Vet65te

Finally got around to using my new Innovate LM-2 Air/Fuel Reader. Have a few questions to ask those who already have an LM readers.
First off, let me fill in some of the parameters. This test was done on the 327 in my 61 Vette which has flattop pistons and a #097 solid cam.
I had put about 1400 miles on the car since buying it in late '08 and it ran okay but the 461X valve guides were sloppy and fouling the plugs. The compression test was fine so I removed the cast iron heads and added a pair of Edelbrock aluminum Performers. Not much else was done to the engine other than a slight enrichment to the almost new Holley Avenger 670 (factory jets were 65/68's, now 67/74's). It does have headers and a 2.5 exhaust system with Flowmasters and an MSD 6A ignition. Timing at 16 degrees initial with vac adv disconnected. Got the engine fired up a couple days ago and drove it around about 25 miles and tested it with the new LM-2 yesterday. The engine was warmed up to 160 degrees, choke off, cool day temps at 60 degrees. Idle rpm was about 850 and steady and mixture screws were about 1.5 turns out. Vacuum reading was around 16 inches, taken the day of the test drive, not while testing.
Did the free-air calibration as required and then hooked it up to the steel bung just aft of the header collector and saw an AFR reading that fluctuated from mostly 12.7 to 13.0 with an occasional dip to 12.6 at the low end and a 13.1 at the top end. Do you guys running cams with more duration than a stock 300 horse cam also see this wandering AFR reading? I then turned in the mixture screws about an 1/8 turn and noticed an every so slight change to 12.9 to 13.2. Tried yet another 1/8-turn in and the reader showed 13.1 to 13.6 but there was an obvious drop in rpm. No vacuum gauge or tach hooked up during testing and I didn't walk around to see what the actual rpm dropped to on the dash tach but it was easy to 'hear' it had dropped. Here's a pic of No 6 plug after the 25 mile run around town (idle on up through 2500 mostly).

And this is the same plug after 30 minues of idling yesterday during the Air/Fuel Ratio testing.


I did notice that when I manually pushed the throttle to get the rpm up higher, say maybe around 1500 or so, the AFR numbers rose up to high 14's and into the 15 range but I'm only guessing at this point since I didn't have my tachometer hooked up.
My questions are: do you guys with non-stock cams typicaly see the AFR numbers vary like this and secondly, what's the correlation between AF ratio and vacuum readings? I would have expected the rpm to rise at the same time the AF ratio was getting closer to the magical 14.7 number.
One other comment to make...before I made the head-change, I used to have this odd slight hesitation when taking off. Accelerator pump adjustment checked, float level checked, all okay. Even though I had the highest vacuum reading with the mixture screws at about 1 1/4 to 1 3/8 turns out (pre Edelbrock head installation), adding a quarter turn 'out' to each screw helped a great deal with about 80-90% of the hesitation gone. Well, with the new setup I have that hesitation back.
I'll do some more testing but suspect I might have to go back to the stock 65 jets since it only took 30 minutes of idling to color up those new plugs.
Thanks, Mike T.

Plasticman

Mike,

I don't even try to adjust the idle mixture with the LM-2.

I use the highest possible vacuum at the highest achieved idle RPM method to adjust idle mix (then back the idle speed back down - if needed, and adjust the idle mix again). Once my idle mix & speed is where I want it, I then use the LM2 to adjust the jets (or rods) as needed to gain the best cruise mixture for the gas I am using (I have found that ethanol needs a richer mixture than straight gas).

Note that if at anytime I start seeing lean surge (at any cruise RPM), I will richen up the mixture a jet (or rod) size until the lean surge goes away. Even then, long term testing may see a raising of engine temps, and force another jet/rod (richer) change.

Once cruise has been satisfied, I go onto full throttle mixture tests.

Yes, this all takes time, and can be done much quicker on a dyno, but I like to tinker.

As for your off line hesitation, think the quickness (ramp) of the accel. pump should be adjusted.

Plasticman

waltonb123

I'm not familiar with 02 scanners but have been trying to catch up on the technology. Why can't they be used to adjust carb at idle?
Bob

wmf62

they can, but the simplest way is with a vacuum gauge. an O2 sensor is better suited to tuning for economy and performance at higher rpms.
Bill

LastC3AZ

The wide bands are a great tool ... but remember, it's just a reference. Many times we find ourselves chasing a "number" and not listening to what the motor wants (needs) for best performance. Every set up is different as is every engine ..... checking the plugs periodically and just feeling how the engine is performing is the key ....

jim lockwood

Chicken and egg.

If you know what AFR your engine likes at idle, you can use a wideband O2 gauge to adjust to that setting.

However, to find out what your engine "likes", you first have to adjust for the best operation. Then you can make a note of that value for future reference.

Keep in mind, though, that the AFR for "best" idle will be a moving target: a function of temperature and atmospheric pressure. Maybe not so much that you will need to tinker with the idle screws, but still a moving target.

Bottom line: Adjust the idle mixture screws the old fashioned way... for the best idle.

Jim

Matt Gruber

so what AFR at 2000 rpm cruise?

wmf62

that is gonna depend on a lot of things: engine torque, OD, rearend ratio, etc. anything that lugs the engine will require a richer mixture.

i have mine set at 14.71
Bill

Vet65te

I appreciate all the input I'm getting, this new 'tool' is going to take a while to fully understand so let me throw out a few questions.
If the consensus is that the LM-2 reader isn't really needed when setting up the idle mixture I will keep doing it the old way by finding that right rpm with the highest vacuum reading. What other rpm points should I focus on for test purposes, understanding that the idle circuit/primary jets are in play on up through steady rate cruise which can be all the way up to 2500/3K? We're forecast to get wet weather for the next 5 days so it won't be till at least Sunday before I can take the car out for another test drive 'but' if I do find, with no mixture changes being made, that the plugs clean back up from their now slightly darkened state after all that idling in the garage 'and' the AFR seems to stabilize in the high 13 to high 14 AFR range while underway, then would that be a good indicator that my increase in the primary jets from 65 to 67 was not over-richening the fuel delivery? That is really only a 3 or 4% increase in fuel.
Thanks,
Mike T.

wmf62

stoichiometric efficiency is nominally 14.7 AFR but will be effected by all sorts of things... but, it is the target to shoot at; you might actually be able to go a bit higher...

LM-2 ASIDE, you will know when you get too lean for the conditionS if you experience a lean search or possibly a spark knock
Bill

waltonb123

I dont want to highjack the op's thread but...One of the things I'm trying to do is to get rid of the exhaust smell. The car seams to run pretty good except the exhaust will burn your eyes and makes your clothes smell. It doesn't smoke and had no fuel leaks, it is strictly exhaust. It has a little Lunati Bracket II hydraulic cam (218 duration@50, 457 lift and 110 sep) and a Holley 600 single feed with vacuum secondaries. I also have a PVC valve running from back of block to carb. So I thought I would try an 02 sensor to try to get a better tune on it bit if it doesn't work at idle why bother?
Bob

wmf62

it will work at idle and should tell you what your AFR is; but regardless you are limited to how lean you can go by turning the idle mixture screws and still have the engine idle.

i'm not familiar with the characteristics of that cam, but if it has a lot of overlap it is just gonna smell at idle...

try raising the idle by the idle screw and close up your idle mixture screws... but ya gotta watch out as you can least it so much that then you end up with an tip-in stumble when you accelerate from an idle.

my .02...
Bill

jim lockwood

Definitely possible with FI. I run my Rochester units at 15.2:1 - 15.5:1, IOW about 5% lean.

The same isn't possible with carburetors since mixture distribution isn't uniform to all cylinders. My personal S.W.A.G is no more lean than an indicated 14.7 with carbs..... but that's just a guess.

Jim

Vet65te

If most people don't use the AFR reader for idle, at what rpm will they check the AFR, 1500/2500 and then at WOT? Unless it's just the characteristics of the #097 solid cam, I'm curious why I saw the AFR climb to the high 14's once I raised the idle rpm up to a (perceived) 1500.

And, since turning the mixture screws does not change the ratio of fuel to air being provided at idle, but only the amount of the two, would it be safe to say that seeing an AFR like I did at idle that is in the high 12 to very low 13's would mean the primary jets need to be reduced in size?

Mike T.

wmf62

again, my .02... your AFR will climb at higher rpms, in neutral, because there is no load (just like manifold vacuum does...)

to the best of my knowledge, the idle circuit has no jets in it, just a needle valve & seat; primary jets have no effect on idle unless the throttle plates are opening.
Bill

65tripleblack

First: Adjust the idle mixture screws to obtain highest idle speed and vacuum. You can note the A/F ratio at this point, for reference as your cruise A/F ratio will be in the same vicinity. Road test the car, and if the accel pump is adjusted properly and you still get an off idle stumble, then open the idle mix screws in 1/8 turn increments until the stumble goes away. If the stumble remains after compensating with the mix screws, then the accel pump needs to be enriched.

Second: Check A/F ratio at steady highway cruise. A mild cam like the 097 should see 13.5 -14.0:1 cruise ratio. Assuming that your PV is correct, then this ratio is directly affected by the pri jets.

Last: Check WOT ratio. This should be between 12.5 and 13.0:1. This is directly affected by both pri and secondary jets, but since your pri jets are already optimized, you should change sec jets only in order to adjust WOT ratio.

LastC3AZ

Good basic old school tuning ..... I love it .....

Again, you can certainly check your AFR as a reference .... but the best is by pulling a plug to see whats going on ..... I've only gotten into the WB referencing due to CFI ECM tuning during the last year or so ..... and its sooooo easy to get caught up in the numbers ..... Your engine will let you know what it likes .....

as for fuel smell .... generally speaking, if you're stinkin up the garage with fuel, you're probably running way rich at idle .... especially when cold .... you can crank back the choke a bit or you the adjustment screw needs attention ....

very informative thread !!!

Matt Gruber

It is a GAUGE not gospel.
all checks are for reference.
It is a GREAT tuning aid!
a cruise 14 AFR should go to ~15.3 when changing from 67(.068") to 65(.065"). This is based on calculating the area of a circle. If the 670 takes a lot of pedal to react with 65's i'd use a high power valve for quick response. Try a 2 stage PV where stage 1 kicks in at 12" Holley part #125-212.

plaidside

All carburetor idle circuits have an orifice or a changeable jet.
All idle fuel goes first through the main jet and then through the idle orifice or jet, which usually is .025-.035". Then it picks up air from the idle air bleed which mixes with the fuel and then through an idle mixture restriction and then to the idle port in the throttle body. The idle ‘mixture’ screw is tapered at the end and moves in and out of this port which changes the idle mixture.
Joe

65tripleblack

Holleys/Holley clones don't use an idle mixture jet. They use an idle feed restriction passage which is usually sized according to the carburetor's CFM capability.

If one can obtain the engine's "best" idle with the idle mixture screws turned (approximately) 1 1/2 turns out (ccw) from the seat, then that tells you that the carburetor's IFR is sized appropriately for your engine. If the idle mix screws require much more than 1 1/2 turns ccw to obtain highest idle speed, then the IFR can be enlarged. If the mix screws require much less than 1 1/2 turns ccw from the seat for best idle, then the IFR may be reduced. This last scenario is easy to accomplish and easily reversible if wire is inserted in the IFR rather than sealing and re-drilling. There is no "problem" if the IFR is inappropriately sized. The sensitivity (emulsion delivery rate change/degree of rotation) of the idle mix screw adjustment is best @ 1 1/2 turns for most stable idle, and the further ccw they must be turned, the lower the sensitivity with the opposite being the case when best idle is obtained with the screws much less than 1 1/2 turns ccw from the seat.

Idle and "main" bleed (emulsion) jets are brass inserts in less expensive Holley/Holley clones. They are screwed-in jets in more expensive models. These can be fine tuned to provide better atomization by introducing more air into the air/fuel emulsion in both the idle and the main fuel wells. This emulsion exists in both wells BEFORE the mixture is discharged through either the idle/off idle port/slot or the main booster.

If your Holley/Holley clone has "four corner" idle mixture adjustment, then you might tweak the secondary idle air bleed jets as well as the primary idle air bleed jets.