For the guys who road race or autocross or the carb gurus I have a question I have a 69 vette small block 434 650 hp holley xp 950 and when I stop real hard or in quick tight turns as I autocross the motor stumbles like maybe the float bowl fuel is spilling out of carb causing stumble any fixes besides like a sniper fuel injection set up .

 

what are you using for a fuel pump, do you have a return, are you using a fuel log type supply, and do you have a thermal wood 4 hole spacer? Is it the Ultra xp 950?

What kind of tranny and rear end do you have? What kind of heads are on the motor?

 

It is a ultra XP 950 no return line with a fuel log yes pump is a Holley 12-150 no spacer heads are AFR 220 370 rear soon to get 12 bolt with 31 spline trans is a t56 6 speed I lowered the floats to the bottom portion of site glass and works pretty good but still at hard stop wants to stumble I was thinking of adding about a 2 inch piece of rubber fuel line to over flow tube to see if it helps .

 

New floats with jet extensions may help some. Maybe mod the overflow with tubing another possible aid in what you are dealing with.

 

I am going to call Holley and ask about the extensions and also the notched float bowls to clear jets and it worked in a cobra I had .

 

I use BG jet tubes that taper under the float. I use Moroso anti-slosh foam with bowl extenders, etc.

 

Your carb is WAY too big and not made for the street. I read a 12 page article on the Holley ultra carbs.

Auto-X racing with big squirters and instant opening secondary 950 CFM on top of a single plane intake is why you are having stumbles. My 830 demon with progressive linkage with lots of flow work wasn't ideal. I had to learn how to not jump on WOT and ease into it.

You need a 650 or 750 carb to make a 434 work. Ultra carbs have big needle and seats and only require 5-6 psi fuel. The NO vacuum ports is why they are NOT intended for street. They are a WOT carb only for racing. Right on the Holley sight they advertise 3 kinds Drag racing Circle track, and marine

Jet extensions are a drag racing idea on the secondaries. If you are braking so hard that the gas sloshes away from the primaries, who cares because you are not at the same time going WOT

If you really have a 650 HP 434 you have very little vacuum at low rpm. My 434 with a big solid roller is not made for parking lot racing even with a 5 speed and 4.11 rear end. It's made to operate between 4000 and 7500 around a road racing course

 

Low Tech; try a loop of 5/16 fuel line arched up between the float bowl vents. Like about a 6” radius. Nip a vent notch up towards the top of the arc. Used that way back for preventing fuel from sloshing out from the vents during hard stops.

Steve O.

 

I road race my car. Yes, you could have fuel spilling out of the bowls. To fix this, you need to get a couple of "Vent Whistles". These rivet to the top of the metering block and prevent fuel from spilling out the little square hole at the top under hard braking and acceleration.

 

All those fuel slop are from the dark days of older Holley carbs with small end bowls

 

OK so I autox'd and Pro-Solo'd an LT-1 Z28 with a stock 780 Holley for 27 years. It took a while to sort it out.

You do not need jet extenders, they are for drag racing.
You do need to drop you fuel level a little, keep experimenting, I wound up slightly below the sight plug hole. You may or may not need to adjust the jet sizes because of the lowered fuel level.
You absolutely need the white plastic vent whistles. Mine got fuel slosh in high-g turns or high-g braking until I used those. That stopped most of it.
Getting the float level correct stopped the rest. And I measured as high as 1.27G.

I thought about adding the vacuum tubing to the vents like the race boats do, but never had to add them.

Autocrossing with big-n-sticky tires is "violent"! The fuel gets sloshed around! a lot.

Run a good mechanical quick responding oil pressure gauge also, with a large diameter braided line (1/8" IIRC) not the plastic tubing stuff.
I had oil cavitation / oil pressure problems also until I got that stuff sorted out. My oil psi would suddenly dip in a violent turn. You definately do not want that too happen for very long! A good Z28 oil pan & baffle, a carefully height-set oil pickup, and welded, solved most of those problems. & driving smoother! LOL. I would have gone to an accusump if I needed to, but never did. I am not sure if a Vette LT-1 pan has the same baffling as a Z28 pan, or not. If an aftermarket pan, I would choose very carefully, you do not want a common drag race pan.

 

I have a t56 6 speed manual with a 266 first and a 370 rear with a 30 inch tall tire and is very streetable my cam is 598 intake 583 lift and 253 intake 261 exhaust 106 intake center line 110 lobe sep .

 

I did lower the float level to bottom portion of site glass and works way better but when I really stop hard it wants to die and where do you get the vent whistles at .

 

Read up on this!!!! It's not your mom's carb and neither is my 830 race demon!

Back when I had the 700R4 3800 stall I considered a 650 cfm for more instant response instead of all out HP on some dyno graph. TQ is the answer to going for going forward. I used the best 8 quart RR pans and had oil starvation in long turns and went to a 4 quart Accusump

Holley 0-80805RDX 950CFM Ultra XP Carburetor

 

Just another question I raced a "509 Rat" what do you know about them? describe it please

 

I had a stroker big block chevy that was a 509 so I named it 509 rat and I have a **** ton of torque down low as well 603. 6 at 4900 rpm 533 torque at 4300 rpm this motor rips .

 

the 434 rips the big block I never dynoed .

 

What was the bore and stroke to make 509. I bought mine already installed in an endurance race boat.

I still say that the 950 is a very poor choice

 

I agree with Gkull you may need an accusump in a long high speed track racing corner. But likely not in an autocross, everything happens much more quickly, transitions are quicker, and corners are way shorter.


This is the whistle. They are cheap and everywhere. If you haven't done it yet, you need it. You can brake way harder than you can accelerate I do not care how big your engine is, it's 4 wheels vs 2. Unless it's a well setup drag car. It's just physics.

https://www.summitracing.com/parts/h...BoCZTsQAvD_BwE

And the carb is probably oversize and hurting your TQ below 3500. According to common calculators, an 800 would be big enough if you spin it to 6500 and an 850 if you spin it to 7000. More carb than that does not help high RPM power, and does cost low end & even mid-range, even though many of the drag racers and their engine builders run really big carbs, and they are sometimes too big, and my drag racing tuner/engine builder has made many cars run faster by downsizing carbs. But does it really matter? That is a lot of power out of a 434, probably more than you can hook up at an autocross anyway right? Must be fun to try and control that! What gear will it finally hook up the tires in? 1st ? 2nd? 3rd? My 425 HP LT-1 would not consistently hook unless it was in 2nd. 1st was very iffy even with 11" wide DOT zeros. And you have like 50% more power. I know you have some really big tires, but does it ever hook?

 

I don't remember bore and stroke and as far as carb goes I think it drives very well any where I can cruise on the freeway at 1800 rpm no problem or rip on it runs great .