I have a 700 Double Pumper on my 355 and I am going to be starting it up and breaking in my new (second) cam (see SIG).

I have the secondary throttle plates barely cracked open, the primary jets are 69's and the secondaries are 78's, a 28 front pump nozzle and a 31 rear, all as-is from new purchase. I changed the power valve to a 4.5, and the pump cam to a blue cam in the second hole.

Does this sound like a good starting point or am I way out of the ball park?

I am at about 600 to 800 feet above sea level (northern IL) if that matters.

 

If it's the 282S cam, use a higher number PV; 8.5 would be the minimum. Otherwise you'll have a GIANT flat spot at tip-in from cruise.

Increase the primary jets to 72.

Other than that, it should be good.

 

It is a Comp 282S, I was going to get it with a 4N7 swap but I was worried about warranty, after reading some horror stories about Comp.

I have an 8.5 power valve, and I will try the 72 jets.

Thanx for the info.

EDIT: Uh... I actually couldn't justify to the Ol' Lady the extra $100.00 for the custom ground cam.

 

Power Valves on a " Properly Set-up" carb have nothing to do with " Tip in". If you're relying on Power Valve enrichment to cure a " Tip in " problem at cruise, then you have other problems, and a higher PV# is just a crutch. Uses more fuel as well. Proper method is to optimise the Transition, Idle circuit and accelerator pump circuit.

Power Valves are intended ONLY for Medium to High load enrichment.

 

I have 4 different power valves laying around and they are all new. I have a 4.5, 5.5, 6.5, and an 8.5. Should I start off with the 4.5 and then check the vac at idle in gear then go from there, or what?

 

"Tip-in" was probably the wrong word.... the PV shouldn't activate until a fairly heavy load is placed on the motor. Something more like "passing maneuver" would be closer to the general idea.

What you don't want, is for your PV operating point to be at a vacuum so far below normal vacuum, that you have to literally floor it to get any enrichment.

Start with the 8.5.

IMO the accelerator pump is just another "crutch".... a properly set up carb should work right with a relatively small one. All it's intended to do, is to make up for the time lag between when air starts to flow as you open the throttle, and the fuel starts to be delivered. A fraction of a second is all. If you have a flat spot that lasts any longer than that, twiddling with the pump (even if you can manage to cover it up that way) isn't the right answer.

I ran that cam for a decade at least. It's a pretty familiar one. Your idle vacuum should be around 13-14", and cruise vacuum should be a bit higher than that, if you have a gear that lets the engine RPM get up where it needs to be (above 2000 or so).

 

Got a 3:73. That should keep the motor, I am figuring, in the 2500 range while cruizing.

 

A #28 shooter in the Primary will likely not be enough. I'd start off with a #31, and you may have to go up to a #35 with the Auto box.

 

Agree with your "revised" wording about PV. However, I don't agree with totally with your your statement that the time lag for fuel flow is a fraction of a second.

While you should always use the minimum Pump shot that you can, the accelerator pump shot lasts a fairly long time. Fuel has much more mass than air. In relative terms to air flow, drawing fuel up through the emulsions tube wells and through the discharge ports in the boosters takes time. How much time? With the motor off, look down the carb and slowly roll the throttle on. Note how long the pump shot lasts. You may be quite surprised at just how long the Pump shot lasts. It is much more than a " fraction of a second".... and yes, it is necessary.

Granted, while many people use "too much pump shot" as a crutch to mask poor tuning ability, it is a necessary circuit for ANY carburator. The more efficient the carburator metering circuit, the less pump shot will be required. IE: Q'jets and Edelebrocks have much more efficient booster designs and use much less pump shot than a Holley 4150\4160 series. They are also three circuit carbs whereas the Holley is a two circuit carb....which is one of the main reasons why Holley's rely so heavily on the pump shot to cover the "hole" in the metering curve until the main boosters start to flow.

The other part of the equation is Booster design. Holley booster design is inefficient, unless you have Annular boosters. Annular boosters create a much stronger booster signal than the "Dog Leg" and [Yuck] "Straight Leg" boosters. Switching over to Annular boosters on the Primary side typically enables you to jet down 2 or sometimes 3 jet sizes compared to the regular boosters. You can also get away with using less pump shot...which is good. Annular Boosters also allow you to use a lower PV opening point than normal.

Some high end carburators, such as Webers, Mikuni Solex and Dellorto, do not even use a Power valve circuit. These carbs are so efficient ( and pricey!! ) that the fuel metering can be correct without the aid of an additional PV circuit. However, they are usually installed on " Independant Runner" manifolds which create HUGE booster signals.

 

So I may have to try a 31 front and 35 rear shooter combo?

I will prolly mess around with the shooters after I break in the new cam.

As for the "auto box" I have a 2700rpm stall speed if this means anything.

 

No, leave the Secondary shooter as is ( for now) . Primary shooter is likely going to require at least a #31....or possibly a #35. Get the " Tube Type " shooters. They direct the flow more accurately.

 

Is it common to have a primary shooter that is bigger than the secondary?

EDIT: My carb right now has a 28 primary and a 31 secondary.

 

I'd recommend leaving the shooters, and everything else about the accelerator pump, alone, until you get the main system, the idle sytem, the transition system, and the same things about the secondaries, set up right. The pump is the VERY LAST THING to calibrate.

 

Thats kinda what I was figuring.

 

Yes, primary shooter often larger than secondary.

 

But when you have information on what BASELINE setting works, you may as well use that info. Isn't that what this board is about?

Do a search on Accelerator Pumps on the Forum and you will soon find out that a #28 is going to be too small.

Seems it's OK for " RB83L69" to recommend jetting changes ( which IMHO may turn out to be to rich....but I bit my tongue on that earlier ) but it's not alright for someone like me to recommend pump settings??? Whatever

BTW....I do not agree that the pump is the "very last thing" to calibrate. The Pump circuit plays such a large part in tuning a Holley that should be set as soon as possible. Even with the main jets dead accurate, a bad pump calibration ( Shooters AND Cam ) can cause all sorts of problems. Here's a thread that shows just how important pump calibration is. Look for the post from Heavy_Chevy29 near the bottom of the page ( Post #52). That should answer the question about how important a baseline setup for pump calibration is.

https://www.thirdgen.org/techbb2/sho...hreadid=202171

RB83L69: Seems that we're going to have to agree to disagree.

 

To the best of my knowledge, air passing through the venturi of a carburetor creates a low pressure; this low pressure is used to cause the carb to introduce fuel into the air. This is the first, most important system of the carburetor. It is called the "main" system, for good reason. This is the one that should be set up first. It consists of the boosters, venturis, and jets. Obviously the boosters aren't user-selectable in these carbs, and the venturis are a property of the casting, leaving only the jets that are field-accessible.

As we all know, 14.7 lbs of air are required to burn 1 lb of fuel completely and perfectly. However, most real-world engines do not produce maximum power at this chemically ideal (stoichiometric) ratio. Invariably, some small amount of extra fuel must be added. This is referred to as "power enrichment". Once the steady-state cruise setup, the "main" system, is properly calibrated, the steady-state "power enrichment" system is next. This consists of the power valve and its restrictions. In a Holley, the restrictions are not readily user-accessible. The power valve determines the break point between the carb's "cruise" (main) characteristics, and its "power" characteristics.

There is rarely enough air flow through the "main" system at idle to produce adequate fuel flow. Therefore, a separate system is included, to supply fuel at low air flows. This usually occurs when the engine is idling. Therefore this system is called the "idle" system. It feeds fuel in direct proportion to manifold vacuum; and as such, its operation is highly dependent on anything that affects idle vacuum. Furthermore, as the engine speeds up off idle and begins to produce power, the bulk of the fuel must change from being metered by the "idle" system, to being metered by the "main" system. The parts of the carburetor responsible for this "transition" (again, not very accessible to customization) are called - you guessed it - the "transition" system. Usually, if a carb is selected that's reasonably appropriate for the application at hand, such as a Street Avenger for the street or a Dominator for a strip-only car, it's plenty close enough. Once the main and idle systems are set up properly, and the idle speed is adjusted correctly such that almost none of the transition slot is exposed below the throttle blades, it usually does its job fairly well.

After all of those things are tailored to the engine's requirements, there's still usually a lag at the instant that the throttle is opened, between the start of air flow and the start of fuel flow, during which time air is coming into the engine but there is no fuel to go along with it. This very brief instant of time is what the accelerator pump is designed to compensate for. It adds a small amount of fuel to make up for what the main system should be flowing, but isn't yet, due to the inertia of the fuel.

How is it that the very first thing you should start calibrating, is the very last thing in the logical chain of operations? Seems logical to me that one would start at the beginning, rather than the end. Or am I missing something here? One starts with jets, moves to the power valve, tailors the idle, verifies that the transition properties are correct, and THEN AND ONLY THEN starts dinking with the pump.

But I'm just being logical, and giving the results of several decades of experience with these carbs, and a certain amount of reading their mfr's published material as well. I don't claim to be an expert though.

 

Well, I was planning on fine tuning the Holley after the cam break in. I will most likely do the break in with the 69/78 jets and the 28/31 nozzles, and the 4.5 power valve. as soon as the break in is finnished I am planning on moving the 31 shooter to the front and getting a 35 for the secondaries, and trying the 8.5 power valve with the 69/78 jets.

See... I had problems with the darn thing running really rich when I first fired the motor up. That was due to the floats being set wrong. Fixed them and started motor again.

Then the #2 exhaust valve went flat in like 20-30 minutes as I stated in another previous post.

With the exhaust lobe almost 2/3rds flat, the engine was struggling to get 10/11 inches vac at idle with a Comp Cams 280H(hydraulic 230* at .050). I used an 8.5 powervalve with some bad advice from a local here in town, and the motor with the flat lobe and 10 inches of vac idled OK but was insanely eye watering rich with the garage door open. It was bad.

So, I at this point found out the cam had gone flat and the carb was removed to change the motor and this is where I am at. The new cam is not in yet. Hopefully next weekend (time is short sometimes) I will have it running because it goes in the shop soon to get the body restored and they want it running so it will be easier to work with.

So I am starting over again with the 28/31 nozzels, the 69/78 jets, and replacing the factory 5.5 power valve with a 4.5. I am just leary of it running real rich again. Then I will start fine tuning it. (ie. jets, higher numbered power valve such as the 8.5, and shooters)

Oh yea, the secondaries are going to have an adjustable lever arm installed before the carb is re-installed. The techs at Holley stated that the secondaries need to be closed all the way then adjust the screw so that it just barly crackes open the secondary throttle plates (mine were opened up a full turn on the base plate adjusting screw). I just want to be able to adjust the secondaries without removing the carb.

Both of you guys are being very helpfull and I am soaking up all the info I am getting from both of you and I am getting a general understanding of what needs to be done. I am not meaning to sound like I am taking one side over another. Sorry if it sounded that way. I know that ""what works for one may be differen't on another"" when it comes to setting up a carb, so I have to take the experiences of both of you and learn from it. I know I can't just ""do what he did and it'll work"" because more times than not that just doesn't work.

 

I am assuming that when you say "exposed below the throttle blades" you mean while the carb is upside down and you are looking at it from the bottom? And under the throttle blades would actually be the topside of the throttle blades? Or no. Fill me in on this.

I assumed that while looking at the carb from the bottom, you wanted the blades as closed as possible to keep as much of the transition slot in the vacuum (plenum) at idle.

Sorry for my lack of terminology. I am new to this carb stuff. Been dreaming of building a muscle car for over 30 years and now I am finding out how little I really know.

 

Yes, when viewed from the bottom. It's a little slot that's horizontal; there's a hole above it that's also part of the system. You want to set the sec throttles such that the pri throttles just are just barely above the bottom of that slot.

You'll find that the various systems of the carb interact somewhat. However, when you get each one set up correctly in its turn, it's muc easier to keep the interaction to a minimum.

I usually start out with a jet that's too rich, it being easier to get a new motor to at least start and run that way, instead of having the problem where the motor dies unless you keep your foot on the gas. THen, once it's ready to drive, reduce the jets in 3-siz increments until it just starts to surge while cruising at the lowest speed it should run at in high gear (about 1800-2000 RPM wiht the cam you're talking about), and then go back up 2 sizes from there. 3 jet sizes is a standard step, 4 sizes is a large step, 2 sizes is a small step, 1 size is almost imperceptible.

Once it cruises right, you'll have a reliable vacuum reading. Use a PV that's about 2-3" below the vacuum at a normal cruise RPM and conditions. I'm betting it will end up wanting to be a 9.5. 8.5 will be a close enough start.

Once those things are dialed in, you should have good control of the idle mixture with its screws. You should be able to peak the idle vacuum with them, and they should be 1-2 turns out. No more than that, not much less.

Then go to work onthe pump circuit. You'll probably find that once you get the rest of the carb right, the pump almost doesn't matter at all. Use the least pump shot you can that gives good results, to avoid wasting gas.

 

Yea, I know where they are, but I just have been questioning where the throttle blades had to be. I got a good idea now.

 

Alright, I got it. I went out to the garage and looked at the transition slots and I had the whole thing back azzwards in my head.

I got it now, I understand what you are talking about, and where the throttle blades got to be.

 

[/Snip]

I wasn't concerned about you taking one side or the other. I just happen to disagree with some of the view points of RB83L69, particularily the importance of accelerator pump tuning. Nothing wrong with that. He's been tuning Holley carbs for decades and so have I. We each have are ways of doing things and there is " more than one way to skin a cat".


He's missed my point however on what is a "Baseline" on a setup, whether it is a carburator, suspension, brakes etc. You asked for a " Baseline " setup on where to start. People with expertise can provide theie experience to give a starting point. That is what I call a " Baseline". It gets you in the " Ball Park" and isn't that what you wanted. Certain things work...other things don't.


Your initial setup sounded pretty good....with the exception that, IMHO,...the Primary shooter is too small. I provided information that I believe to be correct and he replied back not to change it now. Well....eventually you are going to have to change that front shooter....whether you do it now or later doesn't really matter to me. That's just me speaking from 25+ years of tuning these things...whatever that's worth.

No major deal.... If you choose to listen to my advice I would say to leave the Secondary pump circuit alone for now. Tune the Primary accelerator pump only to begin with . Do get all the other settings correct as well though.

I still recommend starting off with a #31 shooter on the Primary side, then going up to a number #35 if required. Your choice of a Blue cam in the #2 hole is good , IMHO. Of course all aspects of the carb should be set up correctly, but accelerator pump tuning is just as important a step as any other tuning. In fact I've seen more people screw up a carb with filthy rich idles, too big of main jets, etc, etc , trying to get rid of a stumble or hesitation, when all a long it was a problem with the pump cam and shooter choice. The board is full of these problems.

Everything has to work together....but the accelerator pump system on a Holley is necessary and plays a big part in the correct operation of the carb.....because the 4150\4160 series is only a two circuit carb as I previously mentioned.


Good Luck. I'm going to be signing off for a couple of days, because I'm building a new computer, so you likely won't hear back from me for a while. Cheers.

 

Thanx for the input. I will most definately use it.