Technical Chasing A Light Stumble Holley

Most carb issues
are best corrected
with/distributor
tuning. !

 

Holley does this on purpose (too rich) to keep a too lean carb from engine damage. My Holley 950 hp needed a ton of work to run somewhat cleaner at idle, light cruise.

 

Yep, and this makes total sense from a liability standpoint. Bunch a guys grenading their motors right out of the box after installation is real bad for the bottom line. Ignition timing or timing curves are kind of the same deal, for the same reasons.

Weak valve spring pressures are a kind of built in rev limiter and were supposedly employed by the OEMs back in the day to try and prevent warranty returns by "overenthusiastic" motorists.

 

If this is in reference to my post about Quick Fuel instructions, re-read the posted Quick Fuel instructions.
I agree that 1 1/2 turns NORMALLY is a rich position for the mixture screws.

 

If there is a takeaway for me
from this thread it is,

“A lean LSAB set up can
make it difficult for carb
to transition to high speed”

Not only is any trace of a stumble gone,
car now seems to leap as transition
takes place.

Tanks to whoever posted this !

 

Interesting discussion. It's spurred my to start my own post as well (yay more carb issues posts).

I run a holley 750 street hp with vacuum secondaries and 4 corner idle. I saw it said a few times about why would you need 4 corner idle on the street. You don't. But it doesn't hurt anything being there either, ASSUMING the user can dial it in properly. The benefit is improved fuel distribution to all cylinders at idle. It's not to say you can't get a 2 corner system to distribute fuel evenly, but the 4 corner makes it easier for a wider variety of combos. Definitely not as simple as a 2 corner idle, but it ain't rocket surgery either. This is why you see it as factory on the HP main bodies, since they're intended for rowdier street or strip setups. Again, you can dial in a 2 corner carb to accommodate a plenty stout setup as well, this is just to aid in tuning. There's such a diverse market for carbs these days based on intended use that it really has cut down on the amount of throwing parts at a setup until it works.

I'm certainly no carb expert, but I will tell you that Holley's fetish of setting all 4 mixture screws exactly the same is bunk. Set them where your engine idles best, meaning makes the most vacuum. They should all be in the same ballpark, but being off by a 1/8 or even 1/4 of a turn is not unheard of.
I've got a pretty large cam, so a lower vacuum reading at idle and weaker signal through the carb. There is simply no amount of idle tuning that is totally going to clean up my rich idle and light cruise (and I will pay money to the person who could) while maintaining a strong idle, but that doesn't mean you should be burning your eyes standing behind your car. Every combo is different, so every combo requires unique tuning. BUT, adjusting your mixture screws should affect idle speed, or there's other problems. In my case, they did not have much affect on idle speed until damn near run all the way in. So the solution for me was to open the secondary butterflies slightly, allowing me to close the primaries and prevent fuel from being pulled from the primary circuit due to excessive opening of the primaries to achieve an acceptable idle speed.