tuning with o2 gauge

[LJDB 103]

I recently got my hands on a innovate mtx a/f gauge kit. Ive managed to get the a/f around 12-14 everywhere except at idle and wot. Iv got a lean a/f at idle around 16-17 and a constant 10.7-10.9 at wot. The carb is a holley 570 street avenger, fiddling with the mixture screws have little effect on the idle a/f ratio and ive tried to lean out the mains but they have little or no effect ay wot. Jetting is currently 62 in primary and 64 in secondary 8.5pv. It was previously 64 in the primaries and 70 in the secondaries with 6.5pv before I started leaning in out. I tied the secondaries shut and still get high 10-11 at wot but my cruise a/f is around 14.

[slydog 7,873]

try a smaller PV

[ando76 4,354]

drill out the high speed air bleeds or put in bigger ones in if they are removable bleeds.

 

Make sure that you have the throttle blades closed at idle - or just open. to make sure you actually have it on the idle circuit.  If you are seeing no changes with the idle screws and the blades are closed or near closed then have a look at the idle air bleeds and fit smaller bleeds.

 

I'd say you have the blades to far open and thereby exposing too much of the transfer slots and taking the idle circuit out of the game.

[gerg 10,871]

PV will still be open at wot no matter what number you use.

 

Agree with Ando on the air bleeds.

 

Lean idle could be a vac leak somewhere. Is the PCV ok? I know that's an obvious answer but whenever you're stumped by something, go back to basics. I know Holleys can be a bit hit-and-miss but it's hard to believe that a street avenger would be that far off.

 

Double check that you dont have any blockages in the air bleed passages. Plenty of stories out there about carbies having swarf in them from new.

 

What sort of cam are you running? If it has a lopey idle, you'll be getting incomplete combustion and that will read lean on your meter despite it smelling rich out the exhaust. You can pump as much fuel in as you like, if it's not being burnt then it will always read lean. An oxy sensor senses oxygen, nothing else.

 

Try bumping up your base timing

 

If you cant get any adjustment from the idle screws, you might have the throttle cracked open too far, also as Ando pointed out. One trick i use is to block an air bleed with my finger when it's idling and that forces the circuit to go rich. If that makes any change to your AFR then you're on the right track.

[slydog 7,873]

Annnnnnd by fitting a smaller 2.5 from the 4.5 my AFR's went from 10.8 to 11.2 on my 650 DP.

 

No other changes just saying

[gerg 10,871]

Sly could the 2nd PV you fitted have been a tad more restrictive? There are different types available, some standard and others high-flow. Really your power valve channel restrictors should control the amount of enrichment, not the PV itself. But good advice as you just never know 100% what a particular change will do.

[Clevo120Y 815]

I agree with all of the above, don't keep pulling fuel jet out you have to tune the air bleeds.

[LJDB 103]

I tried the smallest PV I have at hand a 4.5 and it still was pretty much the same. i went up on the jets just to be sure while i sort out the wot a/f. It does smell rich at idle but reads lean and the screws have little effect. Il check the blades, it has a cam and needs to idle alittle high, how should I set the blades on the secondaries so I don't need to screw the primaries open more to idle. Not to sure on how the air bleeds work. By drilling the high speed will that restrict the PV flow? I have a metering block off a 350 holley would it be wise to wack that in to see if that has any change?

[ando76 4,354]

Have you got the wideband positioned properly?  If it is too far away the idle reading will be off.  Stick to the recommendations that come with that gauge

 

The air bleeds are used for fine tuning.  On most holley carbs they are not removable so be careful when you make changes.  bigger hole allows more air (equals lean). smaller hole equals rich.  They have no effect of power valve functioning. 

 

Power valve functioning is determined by vacuum.  Have you checked what vacuum you have at idle?  Half idle vacuum equals power valve sizing (as a starting point)

[LJDB 103]

Focusing on the rich wot. If the cruise af is ok I shouldn't be leaning out the jetting the only other aspect is the PV which must be the cause of the rich mixture so I need to lean that out am I correct or heading in the wrong direction. The O2 is just after the collector basically under the drivers feet

[LJDB 103]

Focusing on the rich wot. If the cruise af is ok I shouldn't be leaning out the jetting the only other aspect is the PV which must be the cause of the rich mixture so I need to lean that out am I correct or heading in the wrong direction. The O2 is just after the collector basically under the drivers feet

[gerg 10,871]

Yep you're on the right track. Jet for cruise, PV channel restrictors give you load enrichment.

 

Try pulling out the power valve and installing little hooked bits of wire into the restrictors to reduce the flow area. It's very fiddly but it's an age-old method of tuning PVs and you can then determine the next move after achieving the right AFR.

 

You cant make the PVCRs smaller without bushing and re-drilling or tapping for motorbike jets. Might just be easier to get a different meter block (Hume Performance sells them) and go from there.

[gerg 10,871]

You're really getting deep into the realms of carby tuning where the outlay for the various tools, etc may start to become expensive. You'll benefit most from having a set of jet drills, but a good set isn't cheap.

 

If however you're right into it and want to learn all about it, then go for it. That's how i got into it.

 

If you can grab that 350 meter block and compare the PVCRs to your current one, they'll have to be noticeably smaller for it to make the difference that you're looking for.

 

In the interim, you could perhaps jet down the secondaries to compensate. Not the best approach but will do for now.

[LJDB 103]

If I try hooks in the pvcr what are the chances of it getting sucked into the engine? By jetting the secondaries to compensated do you mean run smaller jets in the secondaries compared to primaries and iv heard of people blocking off the PV and rejetting.

[slydog 7,873]

Annnnd thats how I got it to 12.6 via restricting the flow with some mig wire and using a 2.5 PV LOL. Using a block means you will have to step up jetting about 10 sizes aswell to fix the loss of fuel/flat spot off idle you will create.Can use bigger squirters and or acc pump but there is less fuel metering now so you need more jet to solve it.Robbing Peter to pay Paul really.

 

Prob only should only done on drag cars or big HP tuff streeters perhaps ?

 

Just saying  

[slydog 7,873]

If I try hooks in the pvcr what are the chances of it getting sucked into the engine? By jetting the secondaries to compensated do you mean run smaller jets in the secondaries compared to primaries and iv heard of people blocking off the PV and rejetting.

 

Your over thinking it bud...http://www.aussiespeedshop.com/carb-tuning-help/

[gerg 10,871]

No chance of sucking the wire into the engine, as the emulsion wells will stop it.

 

You could plug the secondary PV as Sly said and jet to suit. Many Holleys come without a secondary meter block, they have a simple metering plate instead and the bowl just bolts onto the body around it.

 

I don't know why they bother with secondary PVs as you're at WOT when they open anyway.

[LJDB 103]

Good read on the aussiespeed link exactly what I was after. The secondary plate doesn't have a PV only changeable jets. Il play around with the wire and see how it goes.

[slydog 7,873]

LOL over reading again guys but you get the idea.Limit flow check results.

[LJDB 103]

I put wire in the pv hole and found it slightly better so il try thicker wire when i get some. I also tried the 350 metering block and it was way richer. wouldnt idle unless the mixture screws where all the way in and the af was 12 at best. WOT was low 10's which got my looking at the differences in metering blocks. The main difference i couldn see was the street avenger has a jet/restrictor in the idle feed restrictor location in the metering block. So i followed the channel and found it came out in the choke tower. After some googling i found it to be the idle air bleeds, the air bleed holes on either side are way bigger than the inners or the bleeds on the secondary side. So the street avenger is to lean and the 350 is to rich at idle so i put cut down split pins into the main body to restrict the idle air bleeds the same way as the pv and checked it. What a difference, the mixture screws are responsive it idles way better and the vacum signal was strong at 13. I had to remove the carb to close off the secondary blade because it was idling perfect with the idle screw all the way out just on the secondaries and i can pretty much set the af where i want it at idle. Now just to sort out the wot. Thanks for the help guys.

[slydog 7,873]

350 has smaller holes which mean less air more fuel=richer but your learning the best way possible and getting results by working thru it,props bro

 

P.S What is it on anyway ?

[Clevo120Y 815]

Yeah good work mate, best experience is hands on experience with some great ozfalcon guidence

[LJDB 103]

The cars a Corty with a 250 ACL bottom end 10.3ish comp, mildish cam, tfi with street fire. I decided on the a/f gauge instead of getting it dyno tuned so in the future if I change the build I can retune it myself. I thought I had the jetting pretty close by seat of the pants and plug reading but It goes to show what technology can do.

[slydog 7,873]

LOL...learning how to read plugs is as important as when to pull em.

 

Setting plug gap helps to.Bigger ignition more comp means closer gap.Yes closer not wider.

 

Hows the timing ? Does it rattle or ping ? If not or if only under load 1-2 degrees can trim afrs a fair bit too.

[gerg 10,871]

Good to see you getting your hands dirty instead of dropping the thing off at the dyno and throwing cash at them. You'll stress about it now but you can keep all of this stuff in the old memory bank for later.

 

350s have huge PVCRs, something like those on a 750. Everything in a 350 circuit is bigger because the engine sucks fuel/air through 2 barrels instead of 4.

 

Since you're running so much carby on a small engine (relatively), those air bleeds are too big. A carby that size is tuned for a warm street 350 cube and upwards. Less cubes mean less idle bleed needed for a given rpm.

 

Is it a modern 570 with screw-in air bleeds?