Holley Ultra..Engine Stalls Hard Braking

[Akimbeau]

I installed a new Holley 650 Ultra Double Pumper. Runs fine except I'm having an issue with the engine stalling on hard braking. I'm suspecting fuel slosh is killing the engine. I've lowered the float levels to the bottom of the site windows, raised the idle to 1k rpm, disconnected the PVC system, checked timing and checked the vent whistle. No improvements. I saw some discussions on-line about the bowl vent but also heard the sloshed fuel comes from the air bleeds? I had a close look at the top of the carb and noticed there wasnt a brass piece in one of what I think is the air bleeds -see photo. I looked at 2 other Holleys I have and they both have the brass piece in every hole. Might this be my problem? Fuel coming into the carb throat because this hole is unresticted? Anyboby have a Ultra double pumper they could look into and see how theirs compare?


Kim

[Tommy]

I had fuel sloshing out of the float bowl through the vent tube in the center right of your picture. I placed piece of rubber tubing on the vent to raise the opening another inch or so. That solved my problem. It's a simple fix to try.

[Akimbeau]

Thanks Tommy, tried that. I even put a longer tube on it and directed it away from the carb. No go. i'm thinking I have a defective carb.

Kim

[Texasdoc]

Stalling: Your stalling on decel isn't because of that missing insert. Maybe if it was the idle air bleed, but not likely since it is the main bleed. Do you have the vent tube "whistle" inside the fuel bowl? Get the whistles if you don't have them. You can also try to lower the fuel level a bit more - 1/16 to 1/8 inch.

I found this on the racingfuelsystems.myfunforums Holley carb section: Rapid deceleration will cause fuel to spill from the secondary main nozzles. This is why most secondary bowls have a lower sight plug than the primary and why many OE Holleys have a secondary PV, so the MJ doesn't need to be so large.

Fuel spills out the main nozzle, not the vent tube under hard braking.

The other likely/potential cause of stalling under decelleration is a lean idle. Do you have access to a WBO2 meter? If not, try adjust the idle mixture screws. Start by turning one idle mixture screw out by 1/8 of a turn. If your RPMs increase, turn the other three out the same 1/8 of a turn. If the RPMs don't increase, turn that first screw back in 1/8 to its original position. You can also use manifold vacuum if you attach a gauge. Find the setting that gives you the highest RPM or vacuum with all 4 screws the set the same. Then start going slightly richer until the RPM/vacuum starts to drop off. That way you are on the slightly rich side of best idle. A tiny bit rich can make up for inexact timing and helps cool the intake. Again, make sure all 4 screws are set the same. You can check this with the motor off: start with one screw at a time, run it in until it seats lightly - counting the revolutions. Then make a mark on the mixture block so you can reference "zero" visually. Now back that screw out to its original position. Next do the other three individually. Make sure they are all equal. Now if you turn one 1/8 of a turn, you can visually see that the other three need to be turned.

Brass insert: I think you are supposed to have those brass inserts in all four holes. The inner two are the high speed air bleeds (main bleeds) and the outer two are the idle air bleeds. My Quick Fuel carb has screw in bleeds that are changeable. You may be running very lean if the carb was tuned with the brass insert but now it is missing. Is this on the primary side or the secondary side? If it is on the secondary side, you may not notice the lean running except at WOT (depending on how heavy your foot is) which is concerning - you do NOT want to go lean at WOT.

Three potential "fixes" for the missing insert:
1 - If it is a new carb, send it back and get a new one. I suggest calling Brent and telling him what you want.
2 - Take it into a "shop" that works on carbs. Let them tune/fix it.
3 - Do it yourself. Get some micro drill bits and a digital caliper. Use the drill bits as gauges and find out how large the main air bleed is on the one side. Then order from Holley or make the press-in bleed and insert on the side that is missing.

Either way, you want to make sure you aren't going lean.

[jhv48]

Under hard braking, fuel flows forward uncovering the main jet in the primary bowl. Try jet extensions as well as vent whistles and return the fuel levels to the prescribed levels.

[Ron61]

I used vent whistles in mine when it was doing that and that stopped about 99% of the problem.

Ron

[1795]

Thanks for the discussion! You have caused me to take into consideration some adaptions to my carb on the race car build as hard braking and also a lot of lateral Gs will be common and I want to make sure that I have good and consistent fuel flow during racing.

[vector1]

Originally Posted by jhv48

Under hard braking, fuel flows forward uncovering the main jet in the primary bowl. Try jet extensions as well as vent whistles and return the fuel levels to the prescribed levels.

This. You are experiencing a lean condition.

[moore_rb]

Originally Posted by Akimbeau

Runs fine except I'm having an issue with the engine stalling on hard braking. I'm suspecting fuel slosh is killing the engine.

-You can test this hypothesis by going out with an observer, and replicating the condition while they watch for black smoke... smoke puffs mean your assumption is correct, no smoke puffs, mean you are going the wrong direction, and you might be going too lean on the primaries...


Either way, lots of great advice above- vent whistles and extended jets on the primaries should probably both be considered "mandatory equipment" on a holley that is going to be tossed around hard on the track.

If you have a mechanical fuel pump, then one other thing to check is your low-load fuel pressure (rev the engine hard, let the throttle snap shut, and watch the fuel pressure gauge as the RPM's drop.)

fuel sloshing in the bowls during decal (or hard cornering) can slosh around the outer edge of the float, allowing just enough space beneath the float to allow the supply needle to crack open, and if you have a higher (or variable) pressure fuel pump, then it can push past the needle.... Once this extra gas is in the bowl, it has to go somewhere...

Do you have a fuel pressure regulator? From my own experience, Holleys seemed to like no more than about 7 psi maximum for severe off road or road racing/autocross uses. Now, there might be a more current point of view on this (the last time I track raced a car with a carb was in the early-mid 1990's ); but I'll throw it out there for conversation's sake...

Using a good high volume (but low pressure) fuel pump, along with a good high quality pressure regulator, should help with stabilizing your fuel delivery into the bowls during hard braking and severe off-camber maneuvers.

Lastly, (and again, this is just a personal opinion)- If you know any hard-core drag racers who try to give you any advice on how to set up your fuel system for track use, then walk away man, walk away... In a drag racer's mind, a carb only has two settings: idle, and WIDE-OPEN

Constant fuel delivery, at a consistent pressure, should be your goal.

[Akimbeau]

Thanks for all the knowledgeable replies.

1) Already checked the vent whistles...installed correctly

2) Already tried raising the primary bowl level in case it was main jet starvation, no improvement.

3) I already lowered the float level in the secondary bowl well below the sight window...no improvement.

4) I already used a vacuum gauge to set the idle mixture screws. I also tried it by "ear" and got the same settings. I'll double check that they are all set at the same position and a bit on the rich side.

I'm pretty sure it is a rich condition causing the stalling because restart is hard, I have to open the throttle heavily during restart. The missing idle bleed is on the secondary side. And since it is on the inside it is the main air bleed. I guess I should call Holley next.

Texasdoc, who is this Brent you mentioned?

[Akimbeau]

Just called Holley, the missing air bleed is definitely the problem. And of course I have to ship the carb back to them for repair, they have to be pressed in. I tried to get a new replacement carb but no deal. Now Im suspect that I'll lose other bleed screws. I've been running the carb for the last 5 months or so, not sure if the bleed was missing out of the box or whether it popped out somewhere along the way. Comforting to think the carb is falling apart and swallowing the pieces. Just thankful its small and brass.

Thanks again for the replies.

Kim

[A98Coupe]

Akimbeau,

Engine stalling under braking is due to fuel flooding or your idle mixture being way too rich. You say you have set your mixture screws, but did you check the position of your primary throttle blades to ensure they are not exposing too much idle transfer slot? If your primary throttle blades are open too much you will be pulling main circuit fuel and cause a too rich condition. I set my primary blades so that they expose enough transfer slot to make a square shape, then I leave well alone. I then set the secondary blades so that no transfer slot is visible. I then put the carb back on the engine and adjust idle speed by first setting the idle mixture screws until I get the highest rpm or vacuum and then if I need to adjust engine idle speed, I do so using the secondary throttle blade stop screw. Of course this is all done after making sure the ignition timing is correct - otherwise you are wasting your time.

Also, after correctly setting the primary/secondary throttle plates, you should make sure that your idle mixture screws have proper control over your idle mixture. After setting the idle mixture where you get best engine rpm / vacuum, you should be able to wind them out and cause the engine speed to drop and run rough. This proves that the idle feed restrictor (IFR) in the metering block is the right size. If the engine does not respond to winding out the mixture screws, then your IFRs will need to be enlarged either by drilling in 0.002in steps or replacing the small brass jets if your carb has adjustable IFRs. If you find you cannot stop the engine by winding the mixture screws in, this again indicates your throttle blades are open too much and the engine is pulling main circuit fuel.

Another things to consider is cam size / duration. A large duration cam may need the primary / secondary blades to be opened too much to keep the motor running due to poor fuel/air density. This will uncover too much transfer slot and cause main circuit fuel to start - hence too rich. This is where you need to start drilling holes in the primary throttle blades so that you can close them back to the correct position again.

Finally, for carb tuning, I would strong advise you install a wideband O2 sensor. You can then know exactly how your carb is performing throughout the rpm range and under different load conditions. Only then can you properly start adjusting your carb.

Hope you find some of this useful.

Stuart

[Jerry Clayton]

Lots of good info here-------

I once had a cobra guy come to me because his engine also died (quit running) on deceleration-------

So a few basics---is the carb angle flat or tipped down??

Don't lower the primary level-you'll probably need to raise it (maybe)

do you have jet extendors on the primary jets? Secondary? I don't like either way because if you have an issue with fuel leaving the jets you got another problem---

You may just need to do something simple like push in the clutch a little earlier------

try it -------