Technical Chasing A Light Stumble Holley

All holley 4barrel carbs idle on primary & secondary idle feeds , some are adjustable (4corner idle adjustment) most are not ( secondary idle constant discharge)..

 

My secondary throttle blades
are closed for all intents and purpose.

 

You have secondary idle air bleeds & secondary idle discharge orifices , you have secondary idle fuel unless the flaps are sealed .

 

,
2OLD2FAST,

I’m not sure how your comment (s)
apply to my light stumble problem,
now fixed.

My understanding is if P transfer slots are square or under square, and S butterfly is closed, Holley idles off
P. In some extreme builds, S idle plate must be opened to achieve idle.

My S plate is closed to the extent that I adjust completely closed, then, shining a flashlight thru, I open just enough to see the first ray of light, to avoid plate sticking per Holley.

Now, as soon as throttle is opened and in fact up to about 3000, Holleys Cruise
off idle circuit.

Of course, if I’m correct, then why do S idle screws have an affect at dead idle ?

Comments welcome,
I’m all ears !

 

There is a secondary idle air bleed open to atmosphere , there is a secondary idle discharge port under the flaps , air fuel emulsion will be pulled through the discharge port as long as there's negative pressure ( vacuum if you prefer ) in the intake manifold . by closing the secondary flaps , you're actually increasing the amount of mixture

 

Opening S slightly, might
add air and lean out idle,
but could this also exacerbate
any light stumble during
transition to all four ?

 

Depends on if its a lean or a rich stumble or if its a timing induced stumble , experimentation holds the answer ...
Generally , you can cover up a stumble with accelerator pump cams or shooter changes . If that doesn't work , the next step ius to modify circuit timing , doing so can be , at best , arduous .Its also very easy to turn a perfectly good carb into junk trying . Does weather affect the stumble , does humidity ?

 

Timing is 25-26 idle, 35-36 all in at 1800.
Full vacuum Advance.

“Generally , you can cover up a stumble with accelerator pump cams or shooter changes .“

This is what I had done. Way too much pump cam and shooter size was corrected by upsizing P jets 10%,
3 sizes.

Also didn’t know about specific pump cams Holley suggest for Pro linkage. Change to them helped.

As far as weather/humidity effecting driveability, we’ll see as summertime is just around the corner. I’m pretty close to sea level here.

Amen to drilling holes !

I might play around with S plates. Holley says leave setting alone unless
car has radical motor. My cam is 233/241, 530/545, 110’. IMO, not so radical ?

I have full control of idle speed w/idle speed screw, and 1/4 turn of any idle mixture screw effects idle quality.

Another ?, Holley says start with idle mixture screws 1.5 turns out, should be very close. Many have posted idle screws 1.5-2 turns out. I have never been able to run this rich on 3 different cars. All three cars (BBC, SBC, 400 Olds) and 4 different Holleys seem to like 3/4 turn only on idle mixture.

What does this tell you ?

A lot of fun, getting everything out of a DP Holley ! IMO, amazing pieces of engineering. Just wish I could predict the outcome of changes.

Full access to secondary throttle plate adjustment , you can see a thumb screw just above pressure gauge.

(For some reason, my pics always come out at top of post)

 

1.5 turns out is just a bench setting, so the engine will start once the carburetor is installed on the engine.

If that was "very close" they would have just used fixed jets for the idle circuit and saved money. When you read stuff like "It only idles with the mixture screws five turns out" most likely it means something like a restriction in the idle circuit. Just a slight adjustment of the idle mixture screws moves the air fuel ratio a whole bunch. Sounds like you're in good shape there.

 

Turning the idle adjustment screws does not change the idle mixture (emulsion). The emulsion air / fuel ratio is determined by the idle air bleeds & the idle or pilot jet . the idle adjustment screw determines the amount of the mixture ( emulsion) that reaches the manifold .
Jenkins comp , if you're actually running with 7 lbs of fuel pressure ( picture) , you might try reducing that to 5-5.5 ,that plus your float heights can change idle / transition slot performance characteristics.

 

Well if you say so.

The point I'm making here, even a very slight adjustment of the idle mixture screws will move the AFR a whole point. This is a lot. This is one reason they were practically disabled during the smog years, at the end of the carb era. Too many dumb shits out there with the "Golden Screwdriver.".

 

Are idle air bleeds to
adjust air to fuel ratio ?

What do IFR’s, 2 small holes in each metering block do ?

I have never been able to get much info on float level other than “too high, gas spills over, too low, engine starved”. Adjust in middle of window. Any thoughts ?

RE: fuel pressure, I’m told these gauges might be off 2 or more #’s. Must admit gauge is more for “poser hot rod” looks
than for reference. Hahaha.

If fuel pressure drops, it’ll be at big end,
not idle ? Gauge would have to be plumbed to back side of hood or dash
to have any real value.

Should I try idle adjustments at 1/8 turns, I currently try for 1/4 turns.
Set one, then adjust other then repeat cycle until max idle RPM (off dial back timing light) is reached. I can’t use vacuum gauge, cam overlap (?) makes idle vacuum too nervous.

 

Truck 64 , I didn't make that up
J Comp , IFR's control the amount of fuel in the mixture ,IAB's control the amount of air , but they also control the system timing ( how soon the the system activates) . If you enlarge the IFR , you also slow down how quickly it activates .
I run a 10.4 388" SBC with a 232°234° @ .050 , .490 lift , 108° LSA cam. At 900 rpm idle there is some needle fluctuation , however I can still set a satisfactory 13+ - inches of vacuum reading . This is with 2x4 -450 mech. secondary Holley's on an edelbrock street tunnel ram .

 

Nobody said you made anything up.

I just don't want anybody to get the impression the idle mixture screws don't change the AFR, that's sort of what it sounded like.

------
Anyhoo the idle mixture will ordinarily run very rich to extremely rich if not adjusted correctly. They are precision cut and will meter the fuel extremely fine. Plugs can foul quickly with just mistuned idle mixture screws. Throw in the wrong jets, and/or defective or wrong power valve, weak spark - plugs will be ruined quickly.

The engine also has to be tuned correctly - ignition, ignition timing, manifold vacuum, and idle RPM suitable, so that the transition "slot" is covered just right. It gets tricky with performance parts, racing camshafts and such where engines don't idle very well to begin with.

 

“J Comp , IFR's control the amount of fuel in the mixture ,IAB's control the amount of air , but they also control the system timing ( how soon the the system activates) . If you enlarge the IFR , you also slow down how quickly it activates .”

This is consistent w/my experience.

Enlarging (leaning) IFR’s actually induced a light hesitation, slowing down IAB’s reaction time ?

I have MOL 13” vacuum at 1000.

Some good info here !

Thanks to 2Fast2Old & Truck 64

Tuning a Holley, time consuming,
Having a well tuned, neck snapping
Holley....Priceless.

 

I’ve seen the “mixture screws don’t change the A/F ratio” quote way to many times. In a steady state idle condition, with a properly set up carb, the mixture screws do change the ENGINE air/fuel ratio. The emulsion mixture and the engine mixture are two different things. If the engine mixture didn’t change with the “mixture” screws, the idle wouldn’t change when you turn them.

 

@Jenkins Competition I bet you'd be interested in a book on tuning Holley written by Dave Vizard, quite a bit of it is excerpted online so you can take a peek through it and see what you think. Whether you're trying to win races or maximize range and economy, a good carb setup and calibration is a lot easier when the carb circuits are explained.

 

Can y’all get all 4 idle screws at
exactly the same spot ?

I can’t. Seems I have differences of
1/8 turn.

This is a dual plane intake and 2” Super Sucker.

 

Why the fixation with the number of turns ? That's totally meaningless.

 

From Quick Fuel,

“ Regardless if you have a two- or four-corner idle adjustment on your QFT™ carb, start by setting all of the idle adjustment screws the same (1-1/4 turns from bottomed out in the metering block is a good place to start). The engine should be turned off while you are setting the adjustment on all of the screws. Start the engine and turn one of the idle screws in or out. What you are looking for is an increase or decrease in rpm. Make these adjustments ¼ turn at a time. If engine speed increases with that one screw, adjust all of the others to match. If the engine decreases rpm when you are adjusting the others, take them all back to the original setting, because the settings were already correct. However, if engine rpm stays consistent or increases when you equalize the remaining screws, make another ¼ turn adjustment on the first screw and adjust the others to match”

“Adjust all others to match”

Stated twice.

 

Get them as close as you can, they will reach the optimum point very close, though it isn't going to be perfectly identical.

Then go for a beer!

 

Seems everyone is too consumed by recommended adjustment specifications. No two engines are the same. Lippy

 

Can you read sparkplugs? Do you understand mixture? If the plugs are reading rich, then it's running rich. I f you have a stumble it's either starving for fuel Off idle, or it is not. I still don't understand needing a 4 corner idle carb on the street. If you cannot figure a four corner idle carb you don't have any business running one. Lippy

 

Sorry , but IMO that's just plain bull ! It isn't like there's micrometer indexing & a pointer to get them all exactly the same . If you set them with a vacuum gauge or an accurate tach , you'll be good ...SMH

 

2OLD2FAST,

Went from 73>70 LSAB ,
I believe this improved response.

Believe you stated too lean air bleeds
could hamper transition off idle circuit ?

I use the digital Tach on dial back timing light.

 

The reasoning for the quick fuel instructions are to prevent a beginner from doing the following....
Initial setting is intentionally lean. End user now adjusts one mixture screw to the point that the engine runs best, and moves on to the next with engine running. The second screw makes a difference, but not as much as the first.
End user now moves onto the 3rd... etc... diminishing returns because the first mixture screw overcompensated for the other 3. The instructions, if followed by an inexperienced enduser, result in less phone calls back to Quick Fuel. It is no more complicated than that.

 

I'd still use a vacuum gauge....

 

My vacuum gauge looks
like #’s 2 or 3 in video.

Steadies around 1800 or so.




I don’t think I can post video of my vacuum gauge in action

Edit: I’m mistaken concerning level of nervousness w/vacuum gauge. Fuel pressure
gauge fluctuates wildly at idle. VG is slightly nervous at 1000.

I can read digital Tach more accurately.
10 RPM increments..

 

Well, certainly makes sense. Instructions dumbed down
to lowest level user !

 

Most carburetors I've ever messed with, it appears to me the initial bench setting is intentionally rich, not intentionally lean. A look at the AFR gauge (as well as the spark plugs) tends to bear this out.

Notice, for example the engine will stall out at idle if the mixture screws are turned in too far. 1.5 turns out is pig rich, at least in any stock or stockish engine.

Of course all this assumes there aren't any vacuum leaks or a gazillion other engine tuning defects. This is one reason why carburetors get the blame first.
A carburetor will not respond correctly if the other main tune-up specs are way off the beam.