Holley HP750 T slot not long enough

[mbrooks]

Trying to get this thing to idle low enough on a SBF, with the primary just showing a square or less t slot and the secondary near closed and the center air holes shut off it would idle where it should. I think some of the problem is running manifold advance but that works nicely and there is nowhere to hook into timed vacuum.

Problem here is when the secondaries open it goes way lean til the slot starts to pull, and this light car with vacuum advance doesn't take much throttle to cruise the highway so it seems it is usually into the primary idle circuit or transitioning to the secondaries.

So to be able to shut the primary throttle blade down enough to utilize the secondary t slot negates the primary t slot. Therefore if I could shut the front blades down I could raise the rear blades and show both t slots, problem solved. But the slots are not deep or long enough into the manifold to do this.

Removing the base plate and with the blades just resting on the adjusting screws, looking from the top the slot disappears past the blade, from the bottom the slot does not reappear. Actually from the top the top edge of the throttle blades are just a little higher then the floor of the t slot.

So I propose with the center air adjustment just a minimum of air pass through is needed past the blades themselves, enough to carry/pull the fuel from the idle screws and enough to fill the t slot at idle and expose it on throttle opening.

So instead of lowering the t slot, why not knife edge the throttle blade in the area around the slot, and possibly relieving the blade proper to expose enough of the slot to pull fuel at idle, just a slot itself or round relief to the blade, wouldn't have to be much. I think the knife edge itself is going to be close to enough. The throttle blade measures .040 thickness. And then use the center air adjuster as it should be.

I don't think anyone would be any the wiser......

[pamotorman]

some AFB carbs have the butterfly edge beveled for that purpose

[mbrooks]

some AFB carbs have the butterfly edge beveled for that purpose

Bingo!

[MadBill]

It sounds like both P and S throttles blades are practically seated in the bores and with the central air bypass closed I don't see how even with a tiny cam and a 500 RPM idle the engine couldn't be getting some of its air elsewhere, aka a vacuum leak. (Leaky PB diaphragm? defective PCV? The usual suspects?...)

In my somewhat-limited experience, Holleys are not super-sensitive to mixture at the start of secondary opening, plus when I get that far into it its usually just for a millisecond on the way to WOT. Is this carb vac or mechanical secondaries?

Are you SURE it's the idle feed slots causing the trouble? If for a test you cranked up the secondary stop* to expose say 0.080" of slot and it solved the issue, that would be definitive. If not then you'd need to look elsewhere. (*For easier access to the secondary stop, unscrew it and reinstall it from the top. I switch to Allen set screws.)

[mbrooks]

I’m sure.

[groberts101]

Bill has it right. The secondary t-slot is near useless on anything other than a VERY slow transition to the secondary booster. And how often does someone modulate the throttle that slowly and progressively to actually make that happen?

If you're going lean during secondary opening.. then you need to be looking at the pump cam stroke length and squirter sizing to fill in that hole. That's it's job.

I run very big base numbers with extremely short mechanical sweeps(sometimes in the 8-10 degree range) and manifold sourced vac advance, sometimes upwards of 50+ degrees if using a tuner or programmable ignition system, and that amount of advance causes this same situation. So I stick the rear throttle to the bore, back it off a hair.. just enough to unstick it.. and give it tons of pump shot volume since all that advance really gobbles up the extra fuel.

If you're near stuck shut on both ends of the bores and you've exhausted all the air bypass will allow?.. then you have other issues going on that need to be fixed.

[Geoff2]

You don't want to lengthen the T slot. That will lean the mixture at idle & cracked just off idle. This is because at idle, the amount of T slot above the blades acts as an air bleed. More slot = more air = leaner.

If I am reading your problem correctly, you are using manifold connected vac adv [ a good move!! ] but idle speed is still a tad too high. It could be leakage around the t/blades. Try loosening the mounting screws & centering them better in the throttle bores. Also, it is ok to be showing hardly any T slot; it leaves more of the 'sweep' of the T slot for cruise/transition to the main cct.

[mbrooks]

That worked pretty good. Maybe beginners luck.

Just relieving the area around the slot which is .025 wide is plenty without going into the upper deck or face of the blade, 30 or 45 degree bevel.

Adjusted the center air bleed to the middle and opened it up just a little when first started but see it is going to have to be shut down a little also.

And if you see your work needs to be reversed you can always turn the throttle blade over or around.

[turbo2256b]

champhering the throttle blade is what i have done in situations were its needed to uncover the proper amount of the transfer slot.