Oh my gawd, would you please just stop already? I see having a DP carb absolutely no different as I see not having vacuum advance on a dizzy. The engine doesn't decide when it wants air/fuel...I decide. With my right foot.

I want to know EXACTLY where my timing is at any particular time with my 10.5:1 street engine. So I shim/curve my MSD and remove the vacuum advance. But wait...every car comes with vacuum advance. So I must be dumb too, huh.

To the OP:

I had a HORRIBLE problem with my car bogging under hard acceleration. It ended up being as simple as the primary float being too high.

In my case, I had the fuel set to the top of the sight glass of my QuickFuel carb. Under hard acceleration, un-metered fuel would fall out of the bowl overflow and into the intake. It was a mess.

This is one of those things that makes me SO HAPPY for the car teaching me a lesson. I farted around with it for a while and finally asked the right questions. My engine builder suggested that I take the fuel to the BOTTOM of the sight glass, not the TOP.

Once I lowered the float, I was good-to-go!!!

The deal is that the fuel only needs to cover the jets. And the pump should easily refill the bowl quicker than you're using the fuel.

If the engine dies under hard braking, you're too low in the primaries or too high in the secondaries. Because you're starving the primaries by uncovering the jets (front) or doing the exact opposite by splashing fuel directly out of the secondaries (back).

If the engine craps under hard acceleration, it COULD be because you're too high. For the reason stated above.

I'd tinker with that. If you don't have a sight glass, remove the bowl and adjust the float all the way down with the needle valve adjustment. Then back off the needle valve until the float can only come up about 1/4 or 1/3 of the way in the bowl. Put it on without re-adjusting the float and see where you are. Do the same in the secondary, too.

Another question:

Curious what jets you're running. I was surprised by how small of a jet my engine likes.

For reference, I'm running a full-roller 350 (no bore, no stroke) with a high duration cam and 0.590 lift on solid lifters. I have 200 cc aluminum heads and am running 10.5:1 compression. The engine is 450-500 hp.

I'm running a QuickFuel 650 double pump with annular boosters. I'm also running an MSD system on a slow curve...10 degrees initial with 36 degrees total coming in just above 4,000 RPM. No vacuum advance.

I'd have to confirm exactly what I have in there now, but I believe it is 70 in the front and 71 in the back.

I've learned that "jetting up" is simply a mask for poor timing. It's great if you know what you're doing, but I fell in to the beginner's trap of running rich (I got over 80 in the secondaries) to cover my horrible timing curve (all in under 2,000 RPM).

I'm just sharing a few experiences I've had over the past 1.5 years of trying to figure out my engine. Getting back to 'ground zero' with the timing really helped me diagnose my carb issues and get the engine running better.

I AM NOT an experienced or "good" tuner by any means. I'm just trying to relate to you what I found worked with my particular engine in hopes something would spark your imagination and solve your particular dilemma.

Good luck!!!

EDIT:

I re-read your original post and you indicate that the 'issue' occurs when standing still. This would probably negate my thoughts on the float height.

I'll leave it there just in case it tickles you to check something else that is related.

 

this whole statement doesn't even make sense. Maybe you can convince F22, COOLTED, diehrd, bluedawg, 427Hotrod, etc., but not me.

Edit: When you quote someone, use the entire quote, don't edit it like you did to me earlier, it's rude and dishonest.

 

And maybe you can keep me out of your post. I have asked everyone to get on topic to help the OP ...

 

I didn't scrutinize each and every word, but I think I have to go with Mr. 454Luver on this one.

My understanding: It is the high velocity around the butterflies that pulls fuel out of the idle ports that are in the extremely small slots at the base of each bore that atomizes the fuel at idle.

Without velocity, the venturi effect does not exist.

Without the venturi effect NOTHING is leaving the carb bowls. This is why the carb bowls don't spontaneously empty when the engine is turned off and why changing the fuel pressure serving the bowls has no impact on the tune of the engine.

THESE ARE THE BASIC FACTS TO GUIDE THIS DISCUSSION:

ZERO..NONE...NADA...NUNCA fuel enters the engine EXCEPT through the venturi effect.

The fuel is ONLY 'pulled' into the system through a low pressure area generated in the carb which is CREATED by the venturi effect. This happens near the base of the carb around the butterflies at idle. This occurs through the boosters (which are metered by the jets) at higher engine RPM's and loads...barrel velocities that are high enough to generate the venturi effect at the booster and pull fuel through the boosters.

The venturi effect is ONLY created by air velocity. Period. It is defined by velocity. PV=mRT. Pressure and volume are inversely proportional given equal mass, temperature, and R coefficient.

This is basic.

This is fact.

This cannot be challenged.

So...gentlemen...(and ladies, if Mrs. Priya is present)...lets start from there when discussing how the fuel gets introduced and metered into the system.

 

Well I began to try and diagnose the issue of why the engine was bogging down so I first replaced a bad power valve. When I would put the four bolts back in to the secondary fuel bowl I noticed the bottom right bolt would not tighten down. I started the engine and hooked up a vacuum gauge which only read 16 pounds. Is this my problem and is my carburetor shot?

 

This is correct. Having worked in Aerospace for over 25 years, I can tell you, that it has been used in a variety of aircraft designs, and was called 'the coke bottle shape' informally, because the fuselage would have a narrow waist about its mid-point.

The whole reason a venturi works so well, is because either internally (like your carb) or externally (like your F-106 Delta Dart), is that the same volume of air, going through a narrower profile, has to speed up, relative to the larger profile it started with. That increase in velocity, with a small opening, will create a vacuum and pull the fuel out of either the primary or/and the secondary and the shearing effect as it leaves the passage, across a sharp edge, atomizes it.








The F-106 benefited greatly from the 'coke bottle' design and this feature made it even faster, then it's predecessor, the F-102.

F-106 (coke bottle or venturi waist)




F-102 (no coke bottle, slower predecessor).






Great article on Fuel Atomization by Hemmings Motor News

http://www.hemmings.com/hcc/stories/...feature14.html

 

It's extremely unlikely that this problem has anything to do with jetting. It's also not power valve calibration. If you start fooling with those things now, you're going to get sidetracked. Assuming this is a fuel problem, it's being caused by a too-lean charge when you blip the throttle. There are only a few likely culprits above the heads.

First, you might have a damaged accelerator pump diaphragm, bent pump linkage, worn out pump cams, wrong cams, or the need for a larger pump at the primary and/or secondary. The last two items seem less likely to me, because Holley DPs come with pumps and cams that are pretty close to what most people need. You're not running a radical camshaft, and I'll assume the intake is small enough to fit under the hood (not a tunnel ram) and not too over-the-top. In other words, the motor probably isn't suffering from charge dilution or excessive fallout due to overall system design. Dilution and fallout also seem less likely culprits due to the fact that the car bogs whether you blip the throttle at idle or at driving speed.

Second, you might have a vacuum leak. However, if you experience this problem with about the same severity even when the motor is already running at, say, 3,000 RPM on the road, a leak wouldn't seem so likely as the cause. You need to check though, just to be sure. I do this by squirting WD40 around the base of the carb and the intake flanges at idle. Any change in speed indicates a leak. There are other possibilities for leaks, such as worn-out throttle shaft bushings.

Finally, it's possible the intake isn't warming enough to maintain good fuel atomization. Is it possible you have a manifold that's cast without water passages, or maybe water block-offs were used when the manifold was installed? People sometimes do this to prevent fuel bowl boiling on hot days at the track.

To sum up, because the problem sounds relatively severe (it's not just a low-RPM stumble), I would approach it like this:

1. Without the motor running, pull the air cleaner and look at the pump shot when you open the throttle. Both sets of nozzles should start to deliver fuel immediately, as soon as the throttle is barely moved. When the throttle is pulled back quickly to its full-open stop, you should see strong streams of fuel into all four throttle bores.

If the pump shot looks questionable, you should remove the carb and inspect the accelerator pump mechanisms for signs of binding, wear, damage or misadjustment. Don't forget to disassemble the pumps and check to be sure the diaphragms are OK. It's also possible that someone has opened it up and lost a check-ball or reused a worn-out gasket. While you have the carb off, also check float levels like someone else suggested. This might actually be a good time to simply rebuild the carb. Knowing that the correct gaskets have been installed and all the various pieces have been renewed can be worth many hours of troubleshooting time.

Also be sure the carb fits the manifold and base gasket correctly. There are some odd combinations of spread-bore/square-bore running around nowadays. Some require a special base gasket, so you might need to review the manufacturer's directions for the intake.

2. If there's nothing obviously wrong with the carb or its installation, confirm ignition timing. Also use the timing light to be sure the advance systems are working (mechanical and vacuum).

3. Next check for vacuum leaks like I mentioned above. Plug all the vacuum ports on the carb when you do this, including vacuum advance.

4. If all this looks good, and you know the manifold is warming properly, I would consider swapping the carb. If you decide to buy a new or used replacement, I strongly recommend a 650 CFM for street use with the setup you're running. I know, I know...

Good luck with the problem!

EDIT: Here's what you need to understand all the basic Holley carb functions. I think this version might be out of print now, but probably available on eBay.

 

The threads are stripped in the carb body at that location. You'll need to install a Helicoil or replace the carb.


I think your measurement is OK (16 inches). Here's a link that should help. Checkout the "Scenarios" near the bottom of the page:

http://www.secondchancegarage.com/public/186.cfm

 

This C3 forum is the home of the double pumper fan club.

 

And you , a guy who post to much nonsense

 

Wow, not really...... Look, as the throttle blades open, the vacuum in the manifold drops because the velocity has increased from almost nothing to more than almost nothing. The velocity has increased because the restriction is gone, the restriction was the throttle blades. The venturi effect doesn't come into play until the throttle blades are open and the main circuit comes on line. The venturi pulls the fuel from the booster when the throttle is open because of the rush of air when the throttle opens, not when it's closed. The idle circuit doesn't use the venturis, idle fuel is sucked in below the venturis by high manifold vacuum. This is not a challenge to your knowledge, it's just how a carburetor works. I can cite published articles if you want, but you should be able to that I would think.

 

STOP it , no one wants to here yours or anyones superior knowledge debate ,,

HELP THE OP who if he has any sense ran away from all this side talk and B/S

 

Slow your roll. I am helping the OP. When I see total BS I point it out. Keithinspace doesn't know what he's talking about. The best way you and he can help is to stop posting.

 

You really are something. Exactly what that something is I am sure you have been hearing your entire life so no sense in me reminding you ,

But AT least you shut up on your long winded posts that did nothing to further help the OP..

 

You're referring to the sudden intake of air as denser outside atmosphere rushes in to fill the vacuum in the manifold. This is a very short lived effect, and it's not sufficient to pull any significant amount of fuel from the metering circuits. The velocity falls again as the manifold approaches atmosphere, and it won't increase until engine RPM also increases, which causes airflow (CFM) to climb.

Good point! I've always thought idle emulsion was pulled out by the high velocity that exists at the edges of the butterfly in the near-closed position. However, it makes sense that the pressure differential could play a major role. None of this has much to do with the OP's problem though. Regardless of its exact operation, the idle circuit "goes away" when the butterflies slam open.

 

Idle and transition circuits are supplying fuel even under wot. It may not amount to much (relative to the main circuit) at wot but there is fuel flow resulting from atmospheric pressure only i.e. no vacuum at wot and no venturi effect when the throttle blades are fully open in the vertical position. A carb works on differential pressure. Velocity is created by this pressure differential and there is velocity at wot because the DP is now atmospheric or ~14.7 psi.

To the OP. Go spend some time reading up on carb theory and the various functions of the fuel circuits. Also, before you do anything with the carb, make sure your ignition system is working correctly and your timing is set for what YOUR engine likes. Mine likes 22* initial/10* mech'l and I'm not running any radical cam or high compression. It runs far better than with 12* initial and 20* mech'l.

 

I'm not getting into a long-winded retort. I'm tired of wasting my time trying to explain a point to a handful of folks that have no interest in having a back-and-forth discussion where all parties are respectful and assume that the folks bellied up to the table have something useful to share.

I'll say it: I'm 100% certain I don't know half as much about carbs as you do. Certain.

THAT SAID, it doesn't mean what I do have to contribute doesn't have truth to it.

My only point was that EVERYTHING gets into the carb through the venturi effect (except for injected fuel from the accelerator pump).

The idle circuit? Venturi effect from the idle ports at the base of the carb near the butterflies.

Full throttle? Venturi effect from the boosters.

In between? A combination of the two.

Any statement about "velocities falling to zero" and still discussing the admission of fuel simply can't be the case. Without velocity THERE IS no fuel. That was the thought I was trying to relay in my post.

TO THE OP:

Another item I've run into is SIZING of the Power Valve.

The Power Valve is intended to ALLOW extra fuel into the circuit under LOW VACUUM conditions. So when you accelerate and vacuum apparent in the throat of the carb DROPS, the Power Valve OPENS and admits additional fuel into the boosters, bypassing your jets.

My engine generates very low vacuum. I'm just over 7 inches at idle. What was happening is that my Power Valve was almost always OPEN and I was having a difficult time metering my fuel.

I removed my Power Valve and put in a 'blank' (you can buy them) so I could get my jet tuning back to zero.

After I was satisfied with my tune with no Power Valve, I got the 'smallest' or 'lowest' Power Valve I could get...the one that opened only at VERY low vacuum. 2.5 inches, if memory serves.

I could feel the different immediately, and I am satisfied that the Power Valve does what it should...only admits 'bypass' fuel under certain WOT conditions.

Just another one of those little things that isn't normally tinkered with. Hopefully it will help you consider your issues.

 

Look at the section explaining idle: http://www.superchevy.com/how-to/831...uretor-basics/

 

This sounds like you are describing the "area rule"