Stalling under hard braking

[mrn2obelvedere]

Hello all,

So until I can get around to finishing up the EFI setup in my car I am stuck with the carbie.

I have a problem: Under hard braking the car will stall, as if fuel id dumping in form the secondary bowl. I put extension tubes on the breathers to prevent this, but it didn't seem to help. Are there any other potential causes of this problem?

[esenior]

Do you have power brakes? could be a vacuum problem.

[mrn2obelvedere]

nope, manual brakes

[Roadrage David]

scroll down to my post https://www.pro-touring.com/forum/showthread.php?t=67386

[66SuperSport]

I have an AED modified 750 mechanical secondary Holley. They installed road race floats and I run the float levels a tad on the low side just for added safety. Never had a stumble or miss and this car gets autocrossed and driven very hard.

[mrn2obelvedere]

I saw this
https://www.pro-touring.com/forum/attachment.php?attachmentid=38120&d=1273559493
And I think it is a good idea...

It is one step further than what I had done. I placed 1" risers on my carb vents.

I have read a good bit about this in various forums online and it seems like it can come from a few core areas:

1) fuel spilling into the engine from the vents
2) timing and idle circuit issues
3) fuel moving away from the primary circuits during braking, thus actually leaning the engine out

The way I see it, point 3 seems to be the most likely candidate for me since I have already installed vent extensions. It seems that it might be possible to raise the front fuel levels somewhat to alleviate this issue. Any other ideas that I might consider?

[Pro Stock]

I saw this
https://www.pro-touring.com/forum/attachment.php?attachmentid=38120&d=1273559493
And I think it is a good idea...

It is one step further than what I had done. I placed 1" risers on my carb vents.

I have read a good bit about this in various forums online and it seems like it can come from a few core areas:

1) fuel spilling into the engine from the vents
2) timing and idle circuit issues
3) fuel moving away from the primary circuits during braking, thus actually leaning the engine out

The way I see it, point 3 seems to be the most likely candidate for me since I have already installed vent extensions. It seems that it might be possible to raise the front fuel levels somewhat to alleviate this issue. Any other ideas that I might consider?
Try jet extensions on the primary jets, Call Bob Vrbancic at the Carb Shop with any questions They are good people.

909-930-9980

Dale

[mrn2obelvedere]

From what I have read (and this makes sense) jet extensions on the primaries aren't good because they can cause leaning out on acceleration, which is worse than leaning out on deceleration.

[sik68]

I have an AED modified 750 mechanical secondary Holley. They installed road race floats and I run the float levels a tad on the low side just for added safety. Never had a stumble or miss and this car gets autocrossed and driven very hard.

+1

My engine would die under braking or even moderate downhills... adjusted both float levels to about 1/3 the height up the sight glass and it fixed the problem instantly.

[mrn2obelvedere]

+1

My engine would die under braking or even moderate downhills... adjusted both float levels to about 1/3 the height up the sight glass and it fixed the problem instantly.

What kind of carb is this? Also, isn't there some concern that you might lean your engine out with low float levels?

At any rate, my holley has the old style brass screws for the float adjustment. They are adjusted to trickle from the sight hole.

[Vegas69]

The lower the fuel level in the bowls, the greater chance for fuel starvation during cornering. I wouldn't be to concerned with a little lower but not 1/3rd. I always set mine to where you need to shake the car slightly to get gas out of the sight holes. Mine will die under hard braking but it runs like a raped ape everywhere else. :1st:

[mrn2obelvedere]

I just had an interesting idea. It seems that when you are under braking, the fuel in the front bowl will tend to be pushed towards the front of the bowl, which would in turn allow the float to go artificially lower, thus allowing fuel to flow in too fast. This would cause your float bowl to be abnormally high until the engine burned off some of that fuel. Any thoughts?

[Vegas69]

I'm not sure I agree Sean. If it did, it would be minimal. I still think it's the secondary side. They do make bowl vent extensions that extend to the rear of the bowl.(Interior) I'll probably try those eventually.

[Pro Stock]

Read the article in the September 2010 issu of Popular Hot Rodding, pages 76-79, this is a very informative article on corner carving carbs, and mmmmm, they mention something about jet extensions in the PRIMARY and secondary metering blocks, seems like I've heard that before. Anyway, if you call there you need to talk to Bob Vrbancic (ver-ban-sick), he's a great guy and will help you out tremendously.

Dale