Rich 3000 rpm cruise

[PRNDL]

I have tried .020 T-slot, but then I have to richen up the mixture screws and Im back in the same boat.

You are right Steve. It acts like fancy carbs I had in the past with 5 emulsion holes. I got tired of screwing with them, so I switched to this basic metering block style. That’s why I think there is an internal leak or something or it’s defective. It acts like a have a ton of emulsion. Super rich onset of the main jet the goes lean fast.

I have always run .031 HSAB’s in my past carbs.


If I up the HSAB’s to just .028, I get a lean spike and slight stumble before the secondaries kick in. If I go to .031, I get a straight big stumble. I’ll be at like 15 aft at primary only wot until secondaries kick in.

[PRNDL]

Makes sense about vacuum secondaries. Especially the 4160,s with weird different bleed and secondary idle differences, but that’s why I went with this one. It’s a 4150, so the front and rear halves are carbon copies of eachother. Just Like a double pumper. Except obviously for the missing secondary accelerator pump circuit, missing secondary PV, and the addition of the secondary servo actuator. Everything is squared.

[Tuner]

I have tried .020 T-slot, but then I have to richen up the mixture screws and Im back in the same boat.

You are right Steve. It acts like fancy carbs I had in the past with 5 emulsion holes. I got tired of screwing with them, so I switched to this basic metering block style. That’s why I think there is an internal leak or something or it’s defective. It acts like a have a ton of emulsion. Super rich onset of the main jet the goes lean fast.

I have always run .031 HSAB’s in my past carbs.


If I up the HSAB’s to just .028, I get a lean spike and slight stumble before the secondaries kick in. If I go to .031, I get a straight big stumble. I’ll be at like 15 aft at primary only wot until secondaries kick in.

Is .020 T slot a misprint?

Many OE Holleys had .025" HSAB and I think then knew what they were doing because the thousands of identical units made that way run like they should on the original engines.

[Tuner]

You don't want any T-slot showing on the secondary and in fact if the bottom of the slot is near the butterfly edge, when manifold vacuum is high it can cause high enough velocity past the butterfly edge it the resulting low pressure will cause fuel discharge and undesired richness.

When curb idle occurs at a manifold vacuum below 14" air flow and fuel flow both vary as vacuum varies. Above 14 inches sonic velocity is reached at the butterfly edge so flow is choked at that maximum mass air flow, however because of bleed air, pressure (vacuum) in the idle circuit is less than sonic choke, so flow varies with pressure difference, more vacuum = more fuel flow. Any source of idle fuel, T-slots, curb idle screws and piss-holes below T-slots, will increase fuel delivery as manifold vacuum increases, while air flow is choked and constant above 14" manifold vacuum.

If you have 4-corner idle, close the secondary idle screws and close the secondary butterflies as much as possible without sticking. Loosen the screws and adjust the secondary butterflies to center them in the bores. Eye gauge the air leak by looking at light leakage around the edges of the butterflies and file or scrape to fit.

[PRNDL]

Oh no. I misunderstood Steve. He asked me if I tried TFS, transfer slots to .020. I did not see that he said front and REAR.

My rear slots are higher. My secondary curb idle screw is set to only 1/8 of a turn. None anywhere near showing.

I meant to say I tested with only the fronts at .020 but then I had to richen the mixture screws.

I do agree that the .025 HSAB’s have made this carb run 95% better than how it came from the factory.

[PRNDL]

Copy that tuner….

You said:
If you have 4-corner idle, close the secondary idle screws and close the secondary butterflies as much as possible without sticking. Loosen the screws and adjust the secondary butterflies to center them in the bores. Eye gauge the air leak by looking at light leakage around the edges of the butterflies and file or scrape to fit.

What do you mean by center them in the bores? I have the rear idle at 1/8 of a turn. Barely cracked.

Are you saying to close the rear idle mixture screws and idke off the fronts?

[HQM383]

Tried going up in IAB?

[PRNDL]

Factory IAB’s are .071 in all 4 corners. I tried .073 and .075. After resetting the idle, I get a lean transition stumble. It seems to like the .071 the best with these .029 IFR’s.

[PRNDL]

My last Holley was an AED 750 DP with too much emulation and was a nightmare to tune.

This one runs way better and honestly, the vacuum secondaries pull harder.

I just want to be able to cruise on the highway without fouling plugs.

[HQM383]

Factory IAB’s are .071 in all 4 corners. I tried .073 and .075. After resetting the idle, I get a lean transition stumble. It seems to like the .071 the best with these .029 IFR’s.

What the larger IAB do for the 3000rpm rich problem you are trying to address? A lean transition stumble could be overcome with squirter cam change.

[PRNDL]

The IAB seem to only affect under 3000 rpm, under 45mpg steady cruise.

Example: .071 IAB means 14.5 transition cruise at about 2500 rpm. .075 means transition cruise was pushing 16 AFR. In both situations, as soon as I hit 3000 rpm cruise, the AFR’s jumped to 13.5. I truly do think it’s something other than primary transition.

I’m running an orange cam and .031 squirter (factory). They seem dead on.

[PRNDL]

I’m going to start with closing my secondaries all the way.

[Tuner]

What do you mean by center them in the bores? I have the rear idle at 1/8 of a turn. Barely cracked.

Center all butterflies by removing the base from the body, back the idle speed screws all the way to no contact so the throttle plates are closed to touching the bore walls and look at the light leaking around the perimeter of the plates by holding the the base between your eyes and a light. They never fit perfect and sometimes really need help.

Loosen the screws attaching the plates to the shafts and reposition the butterflies to the best fit. Snug the screws just enough the plates can move when you tap the throttle shaft on the ends to move it from one side to the other while you simultaneously hold the butterflies snug to the bores with your fingers.

The numbers stamped in the plates should be parallel with the throttle shaft.

Then, after the best fit you can get with this process you use a fine file or a three-sided scraper to remove high spots from either the butterfly edge or the bore wall, as required.

Make a three-sided scraper by grinding the teeth off the sides of a 5/16" or 3/8" triangular file and remove more for the last inch or more so the three sides radius together to form a point at the tip. In other words, make a classic bearing scraper out of a small triangular file.

Are you saying to close the rear idle mixture screws and idke off the fronts?

Yes

[rgalajda]

"It runs great but my plugs are black and I burn through fuel a lot."

I don't know what would be different about that carb to require a #77 primary jet. On a 750 I usually end up with about #69 primary jet along with about 68 to 70 TSR.

[PRNDL]

Ok, I’ll start there. I’ll pull the carb, remove the baseplate, and center the valves. Then test again with rear valves closes and rear idle mixture screws closed.

Just to make sure, rear idle mix closed all the way or cracked to like 1/8 or something?