]

Well, in short I think everyone here agrees; get your carb setup professionally tuned.

End of story.

[Mdman352]

I am having the same issues, trying to stall under hard braking and aro8und corners. I do have my carb sealed to the hood scoop and it is getting cold air.