S_Johnson

Ok, so, I have a brand new Holley Street Avenger 670. I also have a wideband o2 gauge for tuning.

I set the idle (~14.0 on the gauge), etc... and took it for some test drives.

In 6th gear, on the freeway, I was getting 17.0 on the gauge, so... very lean. I took the primaries up about 3-ish jet sizes, until my cruise on the freeway was about 15.0, depending on RPM, etc... This seems to work GREAT for cruising MPG, etc.

I also took the secondaries up 1 or 2 jet sizes, and my WOT seems about right, at about 12.5-13.3 on the gauge.

Here's my problem, light throttle leans out so bad it maxes the gauge and the car stalls. I'm talking 22.0 on the gauge, trying to gently pull away from a stop sign. If I get on it, it leans out for a second, then catches, and takes off pretty good (which implies to me that the main circuit is just fine). My accel pump arm has just the tiniest bit of slack in it, like it's supposed to, so that should be fine.

Also, conversely, when I get on it at all (and really, even when I don't stomp on it), when I let off the gas, and coast in neutral, it goes SO rich it's 9.0, and smells bad. It takes probably 30 seconds to climb back up to an ok idle mixture.

I'm very frustrated!

sofakingdom

iTrader: (1)

Raise the power valve number. Stock, it's probably a 6.5; prolly wants to be a 10.5 or even 12.5. Your vacuum gauge will tell you what your cruising vac is, and you want the PV to be a high enough number that it starts feeding gas right after you start pressing on the throttle.

Not a lot you can do about the snap-the-throttle-closed problem. It's native to carbs in general: they feed fuel through the idle system based on vacuum, which since if the engine is braking the car, the vacuum goes SOOOOOPER high, so the fuel feed does also. It's one of the brain-dead things about 19th century technology (carbs overall) that you're stuck with to some extent. Factory computer-controlled carbs have a little "kicker" solenoid to keep the throttles from slamming all the way closed when you let off the gas; worth a good 2 mpg around town with a stick shift where you're on and off the gas all the time. Not applicable to your situation of course.

Best thing to do for that, is to teach your driver not to let the throttles SLAM closed; teach him/her how to work the throttle GENTLY. Like, have him/her pretend there's an egg underneath it, and another egg at the throttle stop for it to come to rest against, and teach him/her not to break either egg.

In fact I've worked on my driver to imagine that there's a bank account, and the more sharply/quickly he/she gives ANY of the inputs to the controls (throttle, brake, steering, you name it), the more gets deducted from the account; and you give em JUST BARELY enough in their account to finish the race. They won't finish, let alone win, if they use up their account too fast. If they work the controls smoothly it costs them less. If they work em harsh and sudden, they get dinged in a hurry. Teach your driver the same way, it'll pay off BIG TIME in lots of ways. Tire longevity, brake longevity, gas mileage, slick surface traction, all sorts of advantages.

Read AND UNDERSTAND the Holley Tuning sticky at the top of this forum.

87v6Bird

iTrader: (1)

I'll start by saying Sofa's stickies and advice are very good and you should follow them to begin with.

I wouldn't necessarily have been too worried about your 17 AFR at cruise. The engine is under a very light load. Of course, if it was detonating then you were too lean. I was watching my AFR gauge driving home tonight and saw it hitting 16.5-17 at cruise. I've been driving my car this way for a couple of years now.

Regarding your leaning out when taking off, I had this problem with my carb as well. I ended up solving it by adjusting my Idle Feed Restrictions and Idle Air Bleeds. Neither of these are adjustable on your carb as it came from the factory so I would visit the power valve first. Assuming you haven't changed it, you have a 6.5 and, as Sofa said, your engine probably wants a 10.5 or 12.5. I went with a 10.5 because I could not find anything any higher.

RedLeader289

iTrader: (2)

As far as the lean while taking off issue - are you getting a good shot through the accelerator pump nozzle? what pump cam came in the carb?

You shouldn't have "play" in the accelerator pump arm, at WOT you should have .015" of travel left in the pump, this insures you're getting the most out of the shot but also not bottoming out the diaphragm.

S_Johnson

Ok, so, a couple of things.

Sofa, I'm a bit confused, this is a daily driven car, not at track car, and I AM the only driver, not sure where you got the idea I was working on it for another driver.

Also, when I said it went super rich when I let off the gas, I specified that it was in neutral. Vacuum is not high in neutral, the engine is not braking the car at all, it's just idling.

I think my power valve is 7.5, I had a higher one in there, and it didn't like it, way too much fuel at moderate accel, it dipped into the PV when it didn't need to at all.

I'm going to take the air cleaner off tonight and double check that the accel pump is working right, and I'll look at the cam it has as well.

I've heard that if too much of the transition slot is showing, it will cause this kind of issue, so I'm going to look at that as well. If that's the issue, I can crack the secondaries open slightly more to bring the primaries a bit lower.

sofakingdom

iTrader: (1)

Don't worry about the pump. For reasons I have never understood, every n00b in the world wants to mess with it. Fact of the matter is, if the carb is set up right, it's almost completely unnecessary. The other systems in the carb take care of feeding fuel, and all the pump is even there for at all, is to cover up the brief interval of time while air has begun moving through the carb but the fuel (being heavier) takes a few milliseconds to catch up.

The Holley Tuning sticky covers transition slot adjustment. You are correct that it involves adjusting the sec idle stop, which is almost always too low from the factory, to allow the pri throttles to close more at idle.

The reason that sticky is there, is to avoid repetitive posts asking the same question and getting the same (only possible) answer over and over. It's all right there, in one place, go read it and follow it. Only difference is, since you've got a wideband O2, you have the advantage of a more direct indicator of the actual mixture at all times. What you DON'T have however, is any kind of guide as to WHAT THE ENGINE WANTS; in other words, it's kinda like having a GPS (will tell you EXACTLY where you are down to the foot) but without a map (you have no idea what roads, terrain features, etc. are around you). You know where you are but not where to go.

Follow the sticky IN ORDER. Skip no steps, assume nothing, avoid doing the parts at the end until the parts at the beginning are complete. And that order is, fuel level, jets, PV, transition slot, idle mixture, then repeat starting at jets until it runs PERFECT on the primaries. Leave the accel pump ALONE.

Might want to make sure you have a more correct timing curve than the stock one. That's not mentioned in the sticky, but is CRITICAL to overall engine performance. Timing DOES NOT mean you open some "book", read some "spec" designed for some other engine intended for some other purpose, whup out some "light", look at some "mark", and consider it done. It means the same kinda thing as tuning a carb: work with it until it's as good as it can possibly be, which is to say, that any further change you make to it, makes the engine RUN WORSE. "Better" includes things like throttle response, idle quality, cruise vacuum, pinging, gas mileage, and operating temp. Most notably it DOES NOT include "hard to start": if you find that making the engine run right causes the starter to be inadequate, get a better starter; NOT compromise the timing. Most likely you'll find that it'll want something in the 12 - 18° range "static", centrifugal set to take it to around 36° by around 2800 RPM with the increase starting just off idle like around 1000 - 1200 RPM (no stock dist is set up like that), and 12 - 15° of vac adv on top of that, with the pull-in/drop-out point dependent on your cam and gears and car weight and all that but most likely somewhere in the 12 - 14" area.

87v6Bird

iTrader: (1)

I'm curious to know a little more about your engine. It looks like we could have very similar setups.

S_Johnson

I have a 400 SBC, it's been taken .03 over, and worked over. I think technically it's a 405 now then. It's got an aftermarket crank/rod/piston set in it, but I don't know about anything but the pistons, since the crank/rod combo was already in it. I know it has 5.7 rods, which is a common 400 "mod." The pistons are dished slightly, can't remember how much.

Heads are GM 487X heads, that came on the "LT1" package on the 72-73 Corvettes. 76CC chambers (bringing my comp ratio to about 9.1), with 2.02/1.6 valves, and some light porting work around the stem area.

Comp Cams XFI cam, pretty mild for low end/mid range power. With 1.6 roller rockers.

Air Gap intake, hedman long tubes to top it off.


The induction/fuel delivery has always been my weak spot. I can tell I'm not getting anywhere near the full potential out of the engine, because of bad carb tune, etc...

87v6Bird

iTrader: (1)

It doesn't sound vastly different than my car. Especially when I still had iron heads.

Do you have a vacuum gauge? That will also come in handy when going through the stickies and getting everything right. Use it to tune your idle for highest vacuum and when you're driving around tuning the car to know when you're actually getting into the power valve.

S_Johnson

Yes, I have a vacuum gage, and wideband o2 gauge.

That's why I'm hesitant to raise the power valve, since I've done that before, and got too much fuel at moderate throttle, because it went into the PV when it didn't need to.

This flat spot is "higher" than that. Off-idle, and it still has like, 13" of vacuum.

S_Johnson

Ok, so, as a quick test/first step, I cracked the secondaries open a bit more, and reset my idle position.

This didn't change the lean problem, BUT, it seemed to virtually eliminate the horrible richness after backing off the throttle. I think this is because my transfer slot was too exposed before.

The thing is, the lean off-idle is instant. Moving the power valve "up" wouldn't help completely, because it wouldn't be fast enough.

Even with the engine idling in neutral, if I hit the throttle, it hesitates and stalls before reving. So clearly, I've got a acel pump issue on some level.

(and yes, I'm going to go through the sticky on Holley tuning, I've already read through it a couple times)

87v6Bird

iTrader: (1)

In this flat spot can you hold hold it there? Or does it come out of it after the initial lean out?

sofakingdom

iTrader: (1)

Well, don't worry too much about individual issues you may spot, until you go through it.

The Deal with carbs in general, and Holleys in particular, is that certain adjustments affect EVERYTHING else, and certain others affect only CERTAIN OTHER things, and only a very few others are completely independent and affect NOTHING else. The trick therefore, is to work on those that affect EVERYTHING first, and optimize them by optimizing their effect when isolated from the effect of anything else; then work through the others that affect fewer and fewer other systems, until at last you reach the independent ones such as the accel pump.

That's the reason for doing the tuning process IN ORDER. Fuel level affects ABSOLUTELY EVERYTHING about the carb's operation: no matter how "perfect" you get ANY other adjustment, if you change the fuel level, you will have to START OVER on EVERY other adjustment. That's why you do it first. Jets are next; they affect EVERYTHING except the height of the fuel in the bowl. And so forth. It's almost like a pyramid: fuel level at the bottom, then jets, then the PV, then the idle system including the transition subsystem. The correct procedure is, start at the bottom of the pyramid, get each building block RIGHT in its turn, by ISOLATING its operation. Kinda like going to the gym and lifting weights, in a way; when you want to exercise a particular part, say your biceps, you choose a position and motion that allows you to mostly relax EVERY other muscle and puts ALL the effort into the one you want to work out. Same deal with the carb.

No you DO NOT "clearly" have an accelerator pump issue on ANY level. That's the surest sign of a carb n00b, is the conviction that there's something wrong with that. If you could get the carb PERFECTLY tuned, you just almost wouldn't need an accel pump AT ALL: the correct amount of fuel would be present at all times from all the other systems. The ONLY thing the accel pump should be tasked with, is covering up the difference in inertia (mass) between air and fuel, for a tiny fraction of a second, as the air speeds up and rushes through the venturis but the fuel being heavier, takes a few milliseconds to go from sitting still in the passages to moving with the air. Holley accel pumps are ALL ALWAYS too big (or at least, bigger than they need to be).

You'll see.

RedLeader289

iTrader: (2)

S-Johnson - here's my thread tracking down what appears to be the same problem you have: https://www.thirdgen.org/forums/carb...ng-out-my.html

After going through Sofa's tuning (TWICE!) the problem persisted, I (like you) looked to the accelerator pump, found out my diaphragms and umbrella valves were on their way out (made no difference to the problem though). After having taken the carb off and apart 4. . ..5? times the problem didn't go away.

Replaced my ICM (Ignition Control Module) and it fixed everything.

Just something to consider.

Also, Sofa, your last paragraph really serves no purpose other than to say "all carbs need an accelerator pump"

sofakingdom

iTrader: (1)

Right: a TINY one. Not, enlarge the already UUUUUUJJE one that's there. Not, leave the poorly tuned carb poorly tuned, and jump right over all the stuff that makes it work better, to dinking with the pump instead in an attempt to cover it up.

I would think any reasonable person would not try to tune a defective carb; those rubber check valve mushrooms or anything else. It goes without saying that the carb must be in correct mechanical leak-free operating condition before attempting to tune it.

Your point about how repairing a defective ignition system made your "carb" symptom go away is very good. It is said that 90% of all carb problems reside in the ignition system, just as about 50% or more of all ignition problems are actually in the carb (or injectors etc.). Much like the above paragraph, it doesn't make any sense to tune on a defective ANYTHING. I hope people are smart enough to where we don't have to tell them something as basic as that. Maybe not?

RedLeader289

iTrader: (2)

I see what you're saying, yeah I agree.

heh, we could hope?