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Posted: **Thu Oct 06, 2016 7:08 pm**

Dave,

Please straighten me out if I don't understand everything I know about this stuff.

The carb has vacuum secondaries. The mixture goes way lean when vacuum is really low and I'm thinking the secondaries are still closed. Seems like the first thing to do is get the A/F right at WOT before the secondaries open. When WOT A/F on primaries is good, then increase the secondary jets, if necessary, to get A/F right with secondaries open.

I'm thinking about disabling the secondaries till A/F is good for WOT on primaries.

Jim

Posted: **Fri Oct 07, 2016 9:07 pm**

Talked to Carl and my plan is now clear as mud.

The Holley Carb Tuning book suggests blocking off the power valve and selecting main jets to get cruise A/F right, then using the power valve and restrictors to get WOT A/F right. Carl suggests leaving the power valve alone and get WOT A/F correct by enlarging primary and secondary main jets. Then use the idle mixture screws to lean out cruise and idle. He says idle circuit plays a substantial role in cruise A/F.

Think I'll try the Holley method first and see how that goes. If that doesn't work maybe try Carl's approach or maybe just give up and get a self-tuning EFI.

Posted: **Fri Oct 07, 2016 10:10 pm**

I've never blocked off the power valve for tuning. I've never read doing it that way anywhere, except on some drag-only setups. I understand they're saying block it off temporarily, and I guess I see what they're trying to accomplish, but I have no experience doing it that way. The power valve has a number stamped on it, which tells you at what vacuum point it opens (and therefore provides enrichment). So if your power valve has a 65 stamped on it, it will open at 6.5 inches of vacuum. Using your vacuum gauge + WB02 while driving, you should be able to tell when it opens.

*) Get your ignition timing set before starting anything with the carb. Vacuum advance should be utilized, and it should run off manifold vacuum, NOT ported vacuum.

*) With the engine fully warmed up, start with your idle mixture screws. Put the car in gear, and have someone hold the brakes. Adjust screws, in even amounts, to achieve highest vacuum. Usually 1/4 turn at a time. If idle gets too low/high, adjust idle-speed screw as needed. Closing the mixtures screws should make it leaner, opening the screws should make it richer. If you turn the screws all the way closed and the engine doesn't stall, you're getting fuel from somewhere else. I'd close them at the very beginning just as a sanity-check and make sure it stalls. 1.5-2 turns open should be a good starting point.

The Holleys I've been successful with only had idle mixture screws on the primary side. I struggled with Tony's carb that had 4-corner idle circuits. 4 corner idles doesn't make sense to me, especially on a vacuum secondary carb.

*) Once idle mixture is done, go for a drive. Light throttle, keep out of the power valve. Lean out the primaries till you get lean-stumbles on light throttle, then go back up +2 jet sizes.

*) Adjusting the secondary jets is probably best accomplished at a dragstrip. (or dyno) Make adjustments +/- 2 jet sizes at a time. Adjust for highest MPH. (or HP on dyno) I've never tuned a carb with a WB02, so you could also try adjusting to shoot for 12.5-12.8 AFR @ WOT.

*) You mentioned you have a vacuum secondary, which means there is a diaphragm on the passenger side that controls the opening-rate of the secondaries. The springs are color coded. From fastest-opening to slowest-opening: white, short-yellow, tall-yellow, purple, plain, brown, black. Use the fastest opening spring that doesn't cause a bog. The plain spring (no color) is 'stock'. I'd start with purple or tall-yellow,

All of this takes a lot of trial and error. A session at the chassis dyno will make it a lot quicker. But you and Walt are both retired, so you should have plenty of time...

(I say that out of envy!)

I didn't mention the power valve. I'm not real certain how to decide on correct selection for that. I think if you find yourself giving it more throttle than what you think you need for part-throttle driving, go to a bigger number power valve. This will open it sooner.

All of this trial-and-error stuff is why self-tuning EFI is so popular. However, it has a higher up-front cost and is more work to do the initial install. Fuel delivery is *critical*, and I cannot emphasize this point enough. It may also require some manual tweaking, but it will be done with an electronic controller or laptop instead of wrenchs/screwdrivers. But once it's working, it's great.

-Dave

Posted: **Fri Oct 07, 2016 11:13 pm**

Rule of thumb on power valve is it should be 1/2 of your idle vacuum.

Posted: **Sat Oct 08, 2016 7:12 am**

Speaking for myself only, I've never changed a power valve on any carburetor I've ever owned, at least to make a drivability adjustment (only for rebuilding purposes). Maybe I've just been lucky, but I've had all sorts of motor sizes and different size carbs, but that's never been anything I've needed to chase. Normally I've been able to fix problems with jetting, adjustments for secondary engagement, and accelerator pump sizing for both primary and secondary where applicable.

-BP

Posted: **Sat Oct 08, 2016 10:24 am**

THANKS for all the comments. WXO and I will follow the Holley tuning book procedure with adds from your comments and some ideas of our own. Three pages from the book:

I ordered a high flow 6.5 power valve. Current PV is standard flow 6.5. Couldn't find any guidance on when high flow is necessary but, with the restrictors, high flow can't hurt. Idle in neutral gave 17" of vacuum. Didn't check in-gear idle but I'm guessing 15". (Can't check in-gear idle since carb is apart.) Would have gone to high flow 7.5 PV (15"/2) but none were available. 6.5 will just delay power valve opening a little on hills or light acceleration.

Current restrictors are .050". Ordered .055" replacements. (Artillery method: if first shot undershoots, second shot should overshoot, third shot on target.) Would have ordered a kit with assortment but didn't find one.

New PV and restrictors will be installed. Current timing is 12* initial, 36* all in by 3K. We'll check the timing and adjust for max vacuum idling in gear. Idle mixture will be adjusted for max vacuum and in-gear idle RPM noted.

Secondaries will be disabled and a test run will be done to get cruise and WOT A/F on primaries only and, depending on what we find, we'll go from there.

Posted: **Fri Oct 14, 2016 2:26 pm**

We've been out for two more test drives, which were really repeats of the first. First one didn't have the air cleaner on so no cold air intake. On the second one the clear plastic tube to the vacuum gauge moved, got on the headers, and melted in two. We noticed that the vacuum gauge was stuck on 15 and not responding to throttle changes.

Actually, both ends sealed when the tube melted but I wanted to be sure the severed tube wasn't causing a vacuum leak.

Results of the repeats were pretty consistent with the first runs: rich on cruise and lean on full throttle. However, after about 30 minutes of driving, cruise is close to 15 A/F. Don't understand why it takes so long to see that effect. Engine temp doesn't change after 5 minutes or so.

We think the WOT lean condition begins when the secondaries open. Initial reaction was to enlarge the secondary jets but first we're going to change the power valve from 6.5 to 10.5. 70 MPH cruise is around 20" Hg. The 521 with 3.25 gears just isn't working very hard. It would climb any Interstate hill, other than in the mountains, and hold 15" Hg so Interstate cruising isn't going to be close to opening a 10.5 PV. Since I know I want to make the PV change, and in the interest of doing one change at a time, We'll do the PV swap first. A 10.5 high flow PV wasn't available locally but should arrive Monday from Amazon.

Well, maybe TWO changes at a time. We'll also reduce the primary jets a step or two to see if the first 30 minutes of cruising can be leaner without going too lean after 30 minutes. We'd really like to get the primaries (idle and cruise) working well before making changes to the secondaries and WOT.

Posted: **Sat Oct 15, 2016 8:54 am**

There's another reason for changing the power valve before working on the secondary jets. We did some tests where the throttle was opened slowly until the lean condition appeared and noted the vacuum. Heavy load lean condition begins when vacuum drops to about 10" Hg. 10.5 PV isn't going to solve the WOT lean condition since it's still lean even when the 6.5 PV is open. Secondary jets will get replaced, as necessary, after the 10.5 PV is installed.

Edit: Reviewed the videos and, with a better view of the vacuum gauge, the heavy load lean condition begins when vacuum drops to 15", not 10".

Posted: **Sat Oct 15, 2016 3:24 pm**

How many CFM carb is this?
Is this a standard bore carb?
What are the primary and secondary jet sizes?
Vacuum secondaries?

I would not mess around tuning primaries then secondaries. Get the thing to run at 12.8 is WOT then your can fine tune part throttle and idle conditions.

10.5 is when the power valve is going to open. I think you are wasting time with the secondary power valve. It is a tool to help transition to full throttle. If you are concerned about it get a plug and jet up the secondaries about six sizes. I know that is not how Dave does it but it will get that variable out of your mind.

If you can't get the thing to get below 6.5 on the vacuum gauge either a) the blades are not opening enough or b) the carb is way too small for the engine.

Posted: **Sat Oct 15, 2016 5:27 pm**

MostMint wrote:How many CFM carb is this? 750
Is this a standard bore carb? Yes
What are the primary and secondary jet sizes?
Vacuum secondaries?Yes

I would not mess around tuning primaries then secondaries. Get the thing to run at 12.8 is WOT then your can fine tune part throttle and idle conditions. Seems like everyone has their own method for carb tuning. We're kinda following the Holley carb tuning book. Makes sense to us to do it in their sequence: idle, cruise, WOT. We're enjoying learning as we go with instrumentation to see what's happening. Might do it differently if we ever did it again.

10.5 is when the power valve is going to open. I think you are wasting time with the secondary power valve. No secondary power valve, just primary.It is a tool to help transition to full throttle. If you are concerned about it get a plug and jet up the secondaries about six sizes. I know that is not how Dave does it but it will get that variable out of your mind.

If you can't get the thing to get below 6.5 on the vacuum gauge either a) the blades are not opening enough or b) the carb is way too small for the engine. Its easy to get very low vacuum, just open the throttle. It just doesn't go below 12 or 15 under normal driving conditions.

Thanks for comments but we're getting a process worked out that might work. The theory is we work on idle first since it only uses the idle mixture screws and replaceable air bleeds; cruise next since it uses idle circuit plus primary jets; power last since it uses idle circuit, primary jets, power valve and secondary jets. Seems best to get the simplest ones first. We'll all see how that works out.

Posted: **Sat Oct 15, 2016 8:38 pm**

I'm no help to you guys... I'd put a Quadrajet on it and it would solve
all the problems on its own. I would tell you and WXO that you aren't
allowed to have any wine until the carb is tuned.... You'll get it soon
with that dangling carrot!! Good Luck!!

GM

Posted: **Sun Oct 16, 2016 7:18 am**

The secondaries have a metering block or plate? Either way there is an equivalent jet sizing you should be able to find. The jetting info would be helpful in debugging.

You have a 750 CFM on a 521 cubic inch engine. This is a rather small carb for the displacement. This in itself is not a problem unless you want to maximize power at higher RPMs but it does affect how you approach vacuum secondaries. Have you changed the secondary spring yet? I would switch over to the lightest spring possible to open them earlier to help address that condition.

Posted: **Sun Oct 16, 2016 1:05 pm**

GMJohnny wrote:I would tell you and WXO that you aren't
allowed to have any wine until the carb is tuned.... You'll get it soon
with that dangling carrot!! Good Luck!!
GM

GM, you can't take away a man's wine. We think better with a little fruit of the vine.
Maverick and I have this thing by the tail. Y'all would be proud of us. Our instrumentation is state of the art and our testing is first rate. All we have to do now is wait for the UPS guy.

Posted: **Sun Oct 16, 2016 1:21 pm**

MostMint wrote:The secondaries have a metering block or plate? Plate Either way there is an equivalent jet sizing you should be able to find. The jetting info would be helpful in debugging.
Thought I had posted this before but didn't find it in this tread.

To cure some driveability issues, on the recommendation from QT tech, the original jets and air bleeds were replaced a year or two ago as follows:

Primary air bleeds: 70 to 76
Secondary air bleeds: 39 to 43

Primary main jets: 71 to 74
Secondary main jets: 82 to 84

We plan to go smaller on primaries, maybe also on the primary air bleeds. Haven't had that discussion yet. Confusing issue here is the first 30 minutes of driving is rich on cruise but after 30 minutes cruise A/F is not bad: high 14s/low 15s. Don't want to go too lean on long drives.

You have a 750 CFM on a 521 cubic inch engine. This is a rather small carb for the displacement. This in itself is not a problem unless you want to maximize power at higher RPMs but it does affect how you approach vacuum secondaries. 750 is good for my priorities. It won't see high RPMs. Occasionally 5K, rarely 5.5K Have you changed the secondary spring yet? QF carb has a screw for adjusting vacuum secondaries opening; no spring changes. Not there yet, still working on primaries as per the Holley tuning book. I would switch over to the lightest spring possible to open them earlier to help address that condition. Tentative plan is to open the secondaries so early it bogs then adjust them later till bog is gone.

Posted: **Sun Oct 16, 2016 1:29 pm**

wxo wrote:
GMJohnny wrote:I would tell you and WXO that you aren't
allowed to have any wine until the carb is tuned.... You'll get it soon
with that dangling carrot!! Good Luck!!
GM
GM, you can't take away a man's wine. We think better with a little fruit of the vine.
Maverick and I have this thing by the tail. Y'all would be proud of us. Our instrumentation is state of the art and our testing is first rate. All we have to do now is wait for the UPS guy.

LOVE your confidence!!

We novices are taking it one step at a time with intervening test, study, noodle.

Should we fail miserably, there's the option of $1000 EFI. Fuel system is already close to what's required. Fortunately, the cost of test and tune the way we're doing it is not bad. Fun too.

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