Strange issue with Holley XP carb setup.

[shithotfast]

I have had my Ultima now for 19 years now. Started out with a crate SMC V8 with a holley 750 four port, and now with a new 383 stroker with a Holley XP carb.

the problem I have always had is that during hard braking or very hard cornering I get momentary fuel starvation and engine cut out. I have raised the fuel level in the bowl but that doesn't help.

Strange that I have had the problem with two engines/carbs so it must be something else. Fuel pump seems ok, and fuel tanks are almost full but still it happens.

anyone ever had such an issue please?

Its the only thing that has always bugged me and tried Holley technical support - no solution, tried various race shops, but still no answer to what must be a very simple fix??

Any thoughts ?

[V8Dom]

if you raised the float level that might be the problem
there are 2 things that help
1 real steel do a kit to extend the main jet into the centre of the float bowl so it doesnt ever starve of fuel on surg

also there are 2 vertical pipes used for breather that stick upwards when you take air cleraner off..if floats set high fuel comes out of these pipes on heavy braking and chokes the engine

grab a piece of rubber fuel hose about a foot long. connect these pipes using fuel hose and cut a hole in the pipe to allow to breath.. if i remember ill photo mine tonight.

if not you can tilt the carb or change to one of the mega carbs charles \Dunn sells

dom

Edited by V8Dom on Monday 1st August 14:22

[confusionhunter]

I think you have the opposite problem, under heavy breaking too much fuel goes in and floods the engine. I have a similar problem and have the jet extenders in the back so I first though that car was going lean under braking. But apprantly fuel under heavy breaking comes out the back vent. and of course the throttle is shut so there is no vacuum to vapourise the fuel and no air to burn it.

I had the same issue when does burn outs and braking at the end of the run for when I did a bit of drag racing and just opening the throttle slightly (NOT pumping so the accelerator pump doesnt does anything) adds air the revs pick up and you are back to normally metered fuel..... A bit difficult to do under heavy braking.

I haven't addressed this yet as its mainly a road car but I believe a whistle vent and /or one of these makes all the difference:

Edited by confusionhunter on Monday 1st August 11:01

[confusionhunter]

And what Dom said ^^

[shithotfast]

interesting! Never thought it might be over fueling ? worth a try. I did have the float level much lower (around one third up the sight glass on the mains, bit higher of the secondary's so might try lowering the secondary as well, and try the extensions.

I am amazed that holley don't put the extensions in as standard, if that is the problem. Will try them,

many thanks....

[Storer]

I had heard about the wedge shape plate to 'level' the carb makes a difference but my knowledge is far less than others on here.



Paul

[Steve_D]

I had both a wedge and vent extension on mine before I binned the carb.

Steve

[shithotfast]

I tried the wedge to lift the carb, but then the air cleaner fouls the rear hood. I have looked into the extensions for the fuel bowl and found the carb I have (XP) does not allow them.

One last thing to try is a fuel pressure regulator, which i haven't ever had. I have found my pump provides at least 8psi, the holley manual say anything over 7psi and "you will encounter running problems". Some Holleys run at max 5-6psi it seems. i will fit the holley pressure regulator (£40) which has a 7psi fixed flow for my carb and see what happens.

After that, i wont go around corners nor stop !

hope you are all enjoying your ultimas.

Anyone bought the new flashy Ultima watch ???

[confusionhunter]

Yea I don't think 8 psi will help, Mine is set to 6.5 psi. Also try the rubber tube trick on the vents and if that works consider the proper holed pipe vent above. Given I occasionally suffer the same thing I doubt fuel pressure will totally solve it, but its definitely not going to help.

[V8Dom]

ive got a holley fuel pressure regulator somewhere is you want it...
you will need a fuel pressure guage to adjust it correctly

[Mr Pid]

Had the same issue on mine. It was overfuelling on braking..... Confirmed by the afr meter. Caused by fuel from secondary bowl sloshing into the carb. After a bunch of research I did this........

Put a wedged spacer in to level up the carb. Without this carb tips forward making things worse.
Set fuel pressure to 6 psi with the pressure regulator. Without this pressure varies a lot with throttle setting, due to fuel flow..... And therefore fuel bowl level varies.
Set fuel level to very bottom of sight holes (or slightly below)..... No fuel should come out when the sight plug is removed....even when the engine is running.
My carb already had the vent whistles..... I think they pretty much all do as they come from Holley.

So all pretty simple, cheap stuff.

Now all is well even under heavy braking......

Stu

[shithotfast]

Good news. Lowered the secondaries fuel level and the stall under braking has gone. Thanks for that one.

Still got a bit of hunting whilst driving at low speed. Might be the solid lifters that I havent adjusted yet. Still running in.

[V8Dom]

if you are still having problems give Charles Dunn a call
he is doing the quick fuel carbs now... ive fitted one recently and they are megga and far better than the Holley imo

dom

[k wright]

I used a section of rubber fuel hose to connect the rear and front float bowl vents. At the top of the arc formed I cut a long hole to allow both bowls to vent.

The reason the engine stalls is fuel in the rear bowl pours out into the venturis under hard deceleration.

Rear jet extensions are also helpful.

[Arnold Cunningham]

Hi Mark! Good to see another spyder owner still has his car too!

Have you not considered going EFI? My spyder was my first foray into EFI, now done another couple of EFI systems and it's truly a breath of fresh air! So much easier to set up and get just right.

[Boosted LS1]

sthotfast said:

Good news. Lowered the secondaries fuel level and the stall under braking has gone. Thanks for that one.

Still got a bit of hunting whilst driving at low speed. Might be the solid lifters that I havent adjusted yet. Still running in.

Yep, could be that, poor/low vacuum. If it were me I'd fit a vacuum guage to the inlet to see what I have. Also, if the vacuum's to low the power valve may be pulsing. You can advance the ignition slightly to see if it increases idle vacuum but remove the advance from further up the rev range if it's not wanted. If it's not the power valve it may just be that you haven't got a strong vacuum signal in the first place and you'd want to restore that.