Engine stall when braking hard

[slpro1207]

I have a stalling problem when I brake hard to stop. Not panic braking, just making a quick stop. Engine starts to stall out. Obviously it is a fuel problem. I’m running a sbf 427 with a Quickfuel 750, Holly external fuel pump. Runs perfect under heavy acceleration so I don’t suspect the pump. Possibly float level or pressure regulator set to low? Pressure set at 6lbs. Any ideas?

[scottiec]

I could be wrong. But you’re probably overflowing fuel out of the bowls and into the intake causing it to stall. Assuming your float levels are correct, I think the solution is a tube that connects the two vent tubes. There was a recent thread on it

[GoDadGo]

If I'm mistaken, the Quick Fuel carbs all have a site glass on the bowls, except the single fuel line feed units.
Is this a 4 corner idle circuit version, not a Slayer Series two corner idler style carb?
If it is starving for fuel under braking then the front bowl fuel level is possibly the culprit.
Have you looked to see where the fuel level is set at for the front vs rear?
Also, if I'm not mistaken your carb may be tilted down in the font a bit which would compound the front idle circuits under braking.
Finally, does the car idle well when it isn't moving?
If it idles well, I think forward fuel sloshing may be your issue.

Good Luck From The Dark Dart Side!

[Fixit]

2nd on the above... take a look at these... http://www.jegs.com/p/Holley/Holley-...43692/10002/-1

[mike223]

Try the "Holley Avenger vent tube".

https://www.google.com/search?q=holl...w=1680&bih=870


Also +1 to checking float levels, etc.

[Feen]

Mine does the same thing, but generally only when it is cold. Once I have been driving for a few miles, I dont seem to have the issue. QF 650

[skidd]

I had the same problem with my carbd 302. After checking everything multiple times, a piece of rubber hose on the top of the rear vent tube seems to have fixed it.

[NAZ]

Back in the stone age (before all the fancy store bought stuff like the Avenger mentioned above or vent whistles which someone will mention) we solved this problem by connecting a piece of fuel hose from the primary vent tube to the secondary vent tube. Slice the tube longitudinally at the catenary about 40%-50% thru the diameter and ~2" in length to give you plenty of open area for the two vents. If you have a choke you will have to install a bit longer vent tube in the primary side. The hose is positioned inside the air cleaner and provides enough elevation that the fuel won't splash out of the bowls and into the carb throat no matter how hard you stop. Or you could simply use two short pieces of hose to extend each vent tube individually straight up into the air cleaner to achieve the same result. The reason we used the longer hose connected from vent to vent (and safety wired to the air cleaner stud) is there was less chance of one coming loose from all that vibration these race vehicles see.

An inexpensive fix that worked on desert race vehicles that take a much harder beating than any street driven machine. And these vehicles are constantly being pedaled from WFO to idle over stutter bumps and big whoops. It's also an easy way to experiment to see if this is really causing the issue you're dealing with.

[GoDadGo]

Help The Dark Side Edelbrock Guy Please!

So the fuel is sloshing out of the rear bowl and into the carb fattening up the mixture thus causing the engine to nearly stall?
Just making sure because you all know that I am not the best at messing with the Holley Style Carbs.

Is This The Case?

[slpro1207]

Help The Dark Side Edelbrock Guy Please!

So the fuel is sloshing out of the rear bowl and into the carb fattening up the mixture thus causing the engine to nearly stall?
Just making sure because you all know that I am not the best at messing with the Holley Style Carbs.

Is This The Case?

I’m with you Steve. I’m not strong on carbs. I am going to try the vent tube and see if that clears it up. Hope so.

[Ducky2009]

Does it do it with a full fuel tank? Had a bad baffle in the fuel tank once. When the tank was low on fuel, it would slosh around when braking and suck air instead of gas, causing the engine to stall.

[slpro1207]

Fuel level does not seem to be an issue. Happened again yesterday with 3/4 full tank.

[NAZ]

Some interesting info to ponder. How many G's does your car pull when stopping hard? 1.5? 1.7? More? Maybe you have sticky tires and great brakes and can pull 2.0 G's on hard braking. At 1.0 G setting still on level ground the fuel in your bowls will be level. At 1.5 G's the fuel will be tilted ~47-degrees and at 2 g's the fuel will be tilted at a 60-degree angle when the car is on level ground and braking hard. Of course that's at a steady G-loading, because of inertia the initial tilt of the fuel in the bowl will be greater for a short duration. That means the rear bowl will be puking fuel out the short stock vent tube into the throat. Same thing happens on acceleration (both forward acceleration and when turning on unbanked tracks) the fuel level is tilted significantly when pulling a couple of G's. Probably more than you realize.

[srobinsonx2]

I had the same issue and solved it by adjusting fuel level in the secondary bowl. Tried a few things. I have some good pictures of the carb "guts". I will send some more details and pics this evening. Hopefully it will help explain how the Holley style carb works and what I did.

[srobinsonx2]

Here are some pictures as promised. I am no carb expert but I did stay at a Holiday Inn Express. In all seriousness, I did solve my flooding issues on hard braking. These are pictures of my Quick Fuel SS 750.



This is a picture of the back of the metering block on the secondary chamber (the side that meets the bowl). The black thing sticking out is the vent baffle referred to in earlier posts. It is suppose to remain in the vapor area of the bowl when the fuel sloshes forward on hard braking. These came installed in my carb. You can also see the secondary jets below and on both sides. They are on extensions. These extensions are supposed to help with hard accelerations. They allow gas to be picked up when the fuel sloshes to the back of the bowl.



An alternative to the vent baffle is this vent screen (it may be called something else). It is supposed to do the same thing as the vent baffle.



Just for reference, here is the primary bowl and primary metering block removed.

I had flooding/stall on hard braking. I tried both the vent baffles and vent screens, I also tried a vent connection. Here is a picture of that superbly crafted piece of precision equipment



As you can see, I didn't spend a lot of time fabricating this. I used two spark plug boots and some fuel line. This was just to test and see if it would help solve my hard braking issue.

No single device solved my problem. I finally settled on the vent baffles, as they seemed to help the most when compared to the screens. The vent connection didn't help much at all. What ultimately solved the issue was lowering the fuel level in the secondary bowl. I set the fuel level down to just at the bottom of the site glass.

Hopefully this helps out and I am not covering basics that everyone already knows.

[slpro1207]

Thanks for the pictures and input. I think I will follow your lead and try lowering the level in the secondary bowl first. We are having a few days of heavy rain so now is a good time for it.

[NAZ]

Changing the fuel level in the bowl is not a preferred method to address a rich condition caused from puking fuel thru the vent tube during hard braking. Changing the fuel level in the bowl is a tuning tool that affects AFR and timing of the main circuit transition. Raising the level can cover a lean stumble during transition to the main and lowering it can lean the transition from the idle circuit. Therefore, if you are lowering the fuel level in the bowl you should pay close attention to AFR and you may also unintentionally introduce a lean stumble.

If you truly have a problem with fuel spilling over the vent on hard braking get a 2"-3" length of fuel line and add it to the vent on the secondary. Raising the vent tube WILL prevent fuel from sloshing over the vent and into the throttle bores. If that doesn't cure the problem then you have another issue. Also note that with a fat mixture you would expect to see black smoke from the exhaust so when troubleshooting your issue keep that in mind -- black smoke = rich.

IMG_0845.JPGIMG_0846.JPG

Since photos help in explaining concepts I've added a couple. The first photo is the old school method of elevating the vents so that fuel doesn't puke out the vent and into the carb. It may be hard to see but the hose has been sliced open roughly halfway through longitudinally for a couple inches and that's what the vent tubes breath thru. I got this from another racer 50-years ago and have used it a lot over the last half-century. The hose gets safety wired to the air cleaner stud.

The second photo shows 4" vent extension tubes. Does the same thing as the hose but makes it's a little easier to get to the squirters.

[johnnybgoode]

I have a stalling problem when I brake hard to stop. Not panic braking, just making a quick stop. Engine starts to stall out. Obviously it is a fuel problem. I’m running a sbf 427 with a Quickfuel 750, Holly external fuel pump. Runs perfect under heavy acceleration so I don’t suspect the pump. Possibly float level or pressure regulator set to low? Pressure set at 6lbs. Any ideas?

I had the same issue with my QF SS-650. Under hard braking, revs would drop 4-500 RPM. First make sure your 4 corner idle mixture screws and curb idle speed are set properly. Then adjusted the primary float level up 2 flats (loosen the locking screw and rotate the adjusting nut counter clockwise 2 flats) this has significantly improved the situation. Rev's still drop but only 1-200 or so. Scott