[gibbons]

My Holley 4150 hesitates badly when the throttle is slam dunked from idle. It doesn't kill, it just sort of stumbles a couple of times until the RPMs get up a little. If I roll into it smooth from idle and hammer it at 1500 RPM, the 468/TH400 immediately starts smoking the tires and revs strong and clean to 6000 rpm.

It is built with an Edlebrock RPM manifold, Lunati medium cam, 9.5:1 TRWs, Hooker Comps, Accel mechanical advance ignition, and Harland Sharp rollers. The 4150 has been heavily played with, the choke horn is milled off, I think it is a 850. It idles rich enough to not need a choke when cold, and the State of Utah thinks its idle emmisions should land it in the crusher. But for general driving, the plugs are a nice toasted marshmallow color.

Anyway, I know this imformation is sketchy. My question is, in general, can a 4150 on an engine like mine be tuned to take out the hesitation?

I hate to sound stupid, but I have always had carbophobia. Other than for nostalgia, I much prefer the EFI's in my normal vehicles and boat.

 

[SS454JC]

Hello Gibbons,

Sounds like you are not getting enough "pump-shot" when you wing the throttle open. You need suffcient volume and duration of pump-shot to give the engine a mouthful of fuel while the butterflies are opening, and just before the discharge boosters take over.

The idea is to get a squirter and accelerator cam combination that provides the engine with a long enough pump-shot. Pretty sure bone stock 850's come with size 31 squirters.

The squirter holes control how long fast the fuel is released. Holes that are too big, release the fuel to fast, so pump-shot duration may be too short and the engine will stumble. Or, the squirters may be the right size, but the accelerator pump cam(s) are not depressing the accelerator pumps long/far enough.

The plastic cams on the throttle shafts control the amount of fuel that the accelerator pumps will deliver to the squirters. Also the position of the cams determine when the accelerator pumps "come on" (position 2 comes on later than position 1). The accelerator pumps need .015 clearance between their lever and operating linkage at wide-open-throttle, or you can tear the pump diaphram.

Try to get your hands on a Holley tuning book, and / or call the Holley tech reps. They are very helpful. I can't find their number! If I find it, I'll send it to you.

Good luck....Joe.

 

[GaryR]

Gibbons,
You should be able to make an 850 work on a 468. Is the carb a vac. secondary or mechanical / double pumper? If you are not sure of the size look on the base plate near the mounting bolts, sometimes they stamp the list number on it.
Have you checked all of the basics like float level & accelorator pump adjustment? If you have a mechanical sec carb try removing the rod that opens the secondaries and raod testing it to see if it does the same. That will eliminate the secondaries opening to fast as the problem. The primary acc. pump should squirt fuel as soon as you wiggle the throtle lever. You want zero clearance between the acc. pump bellcrank and the plastic accuating cam. The best way to adjust the acc. pump is to hold the throttle wide open and adjust the acc. pump linkage secrew until you have .015" to .020" clearance between the screw head and the tip of the acc. pump lever while holding the lever down so the diaphram is fully depressed. It only takes three hands, so you might get a friend to help. It is easier to do with the carb off, but not that tough with it on if you have an extra hand.
Float level should be at the bottom of the sight hole (brass plug in side of float bowl on pass. side) with engine running at idle. I use the clear plastic sight hole plugs in mine so I don't risk fuel spilling on the manifold, and I can check it anytime with no hassle. Fuel should not run out of the hole while the engine is running, but if you wiggle the car it will slop out. It should be as close to running out as possible without actually leaking out the hole. The sec. float should be set about .020" to .030" higher than the pri. Another good reason to spend $6.00 on the plastic plugs.
Also check your ignition timming. You probably need around 16 deg. initial timming with about 36 deg. total timming. If you are running less initial than that you could set it up for a road test and see if it improves the performance. If that makes too much total timing it may cause detonation so I'd only get on it long enough to see if it cures the hesitation and then lift. If that helps then have the dist. recurved to provide 20 deg. advance (10 deg. dist advance) and set the initial at 16 to get a total of 36. That is how my 402/th400 with 9.3 comp. ratio, 800 cfm Holley double pumper is set up and it works fine.
If any of this is unclear feel free to e-mail me and I'll try to make it better.
GaryR

 

[COPO]

Holley carb setup.

Mixture Screws: Turn clockwise until the engine starts to stumble and backout screw 3/4 turn.

If your cruising and the car feels like it's surging then ritchen (turn counter clockwise)
the air/fuel mixture screws 1/16" on both sides and go for a drive. Do this until the surge
is gone.

Some guys like to turn the screws out (richen) until they get the highest vacuum reading or highest rpm but I think this ends up being too rich. You can smell the fuel at the tail pipes at idle.

You should have set up the following:

1. At wide open throttle the pump lever should be set to .015 of an inch (feeler gauge).

2. Secondary throttle stop screw adjustment must be set as follows: back off adjustment
screw until the throttle plates are fully closed (secondaries). Turn adjustment screw
until it just touches the throttle lever and turn 1/2 turn more to position the valves.

3. Float level should be to the bottom of the site screws while car is running.

4. Set idle at 800 rpm or 1000 or what ever your specs call for.

5. You should start off having a yellow secondary diaphram spring to see how fast the
secondaries are opening.

Holley Vacuum Secondary Spring Info:

350 CID Engine
Spring colour RPM to Open RPM at Full Open
Yellow (short) 1620 5680
Yellow 1635 5750
Purple 1915 6950
Plain (Std.) 2240 8160
Brown 2710 8750
Black 2720 not fully open at max air flow

All data taken without air cleaner. An air cleaner would cause earlier opening in all cases.


402 CID Engine
Spring colour RPM to Open RPM at Full Open
Yellow (short) 1410 4960
Yellow 1420 5020
Purple 1680 6050
Plain (Std.) 1960 7130
Brown 2380 7650
Black 2390 not fully open at max air flow

All data taken without air cleaner. An air cleaner would cause earlier opening in all cases.

Mark

 

[DeMarco]

I am having a similar problem with my ride. It is a 427 11.5:1 compression, etc with a Holley 850 Doubble Pumper, Mechanical Secondaries. I was getting a hesitation from idle and at besically any other rpm. With the advice of a few friends I changed to a 50cc accelerator pump. Now the hesitation is gone but my idle is completely screwed up. It is extremely rough and wont hold. I went through some basic tuning but nothing worked. It seems that I fixed one problem and caused another. At the same time that I installed the accelerator pump I installed a new power valve. Any suggestions?

 

[1bad67]

All had good advise. One other option is to verify the squirter is not leaking fuel past the gasket were it seats to the carb body.

I have had a few carbs were the squirter retaining screw was a bit to long not fully seating the squirter allowing fuel to leak between the seal reducing squirter volume.If you have this problem lightly file the screw bottom so it seats properly.

DeMarco,
Verify you have the correct power valve for your application. Idle vaccum divided by 2. If idle vac. is 12" install a 6" power valve.
Also my .02 is installing a 50cc pump on the primary is covering up the promblem. Not resolving it.

COPO,
Idle mixture screws are only for idle character and have no effect on driving exept from a stop to transition. Also jetting affects idle and drivability.

[This message has been edited by 1bad67 (edited 03-08-99).]

 

[GaryR]

DeMarco

I read your post about having problems with your idle after changing to a
50cc acc. pump. Just a quick note to tell you that the 50cc pump is thicker
than the standard pump and will hit the intake manifold on some set-ups. Might
want to be sure it isn't hitting and preventing the carb. from sitting flat on
the manifold, causing a vac. leak.

GaryR

 

[mike reeh]

to add a few things, i would also upgrade to a larger pump discharge nozzle after upgrading to a 50CC pump. also check your pump cam on the throttle linkage.. not much i can really tell you to look for but make sure that it starts a nice strong squirt at the slightest crack of the throttle, and you want to remain squirting as long as you can (up until wide-open throttle) you may just have to play around with the cam & lever settings. the key is to make sure the engine is getting a nice amount of fuel to keep it from bogging. also make sure there is MINIMAL constant pressure on the pump (and lever) at closed-throttle.

also, about idle & jetting..

normal jetting will _not_ affect idle, only cruising speeds. the idle mixture screws, if improperly tuned can and will have an effect on idle and off-idle (sometimes even more) causing slight hesitations. in other words, i would run stock jetting for the carb as it is almost 100% of the time what you should be running.
if you doubt your idle mixture setting, run the mixture just a little rich, it wont hurt nothing and will rule out any lean hesitation problems caused by the idle circuit.

anyways 3 out of 4 times a hesitation problem that is carb related is because its just not tuned right. not because it needs new jets, pump sizes, cams, etc. OR there is a vacuum leak.

if anybody needs some specifics on their carb, let me know the list # for the carb and id be more than happy to look it up in this holley catalog i have. it will tell you original jetting, power valve, discharge nozzle size, etc etc

mike

 

[1bad67]

Mike,

I may be wrong but as I understand the idle circuit fuel deilvery is from the main jet. Hence, if your rich you can run the screws fully in and have no effect on idle mixture.
In fact I'm right !!

When you add a big spark and a cool plug to a cylinder or even if you don't your giving away HP if you dont taylor your jetting to your engine.

Factory Jetting is an excellent starting point. It's amazing the increased drivability that can be obtained by changing jetting up or down. provided you have the proper size carb.

Better review your Holley Knowledge.

[This message has been edited by 1bad67 (edited 03-10-99).]

 

[mike reeh]

1bad67 --

Wait a second here. Are you actually telling me that if I put a larger than stock jet in my holley that im going to screw up my idle mixture, possibly rendering the idle mixture screws completely useless? HOW SO?

the main jet is soley for metering fuel into the main wells at relatively HIGH FLOW rates. i.e. cruising speed.

ABOVE cruising speed (WOT) you have the power valve for when even the relatively large size of a main jet cannot keep up with fuel requirements.

BELOW cruising speed you have the curb idle and transfer slots which GRANTED are circuit thats draw fuel from the main well BUT- there is an "idle feed restriction" in the circuit which by itself will not even meter anywhere NEAR what a main jet will.

Now thats what I have to say about the main jets making the difference in the idle circuit that you claim, which they in fact DONT.

On to your claim about running the idle mixture screws all the way in and having no effect on curb idle, WELL if thats the case you have yourself one now-junk carburetor seeing as how the mixture needles DIRECTLY and 100% control the flow of fuel into the engine from the idle circuit. Unless you have done some clueless, irreversable tweaking of your idle system (IE you broke it) or you damaged the tips of the needles, not letting them seal the circuit off, OR you have your idle speed set so high that the transfer slot is now acting as a transfer slot AND curb idle discharge, then your engine WILL die with the mixture screws turned in until they bottom out.


About changing factory holley jets, ive never heard of anybody (that knew what they were doing!!!) change the jets out and have it really make a difference. they will claim putting smaller jets helps fuel mileage but its my theory that the old placebo affect is kicking in. this does not hold true for strip/track only race setups where they are dialing the engine in (which are usually not streetable anyway) for MAX horsepower.

if i were even going to CONSIDER changing jets, i would first install a good O2 sensor and either a DVOM or aftermarket dial to get a sense of how the motor was running at cruising speeds. then go from there.


if you have a so called "driveability" problem caused by a carb, i can just about 100% guarantee you that you dont have the wrong jets if they are stock for the carb. you need to look into something else. have you ever heard of a driveability problem where the car was running too rich? to slight extent that jetting would be a factor? me either. holley doesnt make carbs run at a perfect 14.7:1 stoich. ratio. Carbureted engines love the richer side of life, 14.7:1 A:F ratio is what EFI and computer controlled carbs are for.


looks like you better review YOUR damn knowledge.


michael reeh

[This message has been edited by mike reeh (edited 03-10-99).]

 

[Fred Ont canada]

Sorry Mike I am going to disagree with your theory on main jets.I had a truck with a 534cu engine down sized the jets main and sec. fuel mileage went from 21/2 to 5mpg with no lost power.It only stands to reason a smaller hole passes less fuel than a big one...FRED

 

[mike reeh]

theory's are meant to be dis-proven! 534 cubes is a lot of motor, and like i said you wouldnt notice a power difference because thats what a power valve is for, when you put your foot in it. not at 1/3 throttle going down the hi-way when only the main jets are doing the metering.

good point though

mike

 

[DeMarco]

Thanks for your help. I'm going to give things a compete looking over this weekend. I'll let you know.
Thanks,
Chris

------------------

 

[1bad67]

All I'm stating is if gibbons carb is way out of calibration and jetting is higher than stock say 6-10# which allows greater fuel wieght on the main well it can cause problems. They carb size is unknown ! And it sounds like a squirter or acc. problem. I was refraiing COPO statement on idle mixture.

Your taking my statement out of text! I can say that changing the jetting effects idle mixture in the way that increasing or decreasing jetting 1-4 numbers changes the wieght of the fuel passing through the "restriction" area causing the need to adjust idle mixture. Not rendering it useless unless jetting is off base.

Yes I have seen several vehicles that stink of unburned fuel and stumble because their rich or "fat". A spark plug is an excellent indicator of air fuel ratio. A daily driver should have no carbon build up on the plugs. I personally feel thats were some get confused, is they think a stumble or miss is always a lean condition when it can be the opposite.

As for jetting 80% of the time a daily driver is running on the primary cicuit.Why not remove 1 or two numbers of jet to fine tune the circuit. As you stated the P.V. will increase the fuel at WOT. Holley sells a carb for all applications and they are sent out on the rich side, the safe side. Everyones needs are different this carb will run but for optimum performance tuning is required. Thats why there is a power valve. To lean out the main system for a crisp responsive carburator that doesnt go lean at WOT. So yes jetting can improve drivability.
When squirter,and acc. pump are properly adjusted, or have a lazy un responsive carb. On the other side of the spectrum a power valve has a predetermined flow rate. So jetting can also effect wide open throttle causing a rich or lean condition.On a race engine it is difficult to reach prime fuel ratio at idle and not br lean at WOT.

How my carb got involved I don't know. If your curious it's a holley HP 950. 78 jets square stock. now at 75 because it was rich. It's a very crisp and responsive carburator. the best i've owned, it also has 30cc pumps.

You think people dont know how to jet carburators, The installation of a O2 sensor or exhuast temp in the wrong location can cause problems as well.

Also gas additives can disturb your plug readings. Plug reading is the cheapest way of achieving optimum tuning. I know, I'm wrong and don't know what I'm doing but I would only waste money on an exhuast gas sensor if I were a racer. They are also good for nitrous oxide to verify you dont enter a lean condition.

This is simply a debate and opinions. You have yours I have mine. I know there are several compitant humans that can jet a carb.


[This message has been edited by 1bad67 (edited 03-11-99).]

[This message has been edited by 1bad67 (edited 03-11-99).]

[This message has been edited by 1bad67 (edited 03-11-99).]

 

[Fred Ont canada]

I can and have played with jetting Holleys lots of times there is no use of putting more fuel in an engine then it needs.Heres the way I did my Chevelles 650 I made it lean to the point that it was lazy and would barely pull up a hill near home.Then went up 2 sizes tried it 2 sizes each time until it regained power then down 1 size primaries only.20mpg and lots of power and great throttle response...FRED

 

[71Bu]

Just a thought that if you like your Fuel Injected cars better, maybe you would rather put injection on your 468 (If you want to pay the money)!
Luke

 

[Fred Ont canada]

Come on guys lets hold the insults.If Holleys were not meant to be fine tuned by rejetting why do they have so many different sizes and sell kits in speed shops.I got mine from a stock car racer who was getting out about 20 sets.Take a look in Summit cat page 81 bottom right corner,"fuel flow adjustment made easy".I also use a K&N stub stack with out this my 72 had a slight stumble at about 50 mph.Also on a hard launch you could go slightly over rich due to fuel slosh...FRED