87v6Bird

iTrader: (1)

Engine/Trans/Rearend:
406 SBC
Ported small valve heads, 8.5-9:1 comp., 87 octane
Weiand Speed Warrior Intake
Comp Cams 268H Hyd. Flat Tappet 218/218 .454/.454 110 LSA
12 degrees initial timing. 32 deg all in. Manifold vacuum advance giving me 16 more at idle.
T56
3.42 geared 10 bolt
275/40R17 Tires

Carb:
Holley 750 cfm Vacuum Secondary
QuickFuel Metering Block on Primary Side
QuickFuel Metering Plate on Secondary Side
65 Jets Primary/68 Jets Secondary
0.026 Idle Feed Restrictions in the primaries. Secondaries untouched, should be 0.040.
10.5" PV with 0.028 PVCRs.
At WOT all of this results in an AFR of ~12.9

I've had my car back together for the better part of a year now. This time has been spent slowly tuning it and getting it running like it should. I've came to an issue with the carb that I'm not quite sure what I should do to solve though.

When I'm cruising on on the highway or interstate it runs richer than I would like it to, 12.8-13.5 or so typically. By cruising, I mean from 60 to 70 mph in 6th, or between about 1300-1600 rpm. If I go above or below this speed/rpm range my mixture will lean back out to where I've tried to get it at between 14.5-15:1. Jet changes do not affect the mixture in this range which leads me to believe I'm still on the transition circuit. As well, if I go any smaller on the idle feed restrictions my car goes very lean on take-off and wants to die but it is leaner in the current problem area which also tells me I'm still on the transition circuit.

My question is what is recommended to fix this? Is this something solved by changing the the air bleeds? Any pointers are appreciated.

pancherj

Not sure if yours has the replaceable air bleeds, but I would make a big move up in the size of the idle air bleed (go up .004" or even a little more) to see what impact it has. I like to make big moves to see if I am on the right track and then small moves to fine tune (this goes for air bleed size, jet size, IFR, etc.). If you go too big in the IAB, it can affect the idle mixture.

87v6Bird

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You're correct. It did not come with replaceable air bleeds. I would have to drill and tap them. My own reading has led me to believe that going up on the IAB's might solve this but I wanted to get some other people's opinions before I made the modifications.

bestracing

What is your manifold vacuum level while cruising at 1300-1600 rpms? With that cam I wouldn't think it would be near 10 or 11 but it doesn't hurt to check and rule out the power valve coming in too early.

87v6Bird

iTrader: (1)

I will verify these again since that was early in my tuning process but cruise was around 15" and idle at 20".

bestracing

I figured the PV wouldn't be an issue but you never know.

I just read an article not too long ago about tuning Holley's and the carb builder was talking about drilling the IABs do to the formulation of the pump fuels and the addition of E10. I haven't messed with this too much since I have mostly tuned for WOT going down the race track.

skinny z

What's your AFR at idle and what rpm?
20" of idle vacuum? That's pretty good. Is that with or without your vacuum advance hooked up?
Do you have any info on when the vacuum can starts to pull in advance and when it's all in?
Your PVCR dimension of .028" seems small.
Regarding drilling and tapping the air bleeds. I've done that on my Speed Demon 750 vac carb and am working through the tuning process as you are.
I find that at cruise the idle circuit is so much involved (up to about 2500 rpm in my case) that any change in the idle AFR impacts the cruise.
And I mean in a big way.
In an attempt to improve an idle issue I set my idle mixture to around 12.5-13.0. That change took my normal 20 mpg on the hiway (cruise at 2500 rpm in O.D.) to less than 15 or 16 mpg. That being unacceptable, a change to 13.5-14.0 idle AFR brought my mileage back into the 20+ range. (I've got a measured best of 23 mpg steady state hiway through one tank of fuel). Now the idle AFR thing is an extreme example and the test distance was very small (100 miles) but the results were obvious.

87v6Bird

iTrader: (1)

I took some measurements when I got home from work. I should note that I found out I don't have 12 deg. initial advance as I thought I have 9 deg. I know I had it set to 12 at one point. I'm not sure why I would have changed it and will probably bump it back up this weekend.

Yes 20" or 19.5" if you really want to be picky AFR at idle is set to 13 at 650 rpm with vacuum advance hooked up. Without vac. advance idle rpm's drop to 550 and vacuum is slightly lower. I'm also fighting heat soak. If I raise the AFR to more like 14.7 it will be so lean after letting it sit for a short period of time that the gauge doesn't register it and it dies very easily.

You got me curious so I checked that as well. Vac. advance begins to come in at 8" and is all in at 14".

I agree with you here. Everything I've read indicates I should need larger ones. Any larger though and my car begins to stutter over about 4000 rpm. It could be that I should go smaller on my secondaries so I can come up a little on the PVCRs.

I'm glad to have found someone who seems to be going through the same tuning process as I am. I agree that the idle circuit plays a large role in cruise as well. I think for my case it's dropping out about 2000 rpm. My theory is that 1300-1600 rpm is an unfortunate rich spot as the idle and mains are both "on". I get 20 mpg on average but just know I'd be getting better if I could fix this. It's killing me haha.

87v6Bird

iTrader: (1)

You're right. It's best to make sure.

Oh I've had my WOT issues too but since this is my DD it keeps getting drug to the forefront of my attention.

skinny z

IF that's the case, then enlarging the HSAB will delay the main circuit activation. Unfortunately in your case, it's a one way operation unless you drill and tap.

As far as the adjustable bleeds, I've used mine to good effect in smoothing over a part throttle lean spot. Accelerating at a moderate pace with vacuum dropping to a point above where the power valve comes would produce an AFR approaching 16:1. I keep the idle afr deliberately lean to aid the cruise mpg so tightening up the HSAB helped bring in the mains a little sooner under that particular load condition (rather than enrich the idle). My cruise vacuum is above that point so my mpg stayed pretty steady.

You sound like you've been thorough in your approach but it's worth your while to be doubly sure that's it's not some anomaly with timing or another carb setting. Is your transition slot in the primary exposed the proper amount? Is there something you can do to alleviate your heat soak and are you sure that's what it is? I experience changes in idle AFR waiting at a stop light.The characteristics of the vehicle change noticeably with temperature whether it's ambient or engine temps. With an Air Gap intake I can see how it can shed heat with the car in motion while not so much when idling. I have a cold air induction in the works that I hope will help even things out.

87v6Bird

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The interconnectivity of the two circuits at this point is what has kept me from making any adjustments thus far. I know, potentially, changing either bleed could result in a similar but not the same end result. I'm just going to have to get in there and do some testing to figure out what's best.

Sofa's sticky at the top was one of my main guides in beginning to get everything sorted. The guidance on the transition slot helped with some of my early drivability issues and is also why I'm running a 10.5" PV. I definitely want to move my timing up to 12 initial and see what impact that has first.

I should clarify exactly what I'm dealing with at idle in case anyone else is experienceing this. When I first crank the car it's rich at 11:1. After driving and letting everything get up to temp (170 deg thermostat) it will be at 13 and will stay here as long as the car maintains operation between 170-190 (fans come on above 190). If I go somewhere and turn the car off for say less than an hour and then crank it again my idle AFR's will be in the 15-16 range. After driving for a bit and letting things cool off they will come back down to 13. Before, with idle at 14.7, after sitting a short period of time the guage wouldn't register the AFR. This was resulting in the car dying at inopportune times which I deemed a risk to my well being haha.

87v6Bird

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A summary of Friday and Saturday's tuning activities:

I advanced my timing back to 12. The car pulled much better but this did nothing for the cruise issue. I advanced the timing a little more to 14 deg. The car seemed to like this even more. I stopped here because that puts my total timing at 38. (I know this doesn't add up with what I originally posted. It's what I get for posting something from memory while at work…)

After the timing, I blocked the PV and locked out the secondaries. Cruising between 2,500-3,000 rpm my AFR was between 16-17 and I had a bit of a lean surge. I came up on the primaries to 66 jets and went out again. The AFR was between 15-16 now and the lean surge was gone so I moved on. The rich spot between 60 and 70 was still there and not affected by the jet change.

Next, I unblocked the PV and checked what things were like under load. With the PV open, my AFR was 14-15. I went up on the PVCRs to 0.033's. This brought my AFR down to 13-14 under load. I didn't feel like drilling another set of PVCRs at the time but I do intend to go up on them some more to bring the AFR to 12-13.

I moved on to the secondaries. The popping I've experienced before was back. It starts between about 4000-4500 rpm and I just lose power. I know it's not the topic for which I started this thread but I'm beginning to think it may be valve float.

Nothing I did fixed the rich spot unfortunately. I do feel my tune is much better now than it was. I'm going to get the stuff to drill and tap my bleeds this week. I probably won't have the time to actually make the mods for a couple of weeks though.

skinny z

Good luck. Keep us posted.

87v6Bird

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I wanted to be sure the primaries were the main factor in the AFR. I was driving in 3rd and 4th to keep my speed down. So depending on the gear I guess that was between 40-50 or 60-70. Vacuum was 12-13" so a little less than what I cruise at but still above where my PV comes in.

Vacuum is low, about 5" or so. I haven't attempted to cruise at these rpms. AFR went rich to about 11 with the 68 jets. I decided to check with some smaller 65 jets today but that didn't seem to make a difference. AFR was up to about 12.5.

It starts generally between 4,000-4,500 rpm. It's just a popping and loss of power. I'm not really sure how to best describe it and am at a loss to what it is. This has happened with two different distributors and I've tried closing my plug gap so I don't think it's spark related.

Unfortunately, I don't know the specs of the springs. I got the engine with about 1,000 miles on a rebuild. I tore it down to make sure things looked good but I assumed the proper springs were installed.

skinny z

I realize that I'm not being much help with your problem however I'm a little confused with the information you're providing.
You say the AFR was between 15-16 but the rich spot between 60 and 70 was still there. When (RPM, Vacuum and MPH) is the AFR at 15-16 and when (RPM, Vacuum and MPH) does the rich spot happen?
You'll find that the adjustable bleeds are a tremendous tuning aid though. Sort of like a mechanical fuel injection with a few less degrees of freedom.
I'm deep into the tune on my carb too and am finding unusual things happening as you are. I've gone so far as to rig up a data logger with inputs from the O2 sensor, tach and a MAP sensor. Some software glitches have kept me form actually doing any logging but the info that will be provided is invaluable.
(I find it odd that no one else has come on board here. I suspect that tuning a carb is becoming a lost art and very few people here have a lot of experience with this type of tune. WOT and idle, sure, but mid cruise AFR and such....not so much)

87v6Bird

iTrader: (1)

Ah. I'm sorry. I see I misunderstood what you were asking. By that I meant that if I shifted to 6th and cruised at 60-70 I still have the rich spot. To make sure we're on the same page:

AFR 12.5-13.5: 1300-1600 rpm, 15-14" vacuum, 60-70 mph 6th

AFR 15-6: 2500-3000 rpm, 14-12" vacuum, 60-70 4th

I'm looking forward to tuning with them and seeing what I can do. I may be weird but making tweaks and seeing what they change is just as fun as the end result of a much better driving car for me.

I've seriously considered a datalogger too. I drive nearly 100 miles a day and the amount of info just one day would give would be helpful. I can't keep my eyes on my guages all the time.

I'm surprised no one else has jumped in too. Thank you for taking the time to follow along. The lack of information that I could find is what caused me to post in the first place. I hope I solve this and that it is useful for someone else in the future.

JaBoT

1300 - 1600 is a combo of the idle circuit and the main jets
According to you in a previous post idle is at 13:1. Lean out the screws a little and it will lean out 1300 - 1600 rpm
2500 - 3000 is the main jets
If you are planning on crusing it in the 15's afr i would put a 10.5 powervalve in it otherwise under light acceleration your vacuum wont drop enough to open the valve and cause a really lean condition.

87v6Bird

iTrader: (1)

I checked this once I got home from work to verify my memory. I leaned the idle out to 15-16 (I wanted to be sure I could see any effect this had) and went for a spin. I did notice that at 60 (6th gear) I ran a little leaner. About 14:1. This effect was gone by the time I reached 70 though and I was back at 13:1.

I think this further shows I'm on the transition between idle and mains.

skinny z

That's were your air bleeds will come into the tuning picture. As you know, you can move the onset of a particular circuit or prolong it's inclusion with the appropriate change.
In my case, I've increased my HSABs which delayed the point at which the mains came in. My lean idle circuit then commanded more of the mixture further into the rpm range. (Or so I'm finding).

87v6Bird

iTrader: (1)

Yes. That sounds like the direction I will need to go as well.

87v6Bird

iTrader: (1)

I got my airbleeds drilled and tapped today. I tapped them for 10-24 threads. It's a pretty straightforward procedure but I figured I'd give a few highlights for anyone who's curious.

Carb ready to be drilled:


When you're drilling the bleeds, the brass inserts will come loose. Just flip the carb over and the inserts should fall right out. This makes drilling a little easier.

Freshly drilled bleeds:




The HSAB's on my carb were very close to the choke horn and body of the carb. This made tapping them pretty tricky because the tap wanted to cut threads in the choke horn.

Adjustable Air Bleeds:





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87v6Bird

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Figured I'd split the modifications and tuning into separate posts. I measured my bleeds before making the modifications. The IAB's on the primaries were actually a hair larger than my #48 bit which I measured at 0.075". Here's a before and after of my bleed sizes:

Before:
HSAB's IAB's
Primaries 0.026" 0.076"
Secondaries 0.025" 0.028"

After:

Primaries 0.029" 0.075"
Secondaries Same Same

*I tried in vain for at least 30 minutes attempting to format this into a table. If someone would enlighten me I would be extremely grateful.

While putting my carb back together, I realized I had used the wrong baseplate gasket when I rebuilt the carb. I put the right one in when I put it back together this time. After I got it back together, I fired her up and went for a quick drive. Initial results are quite promising. At 70 mph (6th, 1500 rpm, 14" vac) I'm now cruising at 16-16.5. Off idle transition and acceleration don't seem to have been affected. I'm going to observe over the coming week and get back with some harder numbers.

skinny z

I found that drilled blanks measured differently from the spec parts. It appears that the profile of the hole isn't exactly a hole either. Sort of stepped with a flared entrance not unlike that of a main jet. My point here is that perhaps the numbered drills and the actual size of the bleeds don't measure exactly the same.

Are you familiar with Microsoft Excel? See the attachment below. I had to save it as a pdf file because for whatever reason, this forum won't let me upload it in an excel format.

Very encouraging. A quick look at the numbers shows that by enlarging the HSABs you've reduced the signal to the main circuit and delayed it's onset into the fuel curve. That's quite a jump too. About 25%. Personally I prefer that approach as any change will be an obvious one.
Reducing the IABs would increase the role of the idle circuit in the overall fuel curve and the overlap it has into the cruise mode.
Keep us posted with your progress. I'm pretty sure that a lot of carb tuners will be interested in your results.

87v6Bird

iTrader: (1)

Yes I am. I was just being stubborn and not wanting to do it that way haha. I've seen on other forums and maybe this one where a table was inserted directly into the post but I couldn't figure it out.

I didn't realize quite how big of a jump I'd made until you said that. I was looking at things from a drill size perspective and not an area perspective and didn't feel like I'd came up that much. I was pleased to immediately see a change and a change in the direction I was hoping for at that.

Yes, I hope others find this information useful as well.

87v6Bird

iTrader: (1)

First tank of gas report: 21.5 mpg

This tank was unusual in that almost all of my driving was on the interstate at 70 mph. Typically, it's a split between state highway driving at 60 with the occasional stoplight and interstate. In those cases, mileage ranges between 17 and 20.

Overall, the car runs much better now. I had a flat spot in my acceleration which is now gone. Cruising at 70 mph (6th), AFR's are in the 16-16.5 range and vacuum sits at about 14". Cruising at 60, AFR's are leaner at 16.5-17 and 15" of vacuum. It also transitions to acceleration easier staying in the same gear. Before it would lug a little before vacuum dropped enough for the PV to come in.

I'll report any further observations but it looks like this stage of my carb tuning is complete. Now to move on to figuring out what's causing the upper RPM popping I experience...

skinny z

It's good to hear you worked out the tune. I will say I'm surprised that at nearly 17:1 you don't have other issues. Lean surge and all that.
Was it only the changes you doucmented in your previous post? That was a sizable jump in the HSABs and a modest one for the IABs.

87v6Bird

iTrader: (1)

I agree. If someone had told me they were running at 17:1 I would have expected a lean surge.

The only other change I made was to adjust my idle screws. The smaller IAB's had richened the idle mix a bit. I haven't yet blocked off the PV and locked out the secondaries so I could drive on just the mains. It could be that my enlarged HSAB's have leaned out the mains more than I wanted.

skinny z

My concern with the very lean condition is the tendency for detonation. I'm on the edge compression ratio wise (both static and running) and while cruise isn't so much of a concern because of the lack of load on the engine, rolling into the throttle from a lean state invites a little pinging. That's something I'll have to work through in my own tune. PV, HSABs and IABs, accelerator pump, it'll all come into play at some point.
Regarding your enlarged HSABs, I don't think the bleeds will create a lean condition overall but they will certainly change the point at which the mains become fully active. I would suspect that moving up in the rev range and building a stronger signal in the venturi will encourage the mixture to reach whatever the primary is capable of providing. Same with the smaller IABs but in reverse. Stronger signal, more fuel.
Again I say job well done.
Anytime you want to come over and get to work on mine is ok with me.

87v6Bird

iTrader: (1)

I'm sure if my compression were higher I would be much more likely to experience detonation myself. Luckily (or, really, un-luckily) my compression is low enough that I'm not dealing with that.

I did a little testing over the weekend. While I would say that the major effect of altering the bleeds is the change in rpm at which the different circuits become active, I also saw a difference in my AFRs as well. Before, on the mains, I was seeing AFRs in the 15-16 range now it's more like 15.5-16.5. Not a huge difference but when you're tuning towards the edge it's something to consider.

Thank you for your input. It was nice having someone to bounce my thoughts off of. I think I'll stick to messing my own carb up though haha

bestracing

Thanks guys for posting this up. I have worked on carbs for a long time but never messed with HSABs or IABs since I've only had stock Holley carbs and really didn't have any problems tuning using a Wide Band and dyno for WOT since all we were doing was drag racing. I knew the theory behind the air bleeds but never seen it done (practical application).

skinny z

Not being strictly a drag racer, and a few of my racing buddies are in the same boat, fuel economy is becoming as important as mph at the track. We spend more time tuning carbs to get decent mileage than we do establishing the best AFR at WOT. We've learned the air bleed/IFR/PVCR tuning techniques through experimentation.
I've posted up "definition of a street car" in the past and there's never a mention of being able to climb in your hotrod and drive it across the country and not have to take out a 2nd mortgage to cover the fuel costs. I've done it a few times, once towing a trailer behind the IROC. (Note to self: don't do that again.)
Interesingly enough, the technology has advanced so much in just over the last decade, that my 12 second cross country cruiser is just another "also ran" as the guy and his wife in the Cadillac keeps up and he has his cruise on with AC blowing arctic freeze. Undoubtedly more comfortable too.
I like carbs and all but I can't help but think that the next engine will be EFI...with a laptop and a supercharger.

Thanks for the thread 87v6Bird. Be sure to post up if you find anything new. Give us a mpg figure from a full tank on the interstate.

bestracing

I have seriously thought about a 5.3L swap into my 86 a few times for both fuel economy and power. I too like carbs but these Gen III & IV SBC motors can give you the best of both worlds, economy and power

87v6Bird

iTrader: (1)

Thanks again guys. I've learned a lot reading on these boards so it's a pleasure to give back some.

I have already tweaked some things some more. I went up on my PVCR's some more to 0.043. This brought my AFR under load down to the 12.5-13 range where I wanted it. I also checked my float levels again and decided that they were really lower than they should be. Raising them evenly richened my mixture once off idle by about 1 AFR i.e. 17 -> 16.

I'm probably going to come up on the HSAB's one more drill size (#68, 0.031") as a result. I'm thinking the HSAB's because with my slightly smaller IAB's I no longer have an annoying flat spot in my acceleration, which I feel was fixed by the IAB's. I'm also planning on squaring up my IFR's by putting in 0.026"s in my secondary metering plate. My idle screws are barely 3/4 turn out and most places I read suggest I should be more in the 1-2 turn range. Of course, if this messes up everything else I've done I'll probably go back haha.

My main concentration this weekend will hopefully be fixing the popping I've mentioned before. I've went ahead and ordered new valve springs. I hope they turn out to be the problem.

I actually really wanted to go carbed LS myself. People around here were just too proud of them when I was in the market.

87v6Bird

iTrader: (1)

Two tanks of gas to report: 18.9 and 19.6. It's looking like I'm not going to be seeing the 17's again. I'm happy for that.

In tuning news, I replaced my valvesprings yesterday. It was all in vain though, as the popping is still there. I noticed something new. If I slowly roll into it, in first or second, I can take it to 5000 rpm without it popping. In third, slowly accelerating the popping will come in about 4500 rpm. However, if I just stomp it from the get go it will start popping about 4000 rpm. It doesn't pull out of it.

It seems like it's load and rpm related. AFRs during this are rich but nothing crazy, about 11-12. I feel like they're reading rich due to whatever's happening as it is.

bestracing

Good to hear!

You have greatly improved your gas mileage.

skinny z

Better mileage is a good as better trap speed as far as I'm concerned!


I'm not entirely sure I've got this right but IIRC doesn't an overly lean condition manifest itself as a rich AFR due to the incomplete combustion?
What are the AFRs just prior to the popping?

87v6Bird

iTrader: (1)

I've heard/read that as well. It's something I'm going to check.

My other theories are that it's connected to my secondary opening rate, since I've now realized that slower acceleration seems to delay its onset, and that perhaps it has to do with my PCV.

bestracing

Well guys over the weekend, while coming home from a car show, my engine started to knock. It's pretty bad so I'm definitely going to be putting a new motor in which will require a new carb as well.

Looking at pricing Quick Fuel has a HR series that is cheaper than a Holley and it also has adjustable air bleeds so it would be just like changing jets. Looks like a QF HR650 is now in my Que for a future purchase.

skinny z

Are you thinking that the secondary opening is changing the mixture to the point where the engine misses and backfires? I suppose that would be easy enough to zero in on.
Regarding the PCV valve, I've experimented with a couple of different specs however this was when I was (still am actually) chasing an inconsistent idle issue. I was switching between an 86 carbed Camaro spec Fram FV184 for example (high idle vacuum) or an FV181 which is the spec for the early 70's Camaro/Chevelle/Corvette high horsepower engines which would have low idle vacuum. I can't say I could see much difference in idle quality although I did eventually keep the high idle vacuum valve because as my carb and ignition tune progressed, my idle vacuum became consistently better.
I don't believe it has much effect on cruise or almost WOT conditions.

skinny z

Gen 1 SBC or an LS.....?
As for the carb, Quick Fuel as developed a pretty solid reputation. I don't think you can go wrong with that choice. As we know, it's the adjustablilty that's important.

87v6Bird

iTrader: (1)

Yes. Something along those lines. I've never seen anything to make me think it's going lean but it's better to verify than assume at this point.

I don't know for certain that mine is functioning correctly. My understanding is that they should open when manifold vacuum is high and close when it is low. I figure that if it's always open it could cause problems at WOT.

87v6Bird

iTrader: (1)

I've been very pleased with the Quick Fuel parts I'm running.

JaBoT

I prefer quick fuel or pro form over holley. You get much more for your money and they have better base calibrations as opposed to the new holley hp's with the new metering block setups are a pain to tune.

87v6Bird

iTrader: (1)

I'm baaaccckkk!!!...

Finally some good news for a change. I discovered the cause of the popping.

Last weekend I had taken off the carb and decided to go back up on my secondary jetting as a shot in the dark. I went up to 70 jets. I also used this as a chance to see what secondary spring was installed (purple). This, predictably, made no difference :/

Today, I decided to check and see what my total timing was. When I tried to check, though, my timing light would quit registering a spark above about 2000 rpm. This told me something was amiss. I decided to check my voltage at the distributor. In making a jumper to go between the connector and distributor, I noticed the connector was loose so I bent it back out to tighten it up. The voltages looked good. I didn't think I'd changed anything but I took her for a spin anyway. No more popping! Apparently I had a poor connection that was causing me to drop spark. I've also checked and my timing light isn't acting funky now.

I can actually work on tuning in the upper RPM range now. I'll report back with what that involves.


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skinny z

Having a problem is one thing, having a problem and fixing it...it's almost worth having trouble in the first place.