4 barrel carb adjustment

[RatLabGuy]

Holley 4160 on a 302, classic bronco,
It's never run right since I've had it and spent a lot of time sitting while I went through other things.
Just tore town and cleaned the carb, was really nasty. Now I'm at the fun part of learning all the voodoo magic of how to tune it.

Currently have the needle on the secondary turned all the way down to take it out of the picture. Think I have the float set right on the primary, at least it's not shooting gas out the vent anymore
Anyway right now it hums pretty good at first on startup, and responds well. But once it warms and the choke open up, it'll still idle fine but if I bump the throttle it leans out to die, have to let off immediately to keep it going.
However if I open the throttle real slowly, it will still rev like it should. Like it can draw gas slowly, but not quickly. This sounds to me like some issue w/ the gas flow not keeping up.

 

[Tim C]

Sounds like an accelerator pump issue. Do you see fuel spray into each front barrel when you pump the throttle engine off? If so you may need to change the squirter nozzle or the plastic pump cam on the linkage to adjust it.
Also quite a few holley 4160 carbs have a non adjustable idle circuit just to use fuel from the rear bowl to keep the gas fresh back there. Before they started doing that if you drove like an old lady and never opened the back barrels the gas would go bad then if you pulled out in front of a truck and stomped it it'd cut out on you. I'd go ahead and set the rear float properly so the idle and off idle circuits are working right.
Haynes makes a decent book that describes the basics of carb operation and adjustment, cartech books super tuning and modifying Holly carburetors goes into even more detail and can be picked up at most bookstores if you're interested in reading up on the subject.
I assume you got the ignition figured out in that thing?

 

[RatLabGuy]

Yes, ignition is working fine now as far as I can tell w/o actually driving it (there are many more issues to resolve before its roadworthy), everything was replaced.

This has the typical curb idle set screw adjustment, turning it seems to do its job. I guess I can re-open the secondary, I was just tryin gto isolate things to keep it simple, e.g. get it working right on the primary, then adjust secondary.
With the engine off, there is nothing coming out when I pump the throttle. I wouldn't think there would be, where would it get pressure from? While I had it apart, the diaphram in the accel. pump seemed nice and tight like it was holding vacuum.

 

[Tim C]

In the top of the carb between front barrels there should be a nozzle held on with s Phillips screw. Every time the throttle is opened it should spray a stream of fuel into both barrels if there is fuel in the bowl. The diaphragm under the bowl is the pump.

 

[RatLabGuy]

In

uh

 

[Tim C]

Hit reply too soon on my phone LOL

 

[Tim C]

I edited the original post, but there should be a lever coming off the front throttle shaft where the linkage to the pedal attaches that is actuated by a plastic cam, that lever should press down on a lever coming off the accelerator pump diaphram

They are adjusting the linkage here. The pivot to the right of the wrenches is lifted by the cam which presses the spring down pressing the lever on the accelerator pump cover down. When you reassembeld the carb did this linkage go back together like the picture?

 

[RatLabGuy]

Yes, the linkage all works. I have never touched/adjusted that screw.
Although, now that I play with it closely, it is a little lose w/ the throttle completely closed, I'd say the accelerator pump arm starts to move w/ the linkage pulled about 1/4 way open, it has to be all the way out to be notable. I gather that should be adjusted so the nut just lays on the lever spring...

I'll also watch it to see if the jets are indeed jetting as intended, I am assuming the engine must be running though, will have to check later when I can crank it...

 

[Tim C]

Yes that spring loaded bolt should make contact all the time and should not "bottom out" at WOT. The accelerator pump linkage MUST begin to move the instant the throttle starts to crack open. Double check the plastic pump cam is bolted to the throttle lever too, no telling if it was removed or broke and fell off over the years and with out it the linkage wont move. You can just see the yellowish/white cam held on with a straight tip screw at the far right of the pic I posted. If everything is correct the squirter nozzle held on with the phillips screw just under the choke plate should start dribbling fuel if you barely crack the throttle by hand while looking into the top with the choke held open, and it should flow two decent streams if the throttle is jerked wide open quickly, you'll notice the fuel will continue spraying momentarily as the spring loaded bolt continues moving even after the throttle stops moving.

If the plastic cam is missing you can order them from summit or jegs, also o'reilly auto stocks a kit with an assortment of cams usually. I'd use the natural (yellowish/white) one with the screw in the #1 hole to start with. If the cam is there adjust that bolt until it makes contact and then make sure you can move it down by hand (pressing down on the threaded part in the pic) with the throttle wide open. That will ensure the pump diaphram isnt being over extended when the throttle goes wide open, so it will last longer.

 

[moldman05]

There is a check valve used with pump diaphragm. I can't remember what the holley uses but quadrajet uses a steel ball at bottom of pump. If check valve is stuck open or closed there will be no gas flow when diaphragm is operated

 

[Tim C]

There is a check valve used with pump diaphragm. I can't remember what the holley uses but quadrajet uses a steel ball at bottom of pump. If check valve is stuck open or closed there will be no gas flow when diaphragm is operated

That check valve on most modern Holley's is a red or orange rubber "umbrella" seal the tip sticks through a hole into the float bowl, the seal itself is only seen with the pump diaphragm removed. Older holley carbs had a steel ball and wire cage, I'm not sure when the changeover happened but I've not seen one personally.

 

[RatLabGuy]

I adjusted the linkage so it was right... no change... it all looks like it should. So I then closely inspected the jets on top of the primary... can't see anything coming out when I bump the throttle.

(by the way, when you're a short guy like me it's a fun challenge to sit on top of the engine of a running bronco while holding a flashlight in one hand and moving the throttle w/ the other while trying to peer in there...)

I pulled the nozzle off and checked it, it occurred to me I didn't specifically clean it w/ the other stuff... but it passes air fine. Does look liek some evidence of gas in the little hole w/ the needle in it.

Hm if the float bowl level was just too low would that cause this too?​

 

[Tim C]

Unless the float bowl is empty it won't affect the accelerator pump, it draws fuel directly off the floor of the fuel bowl. It sounds like a disassembly needs to be done to recheck everything. FYI you can test this with the engine off for now for safety, worse thing that'll happen is you'll flood the engine if the pump is actually working. I'd pull the bowl off the front and do some exploratory surgery, that can be done with the carb on or off the truck.

Holley's website has pretty good tech section that'll help you out too. Leg me know what you find, post pics too that may help.

 

[RatLabGuy]

Well as it turns out... the guy who took this thing apart and put back together is an idiot... and forgot to put that little red rubber stopper in you mentioned above... as a result there was no pressure. I'll get it back and started up tomorrow but I'm sure that was it.
This guy

Doh!

 

[Tim C]

That's definitely it. Without that check valve the pump diaphragm just pushes the fuel back in the bowl. Good work.

 

[RatLabGuy]

Wel... in classic Dave fashion... after putting it all back together, last thing I had to do was put that nozzle back on... and managed to drop that damn nozzle screw, and it has vanished somewhere under the engine whowhere to be seen again... argh. Well just glad it wasn't in the intake, lol . That thing is a real PITA to get the nozzle, o-ring, and screw in place w/ the choke plate in the way. Thankfully Amazon will have another one to me by tomorrow

 

[Tim C]

There's a steel check ball under that screw. Don't pump the throttle til you get the nozzle back on or it'll go into the intake

 

[RatLabGuy]

There's a steel check ball under that screw. Don't pump the throttle til you get the nozzle back on or it'll go into the intake

? mine just had the needle under it. E.g. when I turned the carb over on the bench, that's all that fell out.

 

[Tim C]

? mine just had the needle under it. E.g. when I turned the carb over on the bench, that's all that fell out.

OK. That needle is the newer version of the check ball. Older carbs had a bb and a cylindrical weight. The important thing is that it doesn't fall into the intake.

 

[RatLabGuy]

OK. That needle is the newer version of the check ball. Older carbs had a bb and a cylindrical weight. The important thing is that it doesn't fall into the intake.

lol yes, very important. Before taking the float bowl off, I was very tempted to try and run it w/ the nozzle removed but was worried about that exact thing, any pressurized gas pushing it up and out into the intake... then crunch crunch...

 

[RatLabGuy]

Update:
I got the new screw, got it all back together... bolted it onto the truck, confirmed that when I pushed the lever arm for the the accel. pump up w/ my finger, I could hear a little air, it moved ok... cranked it up... seemed to run ok, then choke - same thing. Peaked in there, no gas squirting out when I pulled on the throttle. And, I couldn't depress the lever, like it was under pressure.
Very confused.... I pulled the nozzle off, then tried pushing up on the lever - low and behold, gas came up (pushing that little needle out, had to fish it out w/ a set of hemostats - btw highly recommended tool to have on hand). So then I closely inspected the nozzle...
Well, as it turns out there is an important lesson here.
DON'T do what I did - I was having a hard time dealing w/ holding the little o-ring gasket for the nozzle, the gasket on top, and the screw all together while trying to set it into the barrel to screw in... so I got this brilliant idea to use a tiny dab of RTV to stick the ring to the nozzle, so it coudl all be slipped in together.

Well what I forgot about was... gas dissolves RTV... and the goop had clogged the nozzle. Doh!It on;y took a tiny amount for that .030 hole lol.
so I cleaned it out, this time used a teeny tiny dab of Krazy glue...

Now it squirts like it should! Yay!

Buuuut... it's still not great. Still has a notable stumble when you press it hard, in fact it can backfire and die, if it doesn't make it through just right.
What I've noticed is that even though that adjusting screw pictured above is carefully set, so the lever arm moves as soon as the throttle lever starts moving, if I peer inside and watch the nozzles, gas dosn't actually start squirting out until maybe 10-15% into the throttle stroke. However the pump arm IS moving immediately. It's like there's a little dead space somewhere between the initial movement of the accelerator pump arm and the squirting action.
How do I get rid of that? I assume it's not normal?

At least I feel like this is a step forward

 

[Tim C]

Is the arm moving immediately with the choke fully open? If it was adjusted with the choke closed the fast idle cam will have the throttle slightly open, then when the choke opens and the throttle closes the adjustment may be off causing your delay. Another thing to check would be that needle under the squirter screw. If it doesn't seat tight it'll allow air to be pulled back in as the pump diaphragm is resetting. A common trick it to use a punch and LIGHTLY tap it with a hammer to reform the seat if it is corroded or dirty. Last thing, watch the linkage for the rear barrels, Im assuming this is a vacuum secondary carb, when you press it hard is the rear linkage moving, opening the secondary throttle plates on the rear barrels? Generally the secondaries should not open at all free revving in neutral or only JUST slightly at real high rpms, vacuum moving through the front venturi is what pulls the secondary diaphragm open, once the airflow is high enough. This can be tuned by changing the spring behind the secondary diaphragm. However if you still have the secondary needle shut off and the throttle back there is opening that could explain your lean stumble.

If it was none of the above issues then its time to start tuning the pump system, this is where holley carbs shine in my opinion they're so adjustable. The nozzle or squirter size determines the duration of the pump shot, a smaller squirter will make the fuel spray for a longer period of time but deliver a smaller amount initially, the bigger nozzle will be opposite. Usually the lighter the vehicle and the lower the gearing the bigger the nozzle can be. If you make changes to the nozzle its best to jump 2-3 sizes at a time. The other adjustment to the pump system is that plastic pump cam on the throttle shaft, you can try moving the screw holding it to another hole (theyre marked 1 and 2). If you pull that cam youll see it has 2-3 holes drilled in it, they dont quite line up so if you change to another hole on the throttle arm youll need to slightly move the pump cam to get the other hole to line up. There are also different cams you can chose from with differing profiles so you can really tune the system by changing how soon or how much fuel is delivered.

If it doesnt stumble moving the throttle quickly only half throttle but it stumbles going wot quickly then you may need longer duration on the pump shot meaning a smaller nozzle, if you have a 30 or 31 now I'd try a 28 or possibly a 25 and go from there. If this thing had been run and driven before you got it then the carb shouldnt need much to get right, but if it's a new combo that never got driven you may need to tune it.

 

[Tom@Hilltop_Machine]

I think the problem is its a Holley. Take a large sledge hammer and smash it. You will feel better. Then go buy a Q-jet and you will be happy motoring.