I have posted here lots and gotten my tune up pretty good thanks to everybody's help. I have a holley 750 double pumper on my mild 408 (high 230s duration .525" cam, air gap, street headers, OD 4spd stickshift, 4:11s 28" tire).

I am still battling with a lean spot at 2200 rpm. I have a wideband installed, and I'm currently switching (moved closer to work) from driving mostly highway to driving mostly stop and go

I have been doing some reading and researching and I think I can maybe restrict my main air bleeds to transition to the main circuit sooner?
I have no idea how to go about it in a scientific way.

I have plenty of timing at idle, high 13s:1 afr at idle, curb idle not too far open (not too much transfer slot exposed), and it is not an off idle acc. pump thing- it happens at cruise or whenever I'm trying to drive gently around 2200rpm. I had masked the problem by running rich and cleaning up on the highway blasts but now I gotta dial it in for real to keep my plugs clean.

Do I need to buy a more adjustable carb (HP series?) to address this? The car runs awesome on the highway, and accelerates great. I don't want to make it just a strip car and never drive it, but it is annoying in traffic with this lean spot.

Thanks

take the wide ban and toss it in the trash. If it runs good screw it. You can drill out the air bleeds I guess. Have fun

You don't want to drill them out, that'll make it leaner, those are Air bleeds, so if anything, you'll want to restricted them, but normally you really don't need to jack with the high speed air bleeds, if it feels good driving around, i agree, the heck with the "wideband", just tune for "best" drivability & MPH, if its only going 13.1 on the wideband, that is not lean, matter of fact, 13.2 AFR is about the best for overall power throughout NA, blower/nitrous/turbo cars like alittle more fuel, but if your dropping below 13.0 AFR NA your heading on the downward slide for best power/drivablity.

what jets are you currently using ?

I'd see what in hg of vac you have at the dead spot and see how much pump shot you have @2200 and at that point is that at WOT or squeezing into it moderately.

Interesting, I have a flat spot in the same area on my 414 stroker. Messed with it a bit today, and it got better, but not gone completely.

Thanks for the replies but my question wasn't about drilling anything or accelerator pump shots or wideband tuning. The car comes off idle fine and the squirters get her going like they should. There is no stumble on tip in from idle and the afrs I listed etc were to rule out common tuning pitfalls.

I have read that RESTRICTING the main air bleed can make the main circuit come on sooner. I believe what I am dealing with is a problem with the overlap or lack thereof between the transfer (idle circuit still active) to the mains.

And like I said, the car ran fine when I was mostly on the highway because I was able to run rich on the bottom to cover this lean spot and all the highway running would keep everything clean and happy. Now that I live near work I have to lean up the idle and jetting to be able to run lean enough to keep clean plugs.

I've got it close- good power and clean plugs with great
throttle response and no pinging. I just can't get rid of this lean spot at cruise at 2200 rpm. No pump shot, no pressing the gas, no drilling, no chasing a magic af number. I am trying to find out about turning my mains on 2-300rpm earlier.

Robert you gave me a thoughtful answer- thanks. I need to reconnect my vac. gauge but I can tell you I am fine when pressing the gas and getting a shot and WOT is scary but works just like it should. It is at cruise or light throttle that I have a problem.

Edit: I have a 4 corner idle 750dp with 3 hole 1" spacer. settings are currently as follows:
idle screws 7/8 turn out
main jets 72
8.5 power valve (pvcrs are tiny on the DPs anyway)
idles at 850rpm curb idle screw 3/4 turn in

Rear metering block has PV plug and 80 jets.
WOT works like it should. An agressive launch covers the lean spot with acc. pump shot.

I have heard of using wire to reduce the MABs (main air bleeds) and make the main circuit come on sooner? I'm looking for someone with experience doing it. I have used wire to restrict pvcrs on a 350 holley 2 barrel before for use on a hot slant 6, so Im not completely new to this game.

Thanks

""I have read that RESTRICTING the main air bleed can make the main circuit come on sooner.""

Radar,
I have read the same and Quick Fuel explained the same theory when dealing with the air bleeds. 2200rpms is real close to when the mains should start coming in. If you have adjustable air bleeds, try a set of air bleeds apx .002" SMALLER to bring the mains in quicker (richer). Please check your wideband to see how much quicker they came in and then you can estimate the size easier if you have to make another change. If .002" (x 4 bleeds) makes a 100rpm difference, then you can use that as a reference.

Re4striction? If you want to get in the main circuit faster, use a smaller carb.

You can also tune the mystrious problem by stagering the accel pumpshot cams. 8.5 PV dont sound correct. The PV should come in at the right vacuum.

Check your advance curve.

Thanks man thats the info I'm looking for.
I don't have adjustable bleeds. I guess I'm looking at either sticking pieces of wire in there or drilling and tapping for a brass set screws and then drilling them like jets?

About the smaller carb-I know I could use a 650 but I want to get the most out of my stroker. A 750 isn't too radical for a warm 408 with 4:11 gears in a light 4spd car and I do enjoy righ rpm blasts pretty often (after dark). Unfortunately on the way to work it is a parade of people doing 35 in a 40mph zone. I know I'm trying too hard when I could maybe buy an eddy carb or something but I only know hollys and I want to make it work. My pump cams work fine, and I'm not having any trouble with the secondaries, just light throttle cruise. I can cruise in low gear but that gets old. I know my motor should have power there not a flat spot.

I came this far! Do I need to get an HP holley or proform main body with adjustable bleeds or can I work with the standard double pumper?

rdr

Does it stumble? Is it surging?

If it's just going from 13 to 14 or so, no big deal.

At a constant cruise you may need more idle feed restrictor sizing/diameter, not the air bleed.

If the air bleeds aren't adjustable, you're in for some fun.

Get a really good understanding on how holleys really work and the systems.

Bottom line, You'll likely NEVER get a perfect fuel curve no matter how hard you try.

750 will work fine. I ran a 750hp on a mild 340 worked great. And I meant drill and tap the air bleeds. I wonder if the 1" spacer is an issue? Are you saying that you want to drive in 4th and no have a flat spot/bog? you have to stay in the cams opeating range. If you drop to low no matter how much you tune it, it will have a dead spot.

I'm running a 275HL comp cam (high 230duration)in a 408 it should have smooth torque available at 2200 even if the HP doesn't come in until 3k+ ?

The car does a surging stumbling thing around 2200. At cruise it doesn't ever run well at that rpm. At cruise a pump shot is always a neck snapping acceleration out of the bad spot. The problem is not evident if I launch the car- the acc pump covers the bad spot. If I am cruising and it loses power then jerks foreward I can always look down at the tach and see 2200rpm.
I have no trouble cruising at lower rpm- I have decent power below and above 2200rpm.

This isn't my first time tuning with a wideband and I am not chasing a straight line afr. For a baseline I like to see 13-16 at cruise and 12-14 under power. Anything better than that is fine tuning to me. An occasional dip to 11s or 16s doesn't freak me out and I understand how to lean out the mains to use the power valve under load/low vac. I also understand that the double pumper has small pvcrs and is meant to run a little rich. Thats fine, I just want my plugs to stay clean without blasting them out on the highway every day. I now have that, but the 2200 spot is getting worse at the same time.

I'm just getting real close to running clean for stop and go traffic but this problem at the overlap spot between circuits is becoming more evident now that I'm taking away unneccesary richness in the idle/transfer circuits and mains. If it was a race car it wouldn't be a problem at all but I'm chasing street drivability.

Also I am running an OD 4spd so I can't just run the r's up before shifting without screaming for longer than is fun or running up somebody's butt. I don't actually cruise in 4th unless I'm doing over 70mph- 1:1 third is too much fun on the highway.

Up your float level, and run a hotter plug.

What is your timing at 2200 rpm?
If you have vacuum advance, disconnect it and check if it gets better?

No. Increasing the size of the MABs will make make the mains begin to flow at LOWER airflow (sooner) and decreasing them will make the mains begin to flow at HIGHER airflow (later).

Think about why the MABs are there. Without them, the carb would get richer and richer as airflow increased. The air bleeds are there to modify this behavior. The introduction of air creates an emulsion which flows easier than liquid fuel. Easier flow = "sooner" flow. This is greatly oversimplified, but if you think it through I think you'll see what I mean.

But don't take my word for - I'm just some guy on the internet that you don't know from Adam, and you know how internet advice goes. You're already inclined to mess with the MABs anyway, so just go way overboard and restrict the heck out of them and watch your wideband and see what happens. A well-secured paperclip or wire will do nicely.

I'm not trying to bag on the guys who said they've read the opposite of what I'm saying. I've read it too. All I'm saying is try it for yourself and come to your own conclusion.

Modifying the carb to accept homemade screw-in air bleeds, as you already mentioned, is no big deal at all. For the MABs, 8-32 brass screws work just fine, and the soft metal in the Holley main body is easy to drill and tap. For IABs, you might want to go 10-32 (if there's enough meat in the main body) as the orfices will need to be larger and you usually have very little hex left in a 8-32 by the time you drill to the proper size.

Finally, you mentioned that you might have a "hole" between the main and transition circuits. Possible, and easy to check by changing the air bleeds. Far more common is too much overlap between the transition circuit and the mains, causing pig-rich conditions at light throttle, especially under high vacuum conditions. Are you sure you don't need more main jet?

Based on what you've said, I think you're on the right track by experimenting with the MABs. Just be darn sure you measure very carefully the existing press-in air bleeds so you can get back where you started if you lose the rabbit. Once you do that, you can do no harm as long as you don't screw up when you drill and tap the main body for screw-in MABs.

Installing screw-in bleeds is not hard at all, you should really never have to mess with the sizeing of the HSBs, i'll bet "anything" its that 1" spacer causeing your problems, you need to increase the jets 2 & 2 when running a 1" spacer as a start, your too lean, install some 74/82s & i bet the problems "gone", but don't take my word for it lol.

Thanks fellas. I am going to play with some wire first. It seems dumb but I didn't think to use a longer wire wrapped behind the choke horn and I couldn't see how to keep little pieces of wire from bouncing out and going through the motor. I got a couple really thoughtful pms and an email with instructions and ideas so now I can fiddle around with my existing carb.

Thanks

Small bleeds =more fuel.The pressure that sucks in air thru the bleeds is the same as the pressure that pulls gas thru the idle jet and mains.

are your sec throttle plates opened the right amount?

What happened when your timming was advanced to 18-20 initial? Did it have more time to burn and smooth out. A stroker engine thats close to square has unusual timming demands. Some take alot of advance with little curve to get running proper.

I think your going in the right direction. You could raise the float level too but MAB is the most logical step. Also, if you richen the IFR it will run richer from off idle until the main circuit comes in - might help. But if it is fine at 1800 rpm could be the wrong fix. I would consider that last.

I do agree I would try some wire in the MAB's to see if they are pulling enough fuel yet or if that rpm where it happens is to low and its still on the idle/transfer circuit. Once you are sure where (what circuit) it happens you can work it from there. You want no surge but you also dont want to foul plugs and use to much fuel. Now in the old days (60's and 70's) we would just fatten it some with more jet (had no adj MAB's) back then and if it fixed it well then cool. We drive it a few days and make sure it's not fouling the plugs. But now we have much more technology and should be able to tune for about 13 to 1 or so and eliminate the surge. I didnt pay attention to how new the carb is as I have seen worn throttle shaft's cause problems of this nature as it can get pulled and tilt then move enough in the throttle plate and let it pull air in when it should not. I also dont remember if you said you run vacum advance as it can cause a timing issue if not in sync with the mech advance. Anyway just throwing some more idea's your way but you sound like you know what to look for. Good luck and keep us posted on you progress. Ron

Timing was bumped up to 21 initial 36 total no vac advance. Doesn't kick back but does crank better with the spark off then I flip it on to fire. I'm using a silver and a blue spring in the MSD dizzy IIRC. Should be coming in pretty quickly.

The secondary plates are open a little to be able to idle well without uncovering too much transfer slot on the primaries.

Thanks for the last round of answers/ encouragement I appreciate it. No time to fiddle for a few days I'll report back then.

Kevin