Holley 750 stalls while hard breaking

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1974scamp

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Hey guys,

im having trouble with my 750 Holley HP douple pumper, engine is a 408 stroker.
I just had it jetted and adjusted from a guy who actually knows what hes doing, car runs really great now, the only issue is that the engine stalls when i brake quite hard. I dont need to slam the breaks, its the long deceleration which causes the engine to stall. When the rpm is going down more and more while decelerating and the engine is just about to shut off, i can save it with hitting the gas.

I spoke to the guy who jetted and adjusted the carb on the phone, he thought that probably fuel floats the venturis and recommended to try lowering the secondary fuel level. Did that but no improvement.

Then I called him again and he said try to raise the primary fuel level and lower the secondary and maybe turn out the idle mixture screws 1/4 turn.
I felt turning the idle mixture screws out 1/4 turn improved the situation but didnt fully cure it.

For me it feels more like the engine is starving fuel under deceleration, as when i hit the gas it comes back, any other thoughts or ideas to this?

Greetings from Europe,
Laurin
 
I had Holleys that dump fuel out of the air horns when decelerating or stopping fast. It a common problem. I had to attach a tube to the air horn so it would dump the fuel into the tube to dissipate. It worked perfectly. I might have some pics of my work but you can find some on the internet??
Or it could be the float level or the shut off needle valve.
 
I had that problem in my truck, a long hard braking situation would cause the motor to stall, was fine with any other normal braking.
I had the secondary float level in the middle of the sight glass. I lowered it to the bottom of the sight glass and then reset idle mix with my normal routine. didn't happen after that.
 
Hey guys,

im having trouble with my 750 Holley HP douple pumper, engine is a 408 stroker.
I just had it jetted and adjusted from a guy who actually knows what hes doing, car runs really great now, the only issue is that the engine stalls when i brake quite hard. I dont need to slam the breaks, its the long deceleration which causes the engine to stall. When the rpm is going down more and more while decelerating and the engine is just about to shut off, i can save it with hitting the gas.

I spoke to the guy who jetted and adjusted the carb on the phone, he thought that probably fuel floats the venturis and recommended to try lowering the secondary fuel level. Did that but no improvement.

Then I called him again and he said try to raise the primary fuel level and lower the secondary and maybe turn out the idle mixture screws 1/4 turn.
I felt turning the idle mixture screws out 1/4 turn improved the situation but didnt fully cure it.

For me it feels more like the engine is starving fuel under deceleration, as when i hit the gas it comes back, any other thoughts or ideas to this?

Greetings from Europe,
Laurin
Just the opposite
The idle mixture is to rich. lean it out and it will go away
Keep your idle speed as high as you can without causing issues.
 
Put the secondary float setting back the way it was, raise the primary a hair.
Don't brake so hard unless it's an emergency. A lot of older carbureted vehicles would die during spin outs and so on. G force ..Gravity. That's why happens in a non pressurized system, unlike fi where its pushing fuel through a return and no vents sloshy bowls are involved.
But mostly it sounds like the fuel is going away from the primary jet and it gets air.
Another member suggested jet extensions, that's a little much but I think he is on the right track.
 
In a nut shell, it’s all in the float settings IMO.
I had this same issue on my 340 with an 800 cfm Edelbrock
Beat my head against the wall for dam near a year floats, rods, springs, settings, jets. Finally figured it out the old rule of thumb to set the mixture screws for highest idle and then go a 1/4 turn rich is a bad practice - stalled under hard turning and breaking everytime.
Now I set the screws for highest vacuum and the go a 1/4-1/3 turn lean. no issues
 
I had this same issue on my 340 with an 800 cfm Edelbrock
Beat my head against the wall for dam near a year floats, rods, springs, settings, jets. Finally figured it out the old rule of thumb to set the mixture screws for highest idle and then go a 1/4 turn rich is a bad practice - stalled under hard turning and breaking everytime.
Now I set the screws for highest vacuum and the go a 1/4-1/3 turn lean. no issues
Oh cool! I am having this problem now on the wife’s car. I know it’s ether the floats or the idle screw settings. The “Job” is incomplete as it was just put on and rough tuned. That and the distributor. I’ll get to it later. A bit busy now with other things. But that was good to read.
The carb is a 600 Edelbrock which is way small for the Hyd roller cam, TF headed 360.

Looking forward to a Brawler later on.
 
Does it have the whistles in the main body? If not, get a pair and put um in.
 
What i also did with no success till now was sticking 2 rubber hoses on the vent tubes, but issue didnt change.

I will try to lean idle mixture now, so i will try to go for highest vacuum and then go 1/4 turn or so lean.

The jet extensions seemed to help a lot of people, also read you need another float or turn the one you have upside down?? what you think about that:
AED Performance 5896 AED Rear Jet Extension Kits | Summit Racing

Or better those:
Summit Racing SUM-200150 Summit Racing® Jet Extensions | Summit Racing

Thanks for your help!
 
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