496 Stroker Tuning Questions

[383man]

As you discovered power valve should be sized according to your cruise vacuum, not that 'half the idle' BS. You can have the power valve open at idle, and it won't cause your idle to go rich.. You're on the right track going up to the 7.5. The whole idea of the power valve is to be able to run the leanest primary jet you can without stumbles or adverse effects, and then have the extra fuel only when you need it (at throttle time!)
I would step down the primary jet again and see if it cleans up those cruise numbers, because it's easy. If not, time to get inside and tweak the idle/transition circuit. When you say 'fat at light throttle' that's an indicator of too much fuel there. That can get tedious, lots of trial-and-error, but it's so worth it to get the tune spot-on. And FWIW that's usually the area of the carb that needs the most work. Very common. Plenty of help with that here and FABO if you need it.
To answer your last part, don't worry about what the gauge does during decel. As the throttle eases closed vacuum jumps around and pulls extra fuel from the T-slots, as well as what was flowing through the boosters ...but it's temporary. Once you get the cruise area leaned a bit it may help with that too. Gotta tune from the bottom up, and since you posted some different vacuum numbers, is it safe to assume you hooked the gauge where you could read it and went driving? If so....YAY for you! That's a big help when dialing in a carb but many folks don't think about it.

I basically agree with you but you have to be careful how soon the power valve comes in. You dont need it at light part throttle as it should come in on hard throttle when the vacuum drops enough. If you bring it in way to soon and its open at part throttle then it will be to rich at part throttle. And some will then drop the main jet to lean the PT and then it is to lean at WOT. And as you said it can be open at idle and wont cause it to be rich since its only running on the idle circuit and the PV goes into the main circuit. But if you actually have a PV open at idle it will be to rich when you step lightly on the gas. I am just saying that I would not want to bring it on to soon because you need that extra fuel on hard throttle at or close to WOT. If it stumbles at light throttle right off idle I would feel thats more of an Accel pump or timing problem because the PV should not come in until about half throttle or more. And it sounds like he did find an accel pump problem so thats good for him.

Also I hear so many say the DP is no good on the street and thats just not true. I always run a DP on the street and track and they work great. Nothing wrong with a vacuum secondary carb but you can make the DP work very good on the street. Call me an old time racer but I like the DP because when I nail it I like all 4 barrels open right away which you can do with a DP and run great. Good luck with your tuning of it. Ron

 

[beanhead]

I basically agree with you but you have to be careful how soon the power valve comes in. You dont need it at light part throttle as it should come in on hard throttle when the vacuum drops enough. If you bring it in way to soon and its open at part throttle then it will be to rich at part throttle. And some will then drop the main jet to lean the PT and then it is to lean at WOT. And as you said it can be open at idle and wont cause it to be rich since its only running on the idle circuit and the PV goes into the main circuit. But if you actually have a PV open at idle it will be to rich when you step lightly on the gas. I am just saying that I would not want to bring it on to soon because you need that extra fuel on hard throttle at or close to WOT. If it stumbles at light throttle right off idle I would feel thats more of an Accel pump or timing problem because the PV should not come in until about half throttle or more. And it sounds like he did find an accel pump problem so thats good for him.

Also I hear so many say the DP is no good on the street and thats just not true. I always run a DP on the street and track and they work great. Nothing wrong with a vacuum secondary carb but you can make the DP work very good on the street. Call me an old time racer but I like the DP because when I nail it I like all 4 barrels open right away which you can do with a DP and run great. Good luck with your tuning of it. Ron

Absolutely, thanks Ron. I just didn't get that deep into it....my posts sometimes get too long anyway!
This is why I always recommend driving with the vacuum gauge hooked up in the cabin during testing and tuning. You need to know what the vacuum is doing, when, to be able to really dial things in properly.
I love double-pumpers on my cars too, when they're done right, they work as good if not better than every other design out there...including our beloved Carters!

 

[Warrenavenue]

I basically agree with you but you have to be careful how soon the power valve comes in. You dont need it at light part throttle as it should come in on hard throttle when the vacuum drops enough. If you bring it in way to soon and its open at part throttle then it will be to rich at part throttle. And some will then drop the main jet to lean the PT and then it is to lean at WOT. And as you said it can be open at idle and wont cause it to be rich since its only running on the idle circuit and the PV goes into the main circuit. But if you actually have a PV open at idle it will be to rich when you step lightly on the gas. I am just saying that I would not want to bring it on to soon because you need that extra fuel on hard throttle at or close to WOT. If it stumbles at light throttle right off idle I would feel thats more of an Accel pump or timing problem because the PV should not come in until about half throttle or more. And it sounds like he did find an accel pump problem so thats good for him.

Also I hear so many say the DP is no good on the street and thats just not true. I always run a DP on the street and track and they work great. Nothing wrong with a vacuum secondary carb but you can make the DP work very good on the street. Call me an old time racer but I like the DP because when I nail it I like all 4 barrels open right away which you can do with a DP and run great. Good luck with your tuning of it. Ron

I got ahold of Holley and ended up getting a replacement carb sent out, the off idle transition is way better now that the accel pump shooter isn’t messed up. It’s still wanting to run way rich on light throttle (10.5-11.5) making me think I’m running on the T-slots and the IFRs are flowing too much fuel. I only have Holley metering blocks so I’ll try the guitar string trick and see what I can do to thin them out.

 

[Warrenavenue]

Absolutely, thanks Ron. I just didn't get that deep into it....my posts sometimes get too long anyway!
This is why I always recommend driving with the vacuum gauge hooked up in the cabin during testing and tuning. You need to know what the vacuum is doing, when, to be able to really dial things in properly.
I love double-pumpers on my cars too, when they're done right, they work as good if not better than every other design out there...including our beloved Carters!

Alright, I got a few more questions here. After tuning a bit today and trying to get a baseline for this carb I’m noticing that I’m cruising on the T slots and they’re running a little rich (12.5ish). Today I pulled the carb off to check the T slots and found that the primary T slots were way too exposed and the secondaries weren’t exposed at all, the secondary butterflies were seated in the bores when closed. A couple web searches pointed out that the T slots in the secondaries should be exposed the same as the primaries (they should both be squared out). So I squared both, bolted it back up and fired it up. I worked the idle screws back to a 13.2 or so sweet spot and took it for a drive. It ran pretty good. It’s leaner than it was before at cruise but still too rich. After doing some more reading on forums I’m seeing that the secondaries in fact should not be exposing any of their T slots at idle since their T slots are higher in the bore, but should just be open the same amount as the primary butterflies? Is this right?

Also, if I close the secondaries down more it won’t idle. Does that mean I should drill my primary butterflies with a 16th inch drill bit like I’ve read some guys have done? Will this help lean out my transfer slots at cruise since some air will be passing through the butterfly and missing the slot? I appreciate the help in understanding this better.

 

[RemCharger]

Don't drill them out yet. And both sides of the carb blades should be somewhat equalized .

 

[RemCharger]

You need a QFT SQ950 .

 

[beanhead]

The difference in how much secondary T-slot gets exposed at idle depends on the carb. Some carbs have the secondary slots a little farther up the bore, and some have all four in the same position. The carbs with the rear slots that don't extend down as as the primaries like the rear blades closed (generally speaking, it's not a set-in-stone rule)...
Getting the transition right is usually the toughest part. You get a good jump in vacuum when you crack the throttle blades, which then pulls more fuel through the idle/transition circuit and causes that rich condition.

 

[Warrenavenue]

The difference in how much secondary T-slot gets exposed at idle depends on the carb. Some carbs have the secondary slots a little farther up the bore, and some have all four in the same position. The carbs with the rear slots that don't extend down as as the primaries like the rear blades closed (generally speaking, it's not a set-in-stone rule)...
Getting the transition right is usually the toughest part. You get a good jump in vacuum when you crack the throttle blades, which then pulls more fuel through the idle/transition circuit and causes that rich condition.

So what can I do to lean out that rich condition?

 

[beanhead]

So what can I do to lean out that rich condition?

It's trial and error, because you don't really know if your trouble spot is happening on the idle circuit alone, an early booster circuit, or combination of the two. Float levels and t-slot exposure at idle must be right first, which it looks like you're getting a hold of.
Big engine/small carb pulls air with increased velocity past the boosters to try to feed those cubes, even at low rpm, so the mains may be contributing early.
The first change I would make is to drop the primary jet and see if anything changes. It may help but at the same time lean the rest of the curve so don't go WOT yet. If A/F changes, you're tweaking the right area so get it dialed where you want and then correct the power mixture with the secondary jets and power valve channel restrictors. Power valve size itself is for timing the enrichment only but I think you worked on that. PVCRs may or may not be adjustable..they can be made adjustable with somedrilling/tapping and brass set screws.
If no change, try reducing the IFR size, then re-adjust idle mixtures and test.
In some cases the fix is to add T-slot restrictors which also requires a little drilling and tapping.
Before drilling anything try the small-wire restrcition method, just to see what/if anything changes, so you don't waste effort.

 

[Warrenavenue]

It's trial and error, because you don't really know if your trouble spot is happening on the idle circuit alone, an early booster circuit, or combination of the two. Float levels and t-slot exposure at idle must be right first, which it looks like you're getting a hold of.
Big engine/small carb pulls air with increased velocity past the boosters to try to feed those cubes, even at low rpm, so the mains may be contributing early.
The first change I would make is to drop the primary jet and see if anything changes. It may help but at the same time lean the rest of the curve so don't go WOT yet. If A/F changes, you're tweaking the right area so get it dialed where you want and then correct the power mixture with the secondary jets and power valve channel restrictors. Power valve size itself is for timing the enrichment only but I think you worked on that. PVCRs may or may not be adjustable..they can be made adjustable with somedrilling/tapping and brass set screws.
If no change, try reducing the IFR size, then re-adjust idle mixtures and test.
In some cases the fix is to add T-slot restrictors which also requires a little drilling and tapping.
Before drilling anything try the small-wire restrcition method, just to see what/if anything changes, so you don't waste effort.

I know it’s happening before the mains start coming in because I can see it happen in my wideband. Cruise will be 12.2-12.6, maybe getting into 13. But the second I tip in to accelerate lightly, keeping the pv closed and running right above the pv opening point, my afr is around 14.7-15.2. Last night, before I upped my mains from 68 to 71, this area just before the pv opened was way lean, into the 17s. But I was still having a super fat cruise. So I upped to the 71 primary and got the transition between main and pv to be a little more driveable. Im thinking I’m even going to up my primary to a 72. I’ll try the guitar string trick to meter the T slots, but I feel like I’ve gotta be missing something if I’m running at cruise in the T slot and there isn’t a better way to meter the fuel in this area. It’s like I’d have to have a trailer on my truck to give me enough resistance to cruise in the mains.

 

[hangn0ut]

I know it’s happening before the mains start coming in because I can see it happen in my wideband. Cruise will be 12.2-12.6, maybe getting into 13. But the second I tip in to accelerate lightly, keeping the pv closed and running right above the pv opening point, my afr is around 14.7-15.2. Last night, before I upped my mains from 68 to 71, this area just before the pv opened was way lean, into the 17s. But I was still having a super fat cruise. So I upped to the 71 primary and got the transition between main and pv to be a little more driveable. Im thinking I’m even going to up my primary to a 72. I’ll try the guitar string trick to meter the T slots, but I feel like I’ve gotta be missing something if I’m running at cruise in the T slot and there isn’t a better way to meter the fuel in this area. It’s like I’d have to have a trailer on my truck to give me enough resistance to cruise in the mains.

You have a pcv system? That's one way to lean it out. Beats drilling holes in the blades.

 

[hangn0ut]

I know it’s happening before the mains start coming in because I can see it happen in my wideband. Cruise will be 12.2-12.6, maybe getting into 13. But the second I tip in to accelerate lightly, keeping the pv closed and running right above the pv opening point, my afr is around 14.7-15.2. Last night, before I upped my mains from 68 to 71, this area just before the pv opened was way lean, into the 17s. But I was still having a super fat cruise. So I upped to the 71 primary and got the transition between main and pv to be a little more driveable. Im thinking I’m even going to up my primary to a 72. I’ll try the guitar string trick to meter the T slots, but I feel like I’ve gotta be missing something if I’m running at cruise in the T slot and there isn’t a better way to meter the fuel in this area. It’s like I’d have to have a trailer on my truck to give me enough resistance to cruise in the mains.

Another way to lean out or richen the t slot is by changing the idle air restriction.

 

[Warrenavenue]

Another way to lean out or richen the t slot is by changing the idle air restriction.

I do have a PCV setup on it. But today I called Holley and talked to their techs dept again about it and they guy I talked to recommended going with an 850 Brawler so I ordered one this afternoon. After going around and around it seems like the undersized barrels might be causing more velocity through the T slot, drawing more fuel. I’m hoping an 850 will slow the velocity and be able to lean out. I’ll keep the thread updated with any changes

 

[Warrenavenue]

It's trial and error, because you don't really know if your trouble spot is happening on the idle circuit alone, an early booster circuit, or combination of the two. Float levels and t-slot exposure at idle must be right first, which it looks like you're getting a hold of.
Big engine/small carb pulls air with increased velocity past the boosters to try to feed those cubes, even at low rpm, so the mains may be contributing early.
The first change I would make is to drop the primary jet and see if anything changes. It may help but at the same time lean the rest of the curve so don't go WOT yet. If A/F changes, you're tweaking the right area so get it dialed where you want and then correct the power mixture with the secondary jets and power valve channel restrictors. Power valve size itself is for timing the enrichment only but I think you worked on that. PVCRs may or may not be adjustable..they can be made adjustable with somedrilling/tapping and brass set screws.
If no change, try reducing the IFR size, then re-adjust idle mixtures and test.
In some cases the fix is to add T-slot restrictors which also requires a little drilling and tapping.
Before drilling anything try the small-wire restrcition method, just to see what/if anything changes, so you don't waste effort.

The 850 has cured my problems. So far. I had 2 Holley techs tell me 750DP but after explaining the richness I was getting at cruise another tech told me I definitely should have gone with an 850. The Brawler is no disappointment. It tuned in nicely after setting up my primary throttle blades. Off idle stumbles are gone, cold start issues are gone, richness at cruise is gone. I’m excited to finally get past the idle/primary transition and start messing with the secondaries. Thank you guys for your help! It’s very appreciated.

 

[dcala84141]

Funny. The problems u had with the 750 dB, I’m having the 830 quick fuel. My afr numbers are exactly the same as yours under same conditions. The holly tech told me the 830 is to big for my engine which is a 383 stroke to 450 with cnc heads and headers putting out well over 500hp.
I thing it depends on who u talk to
I’m going to drop down my mains to 71s and c if that improves my cruising afr numbers