Novice holley carb tuning help

[80427]

Hello long time listener first time poster. So long story as short as possible, I am a 40 year old nontraditional student in my final semester of Mechanical Engineering. I commute an hour and twenty mins each way, every day, so I have lots of time to think and experiment with my driveablity of my automobiles.

I have been working on the driveablity and mpg of my 84 Chevy K10, specs below.
1984, K10 reg cab lwb 2.5" of lift
Engine: Chevrolet 383, 9 to 1 compression, 180 cc Dart Iron Eagles 72 cc chamber, Edelbrock eps, 670 holley truck avenger, hei dist 209 intake/216 exhaust, Valve Lift .435intake/.455 exhaust. Lobe C/L 112.
Transmission: 700r4: 1990, 1550 stall speed
Axles: 10 bolts: 4.10s Detroit rear, Truetrac front.
Tires: General Grabber AT3s 33x12.5x15

After struggling with the truck avenger, realizing it doesn't have high speed air bleeds I emailed Holley to receive an answer stating the truck avenger was "never designed for high speed use".

Liking the fact the truck avenger main body has what they call HVS annual boosters, and the fact extra money is sorta slim, I decided to keep as least part of the carb.
So, I went crazy and drilled and tapped the iab and hsab on the primary and secondary sides of the main body for 8-32 set screws. I replaced the metering block on the front with a marine 500 cfm 2bbl block I had laying around and the rear with a block off of a old 4777 650 double pumper I had. The front block had .036 ifr, .078 pvcr and the standard emulsion holes drilled with .026 in addition to a .026 hole in the "knee" that I think some call the "kill bleed", as well as a 4th hold in the middle of the upper angled portion. I drilled and tapped the ifr and pvcr for 6-32 set screws on the primary block. The rear block I left alone.
My starting calibration before tuning was primary block: .030 ifr, .051 pvcr, 66 main jet. secondary block: .039 ifr, no pv, 76 jet
main block was primary side .073 iab, .036 hsab secondary side both air bleeds .o36
First test drive 10-11 afr up do about 2800, cleaned up to 12s at higher rpms.
First I reduced the primary ifr to .027 and primary main jets to 62s, no real change to the idle/transision but the main circuit was 15.4-14.7 at 3000-4500
at this point I decided the rear was adding heavily to the idle and transition circuit so I opened the secondary iab to .055. This showed the af move into the 12s up to about 2800.
I increased the rear iab to .073 and the af changed to 13s in the idle/transistion.
At this point I felt the rear was no longer influencing the idle/transition circuit significantly so I again reduced the primary ifr to .019. Finally the idle screws finally showed a marked difference requiring 1.5 turns out instead of the .6 or so turns it was taking before. Idle to 2500 is: idle at 15.5 slight dip into the 17s just off idle into the high 15s to 15.0 at around 1800 then into the low 14s about cruising at 1850-2100. If I drop it back into drive it is 15.2-14.7 up to where the secondaries come in.

So I am thinking about reducing my iab to about .068 hoping to level of the low speed curve to be closer to 15.0-15.5 from idle to cruising (2000-2100) then possibly opening the hsab to .041 to help delay it coming on to above cruising if I am still seeing a fatter spot at that 2000-2100 range.

My goal is to get 15 mpg out of this old old brick and to understand this higher level tuning so this summer, when I hopefully have a job, I can get my 1050 3 circuit dominator on my camaro tuned to function better on the street. Changing carbs on Drag Week was a pain.

Am I out to lunch? Suggestions?

Thanks for wasting your time reading this long post.

[jmarkaudio]

Is it a 4 corner idle? If so start with around .029 idle jets in both, .070 idle bleeds in both, tune from there. Idle jet and idle bleed are adjusted to have the correct amount of transition fuel AND to make it taper correctly as the mains start. Idle jet controls the overall amount of idle/transition fuel, idle air bleed tapers that amount and set the point where the transition fuel starts leaning out. Idle mixture usually needs less than transition, so the mixture screws control that. You also need the correct amount of primary t-slot exposed so there is no lean spot with small throttle openings off idle. Set that and use the secondary to set idle speed, as long as the secondary is not overexposed you will be fine, less is OK for the secondary. Also might find some info in the forum in my signature.

[80427]

2 corner Idle circuit. It actually idles with throttle closed below the transition slot. In fact it will idle with the screw backed out at 22 inhg.

[jmarkaudio]

Look for a vacuum leak.

[MadBill]

What's the power valve rating? That truck is a bit of a brick and in overdrive with those big tires, low manifold vacuum might be causing the PV to flutter on the edge of enrichment. Dropping back to 3rd likely would bring the vac. above the opening point thus leaning out the AFR.

2x re a vacuum leak; either that or the secondary throttle stop is set too high.

PS: I know guys with way less savvy than what your posts suggest that call themselves experts...

PS2: Make sure you're running lots of vacuum advance with the lean cruise. Too little kills much of the F.E. potential and can cause surge.

[80427]

Power valve is a 6.5. and I am well above that. Typical flat ground cruising is 12-14 in/hg. It will drop below 8 on some hills.

I have spent a lot of time looking for a vacuum leak with no success. It could be under the intake but I have yet to test with my lpg torch in the valve cover. The only reason I suspect that I don't have one is the fact that it is pulling 20+ in/hg at idle and until finally putting a .019 restrictor in the front ifr it has always been fat more than lean. I will keep checking though. Though the secondary butterflies are fully closed, the fitment leaves something to be desired.

I was running about 52 degrees of total timing but I have been having some part throttle detonation. I have it turned back to about 46 total now while tuning but will work on it again once I get the carb more lined out and fine tune my converter "latch".

[MadBill]

<EDIT: Written prior to reading below post.>
The air it's idling on has got to be coming from somewhere. As you mention, bad-fitting throttle blades or maybe a stuck-open PCV valve?

If Mark's advice doesn't cure it, possibly smaller still mains (and of course matching bigger P.V.C.R.s) might.

[Tuner]

Try a smaller primary MAB. closer to the .028" - .032 area. The large MAB will cause the main to start earlier and be richer in the small throttle opening range, level road and slight acceleration. I think this is why it likes the small IJ. More than one way to skin a cat, so whatever gets the good mileage is right. I recently had a problem with a 600 Holley that had a flat spot on gentle acceleration up from a steady speed on level ground in the 30-40 MPH range and it turned out to not tolerate any secondary opening at idle at all. Not sure what that was about, but closing the secondary all the way with just enough touch of the secondary speed screw to keep it from sticking was what it wanted. The butterflies were adjusted to fit the bores with minimal light leak.

I agree with Mad Bill, you are being meticulous, good job.

[MadBill]

..it turned out to not tolerate any secondary opening at idle at all. Not sure what that was about, but closing the secondary all the way with just enough touch of the secondary speed screw to keep it from sticking was what it wanted. ...

I had that on a 2 x 4 two-corner idle carb pair while trying to get the primary blades positioned right. It was a (big volume) GM factory cross-ram with non-original carbs and I later decided the problem was really lean (non-adjustable of course) secondary idle calibrations, but I used the same fix you did and just drilled the front blades.

[80427]

Try a smaller primary MAB. closer to the .028" - .032 area. The large MAB will cause the main to start earlier and be richer in the small throttle opening range, level road and slight acceleration. I think this is why it likes the small IJ. More than one way to skin a cat, so whatever gets the good mileage is right. I recently had a problem with a 600 Holley that had a flat spot on gentle acceleration up from a steady speed on level ground in the 30-40 MPH range and it turned out to not tolerate any secondary opening at idle at all. Not sure what that was about, but closing the secondary all the way with just enough touch of the secondary speed screw to keep it from sticking was what it wanted. The butterflies were adjusted to fit the bores with minimal light leak.

I agree with Mad Bill, you are being meticulous, good job.

Are you saying a smaller hsab will delay the main circuit start? I had it backwards if that is true. I had in my head a straw with a hole drilled in it. To pull fuel up the straw you need to pull harder with a bigger hole. Help me understand.

[Tuner]

Try a smaller primary MAB. closer to the .028" - .032 area. The large MAB will cause the main to start earlier and be richer in the small throttle opening range, level road and slight acceleration. I think this is why it likes the small IJ. More than one way to skin a cat, so whatever gets the good mileage is right. I recently had a problem with a 600 Holley that had a flat spot on gentle acceleration up from a steady speed on level ground in the 30-40 MPH range and it turned out to not tolerate any secondary opening at idle at all. Not sure what that was about, but closing the secondary all the way with just enough touch of the secondary speed screw to keep it from sticking was what it wanted. The butterflies were adjusted to fit the bores with minimal light leak.

I agree with Mad Bill, you are being meticulous, good job.

Are you saying a smaller hsab will delay the main circuit start? I had it backwards if that is true. I had in my head a straw with a hole drilled in it. To pull fuel up the straw you need to pull harder with a bigger hole. Help me understand.

http://racingfuelsystems.myfunforum.org ... out42.html

The emulsion bleeds enter the well at and below fuel level, the emulsion of air and fuel is lighter than solid fuel so it will lift to the spill-over height with less venturi suction. This starts the main sooner and makes it richer in the early part of the flow range.

Put your primary up to a fast idle at an RPM where the main just starts to flow and put your fingers or tooth picks in the main bleeds, see what happens and describe it. Or, just change the MAB to .028" and go for a drive, see what the A/F gauge says.... or, do both.

[80427]

I only had to read it 3 times, good thing I have all those engineering classes to help me. I will reduce the hsab and see what happens. I believed you I just want to "get it". Tell me when you want to start a new thread with the topic "how part Throttle effects peak torque and BSFC" that is for one of my other future projects.

[80427]

Going smaller on the hsab made it richer earlier. Not sure if the annual booster s have any thing to do with it but I will change it back for an A B A test.

[Tuner]

How much smaller? From what size to what smaller size?

[80427]

From .035 to .7mm or approx .02755