Retune my entire carb after installing new heads?

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Square on primaries. The secondary t-slots are way above the throttles compared to the primaries, and aren't involved.
Air comes from the secondary adjustment, four .100" holes in the throttles, and the adjustable PCV. Trying to remember if this carb also has the bypass needle screw in the air cleaner stud hole or not ;)
 
Hysteric said:
:rofl:

So even less air actually going through the idle/transfer circuit.
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Have you actually looked at the baseplate on a Holley? The secondary t-slots are not at the same height relative to the baseplate, they sit higher in the bores! Do I have to show you a picture?

The secondary t-slots aren't making much of a contribution since they are not exposed at all until about half-throttle where the progressive linkage starts to open the secondary butterflies! They're just barely cracked at idle.

The intended amount of air/fuel emulsion (not just air) is coming from the squared-up primary t-slots. Due to the high required rpm for a stable idle, more air is required than that primary throttle opening can provide. So it comes from holes as well as some bypass air.

Still a rude schmuck, I see. Go back to your "Fantasy Pro Stock" theories and leave this thread alone. Now added to my short Ignore list. :icon_fU:
 
DrCharles said:
Have you actually looked at the baseplate on a Holley?
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Never. Nor do I know what size they should actually be to work correctly............Have you measured any?

DrCharles said:
The secondary t-slots aren't making much of a contribution since they are not exposed at all until about half-throttle where the progressive linkage starts to open the secondary butterflies! They're just barely cracked at idle.
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NO ****.....Maybe that's part of your problem.......

DrCharles said:
Still a rude schmuck, I see. Go back to your "Fantasy Pro Stock" theories and leave this thread alone.
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You mean that Prostock engine that ran quicker after they fixed the idle/trasnfer circuit.........Nah that would never be applicable to a stock or modified engine would it. They must have spent billions to get that right.......
 
Hysteric said:
SpeedTalk thread on Carb sizes


From the guy who built that 750:


Now go think about what that 5.5Hg might be doing for that 425 Ford earlier.
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I'm rather surprised you would reference TMP.
While Troy seems to have started with some experience experimenting, he became a pontificating BS artist trying to convert his observations into physics. IU'm sure some would say I'm being charitable. He was one of the few participants that ever got thrown off Speed Talk. That was no mean feat!
Its too much work seperating his actual observations and experience from the rest of his BS. And there's really no need with all the guidance and methodology others have so generously shared. If there's anything useful in a thread of his, its what people like Mark post in response.
 
Mattax said:
I'm rather surprised you would reference TMP.
While Troy seems to have started with some experience experimenting, he became a pontificating BS artist trying to convert his observations into physics. IU'm sure some would say I'm being charitable. He was one of the few participants that ever got thrown off Speed Talk. That was no mean feat!
Its too much work seperating his actual observations and experience from the rest of his BS. And there's really no need with all the guidance and methodology others have so generously shared. If there's anything useful in a thread of his, its what people like Mark post in response.
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I'm rather surprised you didn't actually read it to know I didn't. :lol:
 
1683065799773.png


1683065872309.png


Looks like a direct quote to me.
 
Mattax said:
I'm rather surprised you would reference TMP.
While Troy seems to have started with some experience experimenting, he became a pontificating BS artist trying to convert his observations into physics. IU'm sure some would say I'm being charitable. He was one of the few participants that ever got thrown off Speed Talk. That was no mean feat!
Its too much work seperating his actual observations and experience from the rest of his BS. And there's really no need with all the guidance and methodology others have so generously shared. If there's anything useful in a thread of his, its what people like Mark post in response.
Click to expand...
You’re being charitable.
:rofl:
 
Read the entire thread. GregK built the 750 for that engine that made 700HP.
 
With your four holes in the sec blades, plus the high idle speed, I wonder if you are getting nozzle drip in the secondaries. Less common in H carbs & clones, but it does happen given the right conditions.

Not sure if this could be done but maybe worth investigating: Four 0.100" holes is equivalent to 2 holes of 0.141" diam. Drill 2 holes through the body & the base near the air cleaner stud. Engine will get filtered air & holes in sec blades can be plugged.
 
No nozzle drip, I looked for that. It doesn't start until much higher rpm... if they were, I'd expect to see a rich idle AFR too.
 
@DrCharles I'm surprised you’re surprised by having to retune the carb when the combo changed. Yes, by your own description of afr readings your new combo is tugging on the carb differently from the old combo. Tune it up. Sounds like it needs idle air bleeds, maybe transfer slot restrictions, and cruise jetting adjustments (15.5:1 to me is a little lean in cruise and might cause it to rattle in tip in). And all of that will get you to where you can tune secondary jetting and pvcr for WOT.
 
Thanks... I am not surprised that jetting is likely to change, along with main air bleeds, since the new heads allow a lot more air in to make more horsepower :)

But as I asked initially, with the throttles closed except for idle air, I would not have thought that the (maximum) head flow would have an effect, since the throttles are the major limiting factor at idle.

Anyway, Bewy theorized that the increased port volume is changing the carb signal at idle too - made sense to me.

Edited to add: no, it does not ping at tip-in even on a hot day. I am conservative with ignition timing (and a good accelerator pump squirt).

Also to add: Dwayne tells me his dyno mule carbs work well within limited hp ranges and gave examples such as: 750 good for 400-550 hp combos but lean on 550-600 hp. And I did transition from the first group to the second with the heads :D
 
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Usually the contributing factors to how an engine will tug on the carb at idle are displacement, camshaft profile, and compression ratio. Hp and airflow make little difference in idle tuning.
 
Which was exactly my point... the displacement and camshaft didn't change when I swapped heads. Compression went up just slightly (maybe 0.3 point)...
 
An engine at idle only makes enough hp to run, so it's not moving much air, it's gonna have really poor velocities/etc.. even in stock form nevermind built, probably making less than 1% of it's peak hp potential.
 
Doc,
At cruise you are quite possibly getting nozzle drip from the secs because of the high rpms at cruise. You may not realise this & have adjusted jets etc accordingly to get the mixture 'right'.

At cruise the carb jets, air bleeds etc are calibrated on the air & fuel delivery from the pri bores. You now have a substantial amount of air passing by the sec boosters as the air makes it through the bypass air holes, possibly enough to pull fuel [ unwanted ] from the sec boosters.

This is why it so important that large amounts of bypass air is not routed through the t/bores, but through the body & exit under the blades. The bypass air then does not interfere with fuel metering.
 
Interesting idea, thanks! I hadn't thought about that possibility... but is that quantity of air really that substantial through .100 holes at 15" vacuum? The primary throttles are open further, at highway cruise, so I would think almost all the airflow would be going through those bores as intended.
But even if there is some pullover through the secondaries, how is that a problem? The cruise mixture is lean where it should be... and 71/86 jetting is not unusual.
 
DrCharles said:
Interesting idea, thanks! I hadn't thought about that possibility... but is that quantity of air really that substantial through .100 holes at 15" vacuum? The primary throttles are open further, at highway cruise, so I would think almost all the airflow would be going through those bores as intended.
But even if there is some pullover through the secondaries, how is that a problem? The cruise mixture is lean where it should be... and 71/86 jetting is not unusual.
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When I talk to my mentor later today for my daily butt chewing I’ll ask him about what Bewy is saying.

I just don’t know one way or the other. We have discussed using bypass holes in the throttle plates in the past but I don’t think we ever talked about those holes causing fuel to flow from the boosters.

I just don’t know. I do remember he is pretty particular where the holes are. I don’t remember exactly where he says they should be but IIRC I *think* he said they need to be oriented to the T slot.

I need to verify that too.
 
Let me know what you find out!
I think your memory matches mine (holes in line with the t-slot)... I bought the carb with the holes from Brad Haak, who knows a thing or two about Holley setup ;)
 
I dont think a .100 hole in a throttle plate will flow enough volume to start the booster flow. There has to be more pressure drop than that will make. At least in my head. Everyone has always said they need to be near the transfer slot but that kinda defeats the purpose of dry bypass air if you ask me as they will pull fuel from the transfers.
 
DrCharles said:
Let me know what you find out!
I think your memory matches mine (holes in line with the t-slot)... I bought the carb with the holes from Brad Haak, who knows a thing or two about Holley setup ;)
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Ok, Bewy is correct.

You can get the boosters to drip fuel at a cruise IF the conditions are right. And that’s about where you are.

A big(ger) hole in the secondaries and cruise vacuum at 14 inches or above can cause the secondary boosters to drip.

Bewy was correct.

I don’t think I’ve ever seen it, but I don’t ever recall using a hole more than about .080 in the butterflies.
 
I don't recall ever seeing (or hearing) about the possibility either! Learn something new every day.

But I don't think it's happening simply because the 3000 rpm cruise AFR is over 15 with #71 primary jets (and #86 secondary which do not appear to be in play)...
 
Doc,
The totality of the airflow at idle & low speed cruise is thus.
- leakage from t/shafts
- air from idle air bleeds
- leakage around t/blades, pri & sec
- air on atmospheric side of T/slot
- PCV.

The engine doesn't know or care where the air comes from, as long as it gets enough to run.
Both QJ & later TQ carbs have bypass air that enters the airhorn, passes through body cavities & exits under the t/blades. With no air passing the venturiis, this ensures no nozzle drip.
Tuning with an A/F gauge, you have no way of knowing what the extra flow from holes in the sec blades is doing for overall jetting requirements. The only way to make sure the jetting is working as designed is to plug the holes in the sec blades.
To supply bypass air, remove the PCV & rig up some sort of variable fitting connected to the PCV port. You can vary the flow to get the desired idle rpm. Then test drive.
 
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