Holley with excessively lean light throttle cruise

[skinny z]

You might try just soldering the holes in those plates closed

Step 1 is to do away with the holes in the throttle plates.

This would be my first step as well.
It seems to me this would force a move to open the throttle a little more at cruise and bring the carb around to the way it was designed to run.

[rebelrouser]

If the carb has replaceable air bleeds, making the idle air bleed smaller will pull a little more fuel though that circuit. If it does not, I have drilled and tapped Holley's many times to fix things like you mention. If you are into the throttle enough that the idle circuit is no longer adding fuel, then either you have issues with dirt in the main circuit, or maybe you just need to increase the main jet size a number. If it is an old carb, don't underestimate that you may have corrosion in places you cannot see. You can also tailor the main circuit with the main air bleeds. I don't mess with the main air bleeds unless I can't fix it any other way.
When cleaning a carb always remember to blow the opposite direction the fuel and air flow normally, if you blow the same direction you just pack it tighter.

[F-BIRD'88]

Agree , you do not need those idle air holes in the throttle blades.. Fill them all with solder or braze.
Now reset all 4 throttles for about .020" of transfer slot exposure at idle. (curb idle stop adjustment.

Incorrect PCV valve (too much idle air flow thru it at idle)
Will mess things up too.
A PCV valve for a engine with a stock cam (high idle vaccuum) can get confused at idle with a racey cam w/lower idle vacuum)

Address the throttle blade air holes first.

[Ken_Parkman]

Thanks for all the suggestions and comments everyone. I will make use of these on the weekend to see what can be done.

A bit more background is this carb was speced for my 04 EMC engine. After that for giggles I stuck the carb on the very mild 401 in the street Rambler, and was astonished how well it worked on an application seemingly very far from what it had been intended for. Fast forward a bunch of years and put together more engine for the car, which is fairly close to the original intended application, and now it does not work.

The engine power was actually 567 which I'm quite happy with. In street trim with quiet-ish full 2 1/2" exhaust it runs 126 mph. It is a bit of a science project in streetable power, which right now is not so streetable.

Last night I pulled the carb down and it is very clean. Nothing seems to be plugged and air blew through everywhere. The jets have been bumped 5 numbers front and back in a Hail Mary to try to stop the leanness and obviously this did not work. But I did not realize the transfer slots were so covered, and did not realize the importance. I'll close the secondary as much as possible and see if that allows the primary slot to be uncovered. Maybe I screwed this up playing with the secondary idle speed screw.

All the idle mixture screws were around one turn, but there was some variation.
The idle feeds are 35 and bleeds are 76p 70s
The timing has manifold vacuum advance. I've messed with the advance thinking that may be a cause and it made no difference. When I tip into the throttle just a tiny bit more it runs fine.

What mystifies me is why it worked so well on a milder engine much further away from the intended application. So I won't solder the plate holes first, but maybe 2nd or 3rd.

[HQM383]

The engine power was actually 567 which I'm quite happy with. In street trim with quiet-ish full 2 1/2" exhaust it runs 126 mph. It is a bit of a science project in streetable power, which right now is not so streetable.

I recall on another forum a guy put up sbc 383 engine specs of:
Heads are Edelbrock E Street minor porting
Intake Edelbock EPS
Speed pro pistons
Pertronix dizzy
Flat tappet hyd Mild cam, 218/224@50 In lift .492 ex .501 110 lobe sep 1.6 rockers
468 TQ 437HP.

There were doubters (me one) but when dyno sheet and engine builder was know there was humble pie shared around. Well done on your build to extract the power you have.

All the idle mixture screws were around one turn, but there was some variation.
The idle feeds are 35 and bleeds are 76p 70s
The timing has manifold vacuum advance. I've messed with the advance thinking that may be a cause and it made no difference. When I tip into the throttle just a tiny bit more it runs fine.

What mystifies me is why it worked so well on a milder engine much further away from the intended application. So I won't solder the plate holes first, but maybe 2nd or 3rd.

.035” IFR should do. Worth trying .065” iab primaries, or even all round to see what that does. I’m still curious of mab and emulsion sizes. They may be shrunk right down for EMC. That could delay onset of mains as there will be reduced emulsion air lifting fuel to the booster early enough.

[rgalajda]

"The idle feeds are 35 and bleeds are 76p 70s"

Do you have full specs for the cam including LSA?
Is this basiclly a street car? What is the list no. of the carb? I am guessing 80512 thru to 80519 maybe.

.035" idle fuel restrictor is not large. A lot of 750 holleys were this size. I have measured up to .038" on a 750. I assume you are using pin gauges to measure this.
.076" primary idle air bleed seems on the large side
I would be converting these to screw in IFR and IAB And trying a .038" IFR and a .070" PIAB . You can tune from there.
At 1600 rpm cruise it must be on the idle circuit. At 2500 rpm in 4th is questionable to be on the idle circuit without more info. Such as booster design.
Closing the secondaries to 1/4 turn past contact may help.
Being told to remove the PV and square jet is complete nonsense.

[Roadknee]

Perhaps the milder engine made more off-idle manifold vacuum which would pull more fuel through the transfer slots at low speed. Closing the secondary throttle should help by reducing the amount of air into the engine that must be satisfied by primary transfer slot fuel. If the lean condition persists a 0.037 primary IFR should be enough. The 076 idle air bleeds should be fine. I’d recommend putting the main jetting back where it was.

[F-BIRD'88]

The .76 PRI IAB seems very big.. For grins try swapping pri to sec IAB.. the .070 IAB seem more in the typical ball park for a 750 hp carb.

[rgalajda]

The .76 PRI IAB seems very big.. For grins try swapping pri to sec IAB.. the .070 IAB seem more in the typical ball park for a 750 hp carb.

What would be the sense of making the secondary IAB too large?

[F-BIRD'88]

The .76 PRI IAB seems very big.. For grins try swapping pri to sec IAB.. the .070 IAB seem more in the typical ball park for a 750 hp carb.

What would be the sense of making the secondary IAB too large?

A temperary trial.

If you don't have other IAB s to try. get the primary side dialed in.

[rgalajda]

. " The 076 idle air bleeds should be fine."

Based on what ? We don't even know the carb list number or what the original size IAB was. Are they screw in bleeds or pressed in? Most of these older carbs were pressed in bleeds. Have you seen a lot of Holley carbs with .076" IAB's ?
More info is what is needed.

[Ken_Parkman]

A few more bits of information: There is no list number on the carb as it is a custom from a carb house. It does have screw in bleeds, but not IFR. I only have blank bleeds and right now do not have the ability to drill. Looking to fix this!

The cam is a 232/240 @ .050" hyd roller on a 110 lsa, 1.7/1.6 rockers for .598"/.584", 106 icl.

It is a street car that is occasionally taken to the track. But it is specifically set up as a street car with the intention of very drivable which has not been achieved yet. So that means street diaphragm clutch, the big OD, quiet exhaust, and a small tire for a stock wheelwell. Because of the diaphragm clutch it will never get a slick cause I have to drive it home. So with those limitations the launch is abysmal and the car is not as quick as it should be, only running an 11.0 at 126 mph. I have still run into some unexpected issues as it broke the shocks off and bent the springs, perches, and axle housing. Just put the rear axle all back together with improvements.

[rgalajda]

Do you know how to change the IFR to screw in type?

For now you could change the IAB to .070"
Closing the secondaries to 1/4 turn past contact may help.

[F-BIRD'88]

You can drill a blank IAB to a 062 by using a common 1/16" drill bit (.062"). Now you got .062 primary IAB s.
A good place to start.(it will shift the idle transition circuit (off idle afr) richer.. It will likely now be a hair too rich now VS way too lean.
But the stumble will be gone.
Now fine tune the size of your new .062" idle air bleeds a hair bigger by honing with yhat 1/16" drill bit BY HAND..
A bit at a time and test using a AFR guage (NB type is fine for this).

Watch the afr guage shift as you open the throttle up from idle.
Set the idle for best running vacuum not a AFR.
(it will be around 12.5:1 afr). ±

Watch the afr guage shift as you open the throttle.
It shoulthen stay around 14:1.+//-. .
If it goes richer,, the IAB is a bit too small.
If it drifts leaner the IAB is a bit too big.
I bet it will want between the .062" (1/16" drill bit) and the .070". bleeds you got.

But you need to fill those holes in the throttle blades and reset the throttle opening T slot exposure at idle.
about .020"/.030" each.
You DO NOT NEED those holes in the throttle blades on this mild a motor. the throttle T slot expose at idle must be correct....
Not that hard.

You can solder up any old IAB with a propane torch and typical electronics repair solder.
a good soldering iron works fine too.
Drill with a1/16" drill bit.. A good place to start IAB tuning. (062"=1/16")

The AFR guage tells you when you got the off idle transition AFR IAB size just right.
You do not need a Wide band AFR guage for this.
A heated narrow band simple type is just as accurate for this tuning.

Final check is done with air cleaner installed.
Ya it matters.

Exhaust system leaks cause AFR guage readings to be false leaner.

It will idle and drive best if when the Initial base spark advance is right about 24°~ 26° +/- BTDC base initial at idle. Recurve the diz to allow this, with correct WOT max timing.. 32°-36° +/-. (a 10° ish mech advance curve)

From that do what you want then with added Vacuum advance. (10-12-15°). ported or full manifold. your preference..

[F-BIRD'88]

What AMC/Rambler car is this?