Voodoo, black magic and Holley carbs

three things I know nothing about. So...  I'm no expert but I believe carbs require air tight gaskets because vacuum leaks are bad. So when your gaskets turn to mush that's probably bad. Also brown gunk in your jets is also probably not good. And the entire thing is filthy. 
 

so questions. It appears that my main jets are a 70 and the secondaries are 74. On a 650 sitting on a 9.8:1 350ci v8 with a moderately sized cam is this too much? Too little? It's always ran rich but it's also had leaks, and other issues so any suggestions are appreciated.

the carb itself is a QFT 650 dp with manual secondaries. Got it lightly used from a friend for half the price of new. It ran well on the old worn out engine. Mostly. But it's been sitting now almost 2 years and apparently this ethanol they put in gas is bad for it. Who knew? Any tips/suggestions? Ordering gaskets tonight. Should I replace the power valves? It's had some large backfires since I got it running this spring. Can I test these? I am not sure I quite understand what they do. 
 

any and all help and suggestions are appreciated 

The jetting is what the jetting is - only way to really know is to plop it on the engine and see.

Power valves are just a vacuum diaphragm.  You can check them by sucking on them, or use a vacuum pump I guess.  What they do is, when the intake manifold vacuum is lower than the power valve's rating, the valve opens and allows fuel to flow through the power valve channel restrictor - basically, another path between the fuel bowl and main well.  It adds roughly 10-12 jet sizes to the fuel mixture and functions as a crude power enrichment setup.  Some people say "complexity = bad" and eliminate the power valve, so it runs pig rich all the time, other people tune for cruise efficiency and modify the PVCR for more fuel.   (I like power valves.)

Quick Fuel made EXCELLENT carburetors, the best I've had the pleasure of working with.  If it's one of the higher end models, everything is tunable with jets, not drill bits like a Holley.

In reply to Pete. (l33t FS) :

this appears to have little removable jets e'r'where I look.... I believe this was around $650 when he bought it for his 383 back in 2015. realized the 650 wasn't enough and went to a 750 then to fuel injection. I would LOVE to swap to FI for it but I want this back together and running right first.

All of this is crazy and strange to me. My brain wants to know how it knows how much fuel to give it with no sensors or inputs. IS IT JUST GUESSING?!?!?!?

In reply to bobzilla :

Oh, there are sensors and inputs!  The main sensor is the vacuum pulled by airflow through the venturi (Bernoulli's Law), which pulls on the main wells, which gets air through the air bleed and fuel through the main jet and it gets emulsified in the well.  The main inputs are manifold vacuum and throttle motion rate.

I'm ignoring the idle circuit, which IS voodoo, but unless you're trying to get a 240 degree @ .050" cam to idle cleanly and have good tip-in response because the guy bought the cam at the bottom of the page for a car he only uses to putt around to cruise-ins... it's voodoo that is best not overthought.

From the Holley Website:

How to calculate Carburetor CFM

The formula for calculating how much CFM (cubic feet per minute) your engine requires is: CFM = Cubic Inches x RPM x Volumetric Efficiency ÷ 3456.

Any ordinary stock engine will have a volumetric efficiency of about 80%. Most rebuilt street engines with average bolt-ons have a volumetric efficiency of about 85%, while race engines can range from 95% up to 110%.

Example: Using a 355 CID engine x 5,500 max rpm = 1,952,500

Take 1,952,500 x .85 = 1,659,625

Then 1,659,625 ÷ 3456 = 480 CFM

 

Even with about a 10% cushion, a 500 CFM carburetor will handle this engine great. If you try this formula yourself, be honest with how much rpm the engine will see. You’d be better off with a carb smaller than needed rather than something oversized that can lead to poor drivability and performance.

Posting that as a baseline for how most of us overcarb the engine. The result of too much carb is low air velocity in the ventury and a lethargic off idle response.

Idle circuits are easy enought to adjust, turn all the way in and then out 1.5 turns. with vac gauge and rpms being observed, adjust for  best results. Then repeate for the other idle jet(s) Make sure your throttle  is not open past the transition slots.

Power vavles fail due to manifold backfires. New Holleys have a checkvalve to prevent them blowing out. Symptom of blown power vavle is car running pic rich.  Rule of thumb is run a power valve that is half of your idle vacuum.

The jets mentioned seem to be pretty big. I would be down in the 60s.

If learning this stuff, have lots of gaskets around as you will need them. Bleed out the bowl into a cup by removing the lower bowl screw and catch the fuel as it comes out. 

If my friend Petey is checking this forum, he will have more sage advice than I can offer.

You can add a power valve check valve on older Holley carbs w/o built in check.

https://www.summitracing.com/parts/hly-125-500

Hey Bob, I have some brand new on the card #62 Holley jets.  Do you want them?

LMK

In reply to noddaz :

I don't know what to do with them. Lol. 

In reply to NOHOME :

Yes I know. Never need more than a 500cfm blah blah blah. Tuu hi is had a great off idle response and ran decent on the old motor. It had a 750 edelbrock that couldn't turn. T  hi is came up for under $300. I figured if it worked well on the worn to hell, tiny cam old motor it should work fine on the new betterer motor. Just trying to get an idea of what the hell to do with these since carbs are a foreign language to me

Also check the float levels for proper adjustment.

In reply to NOHOME :

I've had engines run rich with 55s, lean with 72s, in the primary side.  It's best not to overthink the number and just go with what makes the engine happy.  A lot of things affect it like engine size, cam profile, dual or single plane intake, even exhaust type.

The idle circuit is a lot more than the screws on the side of the metering block.  There are idle jets and idle air bleeds, just like on a more tunable carb like a Weber, except they are cast and drilled in place.  This is one of the reasons why Holley made seemingly fifteen different 600CFM carbs.  It's all tunable if you have a pin vise and a set of tiny drill bits, and you WILL need to tune it if, say, the engine makes 7" of vacuum at idle and had to drill 3/16" holes in the primary throttle blades to get the carb to pass enough air at idle that you could close the throttle plates enough that they aren't uncovering too much of the transition slots.  Ahem.  But this is definitely getting into the category of "overthinking".

(The reason WHY you'd have to mess with it, is that the mixture screws adjust the amount of fuel going to the engine at idle, but the "idle circuit" actually encompasses the fuel flow up to a surprisingly large throttle opening, so the idle jets and air bleeds are important to get a good transition to when the airflow gets high enough that the main circuit starts taking over.  Most people just paste over it with larger and larger accelerator pump squirters and cams, but this is Uncouth.  It's also one of the ways that Quick Fuel carbs are freakin' awesome, they have all of this calibration NAILED for 90% of the applications out there)

As for oversizing....  Carb CFM is rated at 1.5" of vacuum across the carb, for four barrel units.  (3" for 2 barrel, which is why a 500cfm 2-barrel and a 750cfm 4-barrel have the same size throttle plates and venturis)  I don't want a performance engine to have ANY vacuum in the intake manifold at WOT, so it's more of a balancing act between getting good low speed drivability.  As a test on a customer's 289 Mustang with a B*rry Gr*nt carb on it that would simultaneously foul the plugs AND lean stumble, I threw a 750 vac-sec on it that I had kicking around.  It had perfect drivability, which to be honest I wasn't even expecting...   That car got a 600cfm Quick Fuel unit, worked very nice

I've become very fond of Edelbrocks. They don't produce the max power of a very well setup Holley but Edelbrocks are much easier to setup & the street manners are better, not sensitive to backfires & impossible to have a fuel leak since there are no gaskets in the bowls/main body. Use their online guide for setup which is very good. On my 69 Camaro years ago, some guys at a speed shop in Phoenix helped me & we got the Holley running great after weeks of trial & error. We had to also drill the butterflies.
Since then I've done several Edelbrocks of mine & a few for other people & had great success.

I think Melba Toast had a 750 double pumper.   You might expect to step up.  

In reply to mdshaw :

The problem I had with the edelbrock was fuel starve while autox' ing. Hard sustained corners would cause fuel slosh and it fall On its face until you straighten it out and then itd go like hell. Fine on the street, never had an issue. But pull close to a g with those 275's and it wasn't working. That is why I went with the Holley style. 

The jets should be on the other side of the metering block circled in yellow.

#62 is a big jump from #70, but then how rich is rich?  

This thread makes excellent marketing fodder for the Fitech and Snipper products. Not that they don't come with their own universe of challenges.

Into year three of trying to get the Molvo to run right. Fitech to Holley street avenger and just yesterday stuck an Edelbrock 750 on to see if I can get rid of the stumble and rich running that has plagued the car. It should not take 3 years to get a mild 302 to run smoothly. 

In reply to noddaz :

I know, I pulled them and cleaned them. The secondaries had brown gunk in them and that was how I learned what sizes they were. I ordered a new WFT gasket kit which comes with new PV's so its getting those now too whether it needed them or not. 

I was making a joke about not knowing how to tune them more than where/what they are. 

In reply to bobzilla :

Make sure the power valves are the same rating as the old ones.  They come in vacuum ratings from like 2.5 to 10.5 in-hg.  Generic kits usually come with 6.5s.

My experiences with Quick Fuel carbs is that they are spot on as-delivered.  Don't overthink things until you have a running engine you can evaluate.

In reply to Pete. (l33t FS) :

I was curious as to the jets and common thought. This carb was originally bought to support a 400hp 383ci small block that ended up being about 450.

Currently the plan it to clean the carb up, put new gskets and get it running. I do need to get a fuel pressure gague on the feed line to see how much pressure is getting to the carb as well. 

I don't know if this book is still in print or not. I happened to get this copy in a used book store. Takes a lot of the black magic out of working on carbs and certainly answers a lot of question. Also if you have trouble going to sleep at night just read a few paragraphs.

https://share.icloud.com/photos/0JZBK1NMy8l9dr3ldV3YJ44dw

This is a great read: http://www.bob2000.com/carb.htm

I just want to throw out there for anyone dealing with carbs and cleaning them. A Sonic cleaner is your best friend. It'll get all the little ports and holes that are difficult without a problem. 

Used to have dyno carbs for engines if the customer hadn't picked anything out yet and we were ready to run them. Carb would gum up sitting on the shelf from the previous use a month or so before. Pop the bowls, blocks, and base off and sonic clean it all. Dry and reassemble, then it'd run like a champ.

Very good info for ya already, explained by someone that knows holleys way better than me! I never got great at dialing in a Holley, since I don't run em on dd's I keep (have to retune em when the weather changes )

but first disadvantage for a dd i saw, is the mech. secondary's. That's best for light stuff and mt or very high stall. Vacuum sec. are easier to get dials in on average streetcar. 
pete is right on give it the jets the engine wants. But off the top of my head, considering what it came off, I'd guess smaller jets are in order. Their cheap enough at summit and such; get ya a few sizes when you order the reusable gaskets... did ya catch that hint?

I would imagine that Quick Fuel already has a power valve check ball to prevent a backfire blowing it out, so I doubt it needs that kit. Used to be a common problem, though. 
Otherwise, clean it, assemble w reusable gaskets, and be prepared to lean out the jets. Folks Pete's advice, and remember... don't overthink it, and listen to good experience, not popular bench racing knowledge. 

Holley Trick Kit is your friend here.

Renew kit plus most tuning parts.

Or, save a ton of money, and buy a few jets, and a few gaskets. 
Heck, let's go all out GRM... I'll send ya the various size jets I have that you might need, and send me back what doesn't stay in it!

Better than just throwing money at it,