carb tuning with wbo2 sensor...cant get it right

[ALLstrokedOUT]

Hey everyone, I recently put in a wideband o2 sensor, and now i am trying to tune the carb closer to where it should be running.
First off, I am running:
BG Demon 850 double pumper (72 primary jets, 4.5 power valve)
408ci w/ edelbrock heads, air gap intake
10:1 compression
comp cams 243/257 .531/.517
What i find strange is that i can get the o2 sensor to read 14.7ish at idle BUT i need the idle mixture screws on all 4 corners to be ~1/2 turn out, anywhere near the 'factory spec' 1 1/2 and i cant get the a/f ratio to go above 12.0. I think i have the right power valve in there; vacuum at idle is 6...
So why is that the case for the idle mixture screws?
Also, when the a/f at idle is set to ~14.7 the a/f is only at 12.5 cruising at 45mph. Is the carb even using the primaries cruising at 45mph; ive read a few articles that say at that point its all the idle mixture circuit.
Thanks for any help!

[Nessumsar]

What does your timing curve look like? Vacuum advance? Depending on your final drive ratio at 45mph, you could still be on your idle mixture, and maybe not enough timing advance (if no vacuum and low engine speed). I would also think that an 850cfm carb may be on the big size for your power/engine size, requiring your idle mixture screws to be on the "lean" side. And 6 murcury sounds very low for idle vacuum (possibly not enough initial timing?).

[ALLstrokedOUT]

i was thinking of changing the timing a few degrees, i belive right now its 12deg initial, and about 36 mechanical only (vac off) at 3000rpm. I run an adjustable vacuum advance cannister as 'low' as it can go. I know an 850cfm is a bit on the big side, but ill be switching over to efi soon, and i got it used for $50 (sold the 750 vac. secondary i had on for $150 haha) so im just trying to tune it better until then...

[Nessumsar]

I would try for 16-18 initial (so long as your starter can handle it), and keep around 36 total mechanical; you should see a sizeable increase in idle vacuum. See where that puts you at idle and your 45mph cruise, and gradually add in vacuum till you get your 15:1. Make sure to check your a/f at highway speeds as well, as you will have more timing advance at that speed; you will also be into your primary circuit at that speed.

[68Formula]

That's a Thumpr cam isn't it? If that's the case, you have a lot of overlap so you'll want a lot more initial timing. You might find it will respond better above even 18 degrees initial. After it's fully warm, raise it in 2 degree increments until you notice it's loading the starter, then back it off a step. The idle speed is going to creep up as the timing improves the efficiency, so occasional adjustment of the idle speed screws and idle mixture screws will be necessary during this process. One you have the initial set, you'll need to adjust the mechnical to keep the 36 total. Then recheck and adjust the other carburetor circuits.

Are you planning on keeping this camshaft when switching to EFI? The map signal with that much overlap is going to be unsteady making it extremely difficult to set an idle speed with closed loop operation.

[Shaker455]

Hey everyone, I recently put in a wideband o2 sensor, and now i am trying to tune the carb closer to where it should be running.
First off, I am running:
BG Demon 850 double pumper (72 primary jets, 4.5 power valve)
408ci w/ edelbrock heads, air gap intake
10:1 compression
comp cams 243/257 .531/.517
What i find strange is that i can get the o2 sensor to read 14.7ish at idle BUT i need the idle mixture screws on all 4 corners to be ~1/2 turn out, anywhere near the 'factory spec' 1 1/2 and i cant get the a/f ratio to go above 12.0. I think i have the right power valve in there; vacuum at idle is 6...
So why is that the case for the idle mixture screws?
Also, when the a/f at idle is set to ~14.7 the a/f is only at 12.5 cruising at 45mph. Is the carb even using the primaries cruising at 45mph; ive read a few articles that say at that point its all the idle mixture circuit.
Thanks for any help!

850's require the most attention and mods to balance.

They need bypass air and transition circuit jetting to balance idle and part throttle/cruise.

With your combo and cam you engine requires plently of fuel and 72 jets is a bit lean for an 850.

The main circuit works right off idle.

I agree with bumping your initinal timming up and your idle A/F should be 13.0 to 13.5

So open all four mix screws up more to get the A/F down.

You do not want your cruise A/F greater then 14.5 as any higher with your cam & combo it will tip lean under sudden or heavy jabs and will cause it to spike higher than 16.0 which at that point you will feel the power fall off.

[ALLstrokedOUT]

Thanks for the replies.
68Formula, Yes, thats the mutha thumpr cam. I've never been any good at tuning, I comprehend how everything works, i just dont think i have the experience to properly diagnose what needs to change.

I'll play around with the timing and carb some more tomorrow. Ive got a timing light, jets, accelerator pump cams, power valves and some time so well see what happens...thanks for all the help so far!

And as far as fuel injection goes,I am aware of the problems that occur with a big cam and a map sensor..initially I'll be keeping the cam, but in 5yrs or so I plan on turboing it and switching the cam. I was thinking of running either a combination of alpha-n and map or using a maf sensor, but am still researching it all...

Shaker455, i had 77's in the primaries to begin with, but put the 72's in when i was doing some trial and error stuff. With the 77's in i was getting an a/f of 11.5 at 'cruise'. Then engine does really like to idle at an a/f ratio of 13ish, thats what it was at before i touched anything..You dont think that an a/f ratio of 13-13.5 at idle is too rich? ..Ive been trying to get close to 14.7 unless under heavy throttle..

[Shaker455]

Shaker455, i had 77's in the primaries to begin with, but put the 72's in when i was doing some trial and error stuff. With the 77's in i was getting an a/f of 11.5 at 'cruise'. Then engine does really like to idle at an a/f ratio of 13ish, thats what it was at before i touched anything..You dont think that an a/f ratio of 13-13.5 at idle is too rich? ..Ive been trying to get close to 14.7 unless under heavy throttle..

No that's not too rich and the A/F will never be perfect with a carb unmodified so you have to find a happy medium.

If you think you were rich before then how did your plugs look?

Were they soot black?

They should be a tan coffee color.

[68Formula]

And as far as fuel injection goes,I am aware of the problems that occur with a big cam and a map sensor..initially I'll be keeping the cam, but in 5yrs or so I plan on turboing it and switching the cam. I was thinking of running either a combination of alpha-n and map or using a maf sensor, but am still researching it all...


This is getting a little ahead, but I'll bring it up since you mentioned it. The MAF system still uses a map signal, so you'll still need an Alpha-N switching strategy if you can't get a clean map signal. If you're going to do this, it would be better to have a system that is wide-band closed loop capable.

Some other food for thought. Since you already know you have to change the camshaft for the turbo application, you may want to consider swapping it at the same time you add the EFI to make the tuning easier. Running a turbo-specific cam for a naturally aspirated engine might not be optimal, but it's not horrible either.

[ALLstrokedOUT]

Shaker455, i know, and can see with the wbo2 that with a carb you cant always get a perfect A/F. But i did think i was running pretty rich, the plugs were pitch black, i suppose i had the idle mixture set pretty good when i was getting the a/f of 13 at idle, but then i definatly need to reduce my mains or work on timing or something because an a/f of 11.5 while cruising is definatly too rich..

68Formula, thanks for the info. ive been paging through MSefi.com and a couple books i bought about efi to try and figure things out. Even though i want to switch over to efi, it probably wont happen this year with school and stuff in the way (all my time off will be put into rebuilding a t56 thats going in soon)
well, Im heading out to the garage to mess with everything so well see what happens....nevermind, i warmed the engine up, then it started raining, looks like itll be reaining all day.

[68Formula]

well, Im heading out to the garage to mess with everything so well see what happens....nevermind, i warmed the engine up, then it started raining, looks like itll be reaining all day.

Hey now, you have a garage... stick the exhaust-end out and get to work! You can at least start mucking with the stuff at idle. Get the initial timing optimized, re-adjust the idle speed/mixture, then pull the distributor to reset the mechanical advance limits to total timing. Should take up a bit of your day. Then when the weather gets better you can start driving it and doing further adjustments.

[ALLstrokedOUT]

..the garage has my dads corvair in it on jacks (replacing the front brakes), its raining off and on, but its slowly stopping..

edit: i got a bit of work done, re-timed it to 16 initial timimg for now. the vacuum didnt change much, but i now have the idle set screws 3/4 of a turn out and its idling around 13.0-13.5 A/F. But under cruise im still getting around a 12.0 A/F. I have a 4.5 power valve in now, could replacing that help my cruising A/F ratio? Ill try setting the timing to 18 after dinner, maybe pull out and replace a plug to see how they look..

[Shaker455]

Cruise will be slightly rich without jetting the transition circuit

[ALLstrokedOUT]

I dont mean to be naive..but how do you jet the transition circuit? is it in the metering block above? ..ive never gone more in-depth to tuning a holley than primary, secondary jets, and ive played with accelerator pump cams...We have an old 'how to tune a holley' book at our shop, im gunna pick it up and take it with me tomorrow too

[Shaker455]

Here's another book you should have if you want to dig deeper into carb tuning:

How to build Horsepower Vol.2 by David Vizard ISBN# 1-884089-14-3

It also covers transition circuit jetting.

[ALLstrokedOUT]

Hey guys, just a quick update. I had to drive a couple hours up to Tampa on the interstate to get back up to school. At 80mph @2700rpm (overdrive!) the A/F was sitting around 13.0-13.5. So it seems the #72 primaries are an ok selection, I might even want to go smaller(?) I still need to work on that throttle transition point though. I sold some books on amazon so with my credit I will be buying 'how to build horsepower' hopefully it'll get here before the semester gets too hectic... *

[ALLstrokedOUT]

Hey, its been a while, havent had much time to tune the carb better. Theres still a part throttle stumble, but it runs better than ever at hwy speeds and full throttle is amazing..anyways spring breaks coming up, so ive been doing some reading, i found a good articel on car craft if anyone ever has a similar issue:
http://www.carcraft.com/techarticles/ccrp_0607_engine_tuning_guide/index.html

"We also discovered that even the tiniest change in the idle-mixture screws brought about huge changes in the A/F which told us we needed to lean out the idle (fuel) feed restrictor. The huge idle-mixture screw swings in A/F indicated the idle-feed restrictors in the metering blocks for the 950 carb were too big for this engine. This left us with two options. The first was to reduce the size of the idle-feed restrictor from its original 0.036 inch to more like 0.034 inch. While it's not much of a diameter change, it's a significant decrease in area, thereby reducing the amount of fuel to the idle circuit, which creates a wider range of adjustment for the idle-mixture screw. This makes it easier to fine-tune the idle-mixture..The second option for leaning out the idle circuit was to experiment with the idle air-bleeds. One of the advantages of the Holley 950 HP carburetor (or any HP-bodied Holley) is that the idle and high-speed air-bleeds use screw-in jets that make them very easy to change. Since these bleeds meter air, they operate the opposite way that fuel jets do: increasing the air-bleed leans the circuit."

Ive known that the 850 is too big for the 408, but i cant imagine being carcraft w/ the 950cfm carb on a 434, although im sure its a more built motor...hopefully in the next few weeks i can order some air bleeds and such to get everything smoothed out once and for all.

[61ragtop]

Hey Tim, just curious how the tuning is going? Have any updates?

[ALLstrokedOUT]

Hey, sorry it took so long to get back to this thread for updates, but not a lot has happened. So far i have not been able to try what i boldened above in the article i found due to classes and needing my car to be up and running every day. I have the needle adjusters barely open and it idles happily at 13:1, a moderate throttle lean hesitation is still there, but its A/F is pretty good at full throttle. The one issue i have is that i (stupidly) am not running a fuel filter at the moment. Because these idle mixture screws are so close to shut, any debris cloggs them and leans the idle, sometimes itll idle at 16:1, sometimes 14.5:1.

I only have three days of exams left before summer where i would have more free time to tune, and install a filter, but recently my hydraulic throwout bearing went out (chaffed hose that i have vise gripped off). So the next oppurtunity i get ill be limping the duster home; the 6spd conversion is getting fast-tracked because of that...

[67cougnut]

great thread

[61ragtop]

Any new news? I am working on dialling mine in too. Hope it is all going well for ya. It is most definitely a lot of trial and error.....