BACKGROUND:
The carburetor is on a 350/th400 combo in a heavy vehicle with 3.42:1 gears and 28" tires.

I cruise at 2500 rpm which is around 60-ish mph. Cam specs are 212/222 (112LCA). It's got 9.1:1 compression with a dual plane intake. Long tube headers carry out the exhaust gas. Also, I'm running vortec heads. The exact motor is the GM 350HO crate engine.

CURRENTLY:
I'm running a 600 cfm 4160 Holley 1850-S carburetor. It makes decent power, but the secondaries don't open soon enough or bog or .... I can fell them kick-in or I get stuck with vacuum on the top end. I "jungle engineered" the carb so the secondaries would open mechanically. This resulted in increased low-end, but (of course) a horrible bog.


SEEKING:
Decent gas milage with more midrange and control over when the secondaries open.

PLAN OF ATTACK:
I want to put her on a wide-band and adjust the carb hopefully saving some time. I plan to use larger idle bleeds (.002" larger) and then jet the carb down a few sizes. I'll jet it down until I achieve a cruise mixture I'm happy with and then enlarge the PVCRs until WOT AFR comes in.

SO?
What AFR should I shoot for? How close to 14.7 should I try and get for cruising?
WOT? What ratio have you had the most success for a ~350hp engine?

I don't want any bogs, but would like to get decent milage by running on the leaner side.

If I have to enlarge the squirters... that's no problem either. I just down want to squirt an excess amount of fuel on my piston rings.

Please advise. How does this sound guys? Damon? Others?

Ben T.

 

14.7 is stoich, which is good for emissions, but not necessarily the best for fuel economy. 15.5:1 is where I'd aim as a "starter" AFR for best economy. Some combos you can run 'em so lean they squeak (16:1 or even higher). Detonation, high running temps, etc. will be your limiting factor.

You don't want to do this with a universal replacement Holley, in my opinion. Not only are they jetted a little fat, but the idle circuits are also very rich in most cases. Remember that you run about 80% of the time on the idle circuits, so having nice lean ones is fairly important.

A stock Qjet off a mid-70s 350 Chevy, rebuilt to factory specs would be a good starting point for a fuel economy carb. They come already set up pretty lean in the idle and primaries, they are tunable a million ways from Sunday that you just can't do with a Holley, and you can still do chenges to the secondary metering rods so that when you lay into it for power it's not too lean to get up and go.

 

That's leaner than I was thinking. Thanks for the response. Maybe pinging, high operating temps, and lack of responsiveness should be my limiting factor. Lean it out until I reach one of those and then go a wee bit richer.

While technically a "universal replacement" it is more adjustable than a standard Holley.

It is tapped and has jets in all four air bleeds. So, I can lean my idle out by increasing the idle air bleed.

I'm just worried about possibly drilling the PVCR too large and ruining a good metering block. Oh, well... break and learn if that happens. There's always the atv jet fix if they're enlarged too much.

Ben T.

 

Secondary idle adjustment is supposed to make for a more balanced carb with the same fuel flowing from each barrel at idle, IF the blades are open the same amount, idle air bleeds are the same diameter, and screws are evenly adjusted.

I feel comfortable setting the idle, but I don't know where to start at for cruise and WOT being this is a double pumper. I think it's possible to run the cylinders 1&2 lean and 7&8 real rich and look 'ideal' on the oxygen sensor reading. So, I'd like to try and avoid this.

Do I want to match every primary and secondary setting; idle bleed, high speed, jetting, pvcr?

Should I unhook the secondaries and dial in the primary side? Then, just transfer all the setting to that side? I know I'd still have idle-fuel coming from the secondary side though.

I think I know where to go, but where do I start?

Ben T.

P.S. This is my first double pumper. I'm dreading even setting the idle speed. TWO screws and one is impossible to reach.

P.S.S "If it's not broken yet, it doesn't have enough features" - possibly Murphy?

 

Ben- you've got the right idea. Unhook the secondary side completely for now. Obviously, if you've got 4-corner idle adjustement the secondary idle circuits will still be feeding some fuel at idle, even when the secondary throttles are fully shut against their stop, but that's unavoidable for now. Do the best you can with it.

Tune the primaries so lean they squeak using your AFR gague and your best judgement. Since you have replacable bleeds and such that makes the carb much more "tunable" than a typical Holley, as you say. DON'T do any tuning unless the engine is fully warmed up (i.e driven 5-10 miles). Tuning on a cold engine is never going to give repeatable results and will end in frustration.

Leave the primary PVCR alone for now. Just say out of the loud pedal for a while. Drive like your grandmother. Get everything dialed in as best you can for everything under about 1/3 throttle. Idle mixture screws (curb idle), idle air bleeds (just off idle) and primary jets (from there to about 1/3 throttle) are what you want to play with at this stage. This will avoid activating the power valve under most/all conditions, especially with an automatic trans. You want to really tune it dead-bang from curb idle to about 30% throttle. This is where almost all fuel economy happens.

After you've got that all dialed in then you can play with the primary side PVCR so that you get acceptable performance under heavy/WOT throttle BUT STILL DON'T REV THE ENGINE VERY HIGH. You're still only running basically a 2bbl carb at this point. You don't want to take it high engough into the RPM range that you start building any meaningful vacuum in the manifold. Full throttle stabs up to about 3500 RPMs or so is all you're looking for. Don't be surpirsed if you end up with some real lean jets and a considerably opened-up PVCR. And don't be surprised if you have to move up a couple of squirter sizes to overcome the "hole" that happens when you nail it with lean jetting.

Tip- Always try upping your primary squirter size FIRST to see if you really NEED to richen up the PVCR or if you can "compensate" for a momentary bog when you "nail it". A short bog-and-go can usually be overcome by playing with the accelerator pump squirter/cam. But if you nail it and it's got a long, protracted bog in the lower RPMs you will probably find the solution is a richer PVCR. In short, you want to try to differentiate between a MOMENTARY leanness when whacking the primaries open (accelerator pump) and a consistent leanness (PVCR).

Once the primaries are dialed in right you should have good ecomony AND it should be pretty damned responsive when you nail it. In short, it should feel damned good from idle to WOT up to about 3500 RPMs and your AFR gague should be reading lean anywhere under about 30% throttle. This should take several days of testing/tweaking/trying different stuff. Don't do this like you're going to a fire.

Speaking of the power valve you may wish to play with it's openign point. A stock valve will usually be around 6.5" opening point. That should still be about right, but if you find that you're regularly below that point when under normal driving/slightly loaded conditions you might consider going to a lower opening point like a 4" power valve. Imagine if you are running on the highway or normal around-town driving and vacuum is often right near the power valve opening point. It's going to be opening and closing a little here and there when it isn't strictly necessary and reduce your mileage. In a car that isn't towing a load, has reasonable gearing and given your modest cam you shouldn't be anywhere near your power valve opening point under the vast majority of normal driving circumstances, but it's something to keep an eye on. Duck-taping a vacuum gague to the windshield (temporarily!!) while you drive around can be VERY revealing. You'll be amazed how much you can learn from a simple vacuum gague.

LAST, do your thing with the secondaries. They are by far the least important to fuel economy since they only open at or near WOT and in mid-high RPM ranges on a vacuum secondary carb. You can go fat to help compensate for the lean primaries. You don't even need to run a power valve- it's basically going to always be open by the time the secondaries kick in anyway. By the time they join the game you're basically calling for "full steam ahead" and fuel economy takes a back seat to making power. And you don't want to run lean at WOT/high RPMs, for obvious reasons.

I notice that you are having problems with your secondary opening rate. First off, get rid of all that "jungle engineering" you did to make it into a double pumper. It won't work. Without a secondary-side accelerator pump you're headed to boggsville. Population: You. Put it all back like Holley intended it to be.

Here's the usual culpirt when you feel like you're walking a "tightrope" between bog and not enough opening: Holley actually did a decent job of sorting all this out on the vac sec carbs. I suspect yours has a problem somewhere in the secondary opening diaphragm assembly on the pass. side of the carb. Make sure the vacuum diaphragm isn't leaking vacuum. Also, make sure the little check-ball in the vacuum passage that feeds vacuum to the diaphragm is actually present and not gummed-up or stuck (you have to take the whole lid off the secondary opening diaphragm assembly which means removing the whole assembly from the side of the carb). The check ball sits in the base of the vacuum diaphragm and should be immediately apparent once you get the lid off it. The check ball acts like a vacuum "bleed" to SLOWLY admit vacuum to open the secondary throttles. If it isn't present or isn't working right you will get this "walking a knife edge" kinda feeling from the secondaries. The secondaries have to open SLOWLY and in a controlled manner because there is no secondary-side accelerator pump. Without the proper functioning of the check ball they will tend to "flop" open when you nail it, causing an instant bog. Then you try to "compensate" by putting in a heavier secondary spring and you end up with secondaries that don't open fully at WOT/high RPMs. Not sure if that accurately describes your situation but it's one I've seen many times. Many shops even remove the check ball ON PURPOSE, but they only do this out of ignorance, not becuase it helps. Check ball's gotta be in there or you'll chase your tail forever an still never get it right. Yes, my friend, a stupid 1/16" ball bearing could be all that's wrong with your secondarys.

You may want to read all that a dozen times or so. There's a crapload of information up there. Try to picture in your mind what is actually taking place in the carb when you read it. It's far more important to understand the basic logic and principles that make the carb work than it is to look at this as a "how-to" kinda text. You're way off in the weeds vs. what most people will ever do to their carbs. In short- nobody can help you if you get stuck. They won't understand what you're trying to do and they'll give you "this is what worked for me" kinda suggestions with specific jets, squirters, etc. You need to THINK about this while you're tuning.

 

Damon, I think I can really visualize what you've said.

I'll keep adding to this thread when I put her on. It was ambiguous, but the new carb is a 4779 with the Proform mainbody. Holley says it's an acceptable size... for operation above 2000 RPM. I have a 2400 stall and cruise without OD, so I'm not worried. If I feel it is "too big," I'll have a reputable shop install some annular boosters and go from there.

Perhaps Tuesday it will go on the truck. The first step will be initially running it, and then going to way leaner jetting. Primary jets should be downjetted first I would imagine. Smaller jets will begin to lean the ENTIRE delivery. After sufficient downjetting then I'll change/drill the airbleeds.

I really know what you mean about being your only friend when you get so far "out in the weeds". Hopefully, I won't get lost.

Damon, can you explain what changing the highspeed bleeds does? My David Emanuel Holley carb book doesn't talk about this one. I would imagine a smaller air bleed would start the enrichening to the secondaries sooner?

If I could get away without being too lean to around 2800, I could SLIGHTLY increase the PVCR and ppssibly bring full enrichment earlier if this is correct. ?

High speed bleeds either make full enrichment come in sooner or make the AFR curve an unlinear function. Please explain.

Comments?
THANKS A MILLION!

Ben T.

P.S. the "jungle engineered" carb is going in the trash

 

Damon, I found some more information.

http://www.carcraft.com/techarticles/81958/

Ben T.

 

Don't go nuts over the bleeds. Their operation is simple. They have basically the reverse effect as jetting does, with a little twist.

You want to play with the idle air bleed to richen and lean your just-off-idle mixtures. Like under 10-15% throttle. THis bleed ONLY affects the idle and off-idle mixture. The reason you want to play with it is it's the only way to "tune" the off-idle portion of your fuel curve. The idle mixture screws only do curb idle and the main jets only do from about 15% on up. It's that in-between from about 5-15% throttle that your idle air bleed will be your tuning tool.

The high speed air bleeds, on the other hand, just do the reverse of a jetting change. Yeah, I know what the article said, but they didn't explain it quite right. It only afftect the mixture once you are using the fuel that runs through the main jets. i.e. when you're feeding fuel from the booster venturis. Basically, when you're over 15% throttle all the way up to WOT. When you're running on the idle ciruits it has no effect.

The "twist" to a high speed air bleed change is that it's not quite like changing the jets. It's effect is more like changing the float height to affect mixture. Sure it richens/leans the mix, but it also affects WHEN the mains start to operate. How much airflow before they begin to flow. In the article they said they saw a rich spike in the upper RPMs that was cured with a larger high speed bleed. Probably that is becuase at extremely high airflows it started to actually pull liquid fuel out of places it shouldn't have. Weird things can happen at high airflows. Changing the bleed can sometime fix the problem. But that't not what you're doing right now- you're tuning for economy first, then to bring the mixture to a reasonably level at WOT second. Concentrate on the first job first and leave the other one for later.

For what you're doing you don't want to touch the high speed bleeds- leave them stock size. You want to concentrate on the idle circuits, idle bleeds, jetting and maybe a smidge of work on the power valve and PVCR when you get into step 2, tuning for WOT power.

 

Thanks for continuing to send your words of experience Damon.

The wideband O2 is not mine, but belongs to a friend's friend. I have to rent it per day. Following your suggestion, I'll play around with only the primary side and no high speed air-bleeds at all on the first day.

Try and get a nice lean cruise with the mainjets and idle bleeds. Then make it seemless with the right cam and squirter combination. Drive around like that for a while and then play with the WOT mixture on another Saturday.

You're right, the idle air bleed controls the idle air fuel ratio. The screw just controls the amount that is let out.

After reading the article above, it sounds like I should try and set my cruise afr first, then work around that and find a nice idle... THEN do the WOT tuning.

One last concern. Will cold starts be miserable with such lean tuning? This mainbody has zero provisions for a choke.

Ben T.

 

Yep. She'll be a miserable old cus on first startup without a choke. Plenty of revving and gas pedal pumping to keep it running for the first minute or so. That's why they put chokes on them from the factory.

 

Damon, it ran! Right out of the box. A few tweaks and then she was purring. I've been driving it around a little lately. Just wanted to make sure it 'worked' before I fix it.

Small jets go in soon.

Ben