[Skip Fix]

Well I swapped a 73 "plain 750 cfm #262 for my SD replacement 800 cfm on my son's 81 400 214/224 @0.050. If you rmember some of the posts on AFR for it it was shocking to see how much the primaries affect WOT AFR.

This carb I started out with 73/43 and a new 019 piston sprimg. Took alot more screws out on the idle mixture than the other carb to get best vacuum (19.5)and a AFR 14.0.AND crusing it is WAY lean 15.8-16.4! With the same DB secondary rods 13.9 WOT.Idle bleed on top is seated, tried it 1-2 screws out and made negligible AFR changes.

Going to pul the top off and go with some 74/42 and a littl estiffer 305 spring to see how that does, as stays lean even with a little tip in that should bump primary piston spring up. It doesn;t feel alot different than the other carb seat of the pants.

Anyone lese using an AFR and what results have you seen with the "classic" Pontiac jetting?

[Junkyard Dog]

What should cruising AFR be?

[Skip Fix]

14.7 is "ideal" for emissions. Most feel low 13s high 12s for power.

[Tom Vaught]

Not exactly on the same thread but Joe Zajac noticed the same deal
in MPH differences with His Carbs and Larry Kauffman's carbs (both 750 comp series Carters) with only the mixture settings different (the
turns out was the only difference).

Asked me about it and I told him why!

Most people do not realize that the idle circuit still supplys some fuel at wot. Good job, Skip!

Like that wide band, huh?

Tom V.

[Kenth]

Did you try the original jetting for this carb, 71/41, CR, #7029529 PP spring, 3/8 turn preload on airvalve spring?
The main circuit air supply in the air horn is somewhat big, .083", usually Pontiac uses .070" here, you might end up with #73 jets.


Carb # 7043262
jet 71
prim.rod 41
idle restr .047
idle tube .038
upper idle air .055
lower idle air .070
idle air bypass .110
idle discharge .110
main air horn .083
main air body .070
main air nozzle .047
sec rod CR .0547
sec pull out .063
sec air tube .032
sec air valve/APT 1.38" / 1/2

[Skip Fix]

Ok after working in 94 degrees on a black car that the engine didn't cool down after 4 hours I rejetted to 74s, put my 42 rods in , but they actually measure around 40.5 on my calipers. put a 305 spring in. So probably a real 42 rod might be the perfect deal.

Stays 13.4-13.8 cruising WOT drops to low 13s.

This carb has one less ported fitting than my SD ,it had one below the fast idle solenoid, I assume the one driver's side that opens into the little chamber in front of the promaries that the gasket seals of is ported?

For those of you out there interested in AFRs the Mustang Dyno comes with an LM1 as part of the package.

[Skip Fix]

Tom can you give me a formula to convert 0-14.7 psi to inches vacuum?

I also sent you an e mail about the 850.

[Ol Goat]

I bought the LM-1 Digital A/F Meter from Innovate Motorsports. What a great device and excellent software! Took every bit of the guess work out of tuning the Q-jet and allowed me to get to the exact right settings for my engine. I targeted 14.7 when holding an even cruise speed of 60mph (3000 rpm) which is basically just primary jets (because the larger diameter of all primary rods is identical). After several test drives I got to .070 jets. Then I did acceleration runs with the secondary air valve blocked closed and the primaries allowed to open only to where the scondary butterflies would start to open mechanically. (I tried doing WOT with the secondary air valve held closed and letting the secondary butterflies open but that leaned it out). I changed primary rods until I got 12.9 A/F. These were .040 rods. Next I adjusted the idle mixtures for 14.7 and retested both 3000 rpm cruise and full primary. Both were unaffected. I then tried different power springs until I got the primary rods to lift at the vacuum level that I wanted. Next was to try different secondary rods until the WOT mixture with both primaries and secondaries wide open was the same 12.9 as with primaries only. This was achieved with CE rods. The goal wasn't to dump more gas in, but give it lots more 12.9 mixture to burn. The final step was adjusting the secondary air valve spring to minimze the lean-out when the seocnadry air baffle opened. It was SO easy to see this on the graph. Even when seat of the pants said it was good I could still see the lean condition and kept adjusting it out. My car now runs the best it ever has and is so much stronger on the 1/4 mile. As an example, this carb tuning was the only mechanical difference I made and comparing time slips with runs that had similar temps and humidities from last year the car was easily 4/10 quicker and 3mph faster. I would say that chances of my settings be right for anyone else's car is about zero because so many component selections impact the mixture (cam, intake, timing, advance, air filter, etc.) so I would never say "use these settings". But I highly recommend the LM-1. It requires a lot of play time to dial it all in but what better excuse to drive the car?

[Skip Fix]

Ol Goat I think you have the primary rods dimensions slightly backwards, they all have the same narrow tip (0.026)which affects WOT as the spring kicks it up out of the jet. Cruising with full vacuum pulls the rod taper down so the thickest part dimension is in the jet, this is typically the number on the rods.

As Kenth as brought up before there are different tapers comapring a plain 44 to a 44B, I even had some other odd ball letters early rods that have even richer P and a dual taper like later rods C set. Plain rods are a little richer taper than the B rods.

I think this combination probably could be tweaked slight leaner, I'll probably go with a smaller primary jet so I can have more leeway with the secondary rods for WOT tuning since it's already pretty lean with the DBs. This carb has the secondary "slot" flaps so I think it may have different AFR blip as some air is moving but no flaps open as the secondary throttle blades are opened by the linkage.

What did you use for a rpm input? Their inductive clamp is worthless.

[Ol Goat]

Yep Skip Fix, you are right. I went back and looked at all the notes I took and I did do the primary WOT first to get the right primary jets for the 12.9 A/F because the primary rods made no difference at WOT (all the same tip diameter as you correctly said when they are up out of the jets). Then I tried different rods to get the 14.7 A/F at cruise because all the rod diameters are different when they are down in the jets. I have the sizes I ended up with shown correctly, just stated the procedure backwards.

I used their inductive pickup. I didn't get it to work right until I also carried along my laptop so I could view rpm readings and see if they matched what my tach said. Apparently noise is an issue and the rpm readings could be way off. I had to move the pickup wires around and tried different paths until I could find a route that had a clean enough signal to read correctly. Once I verified on the laptop that the LM-1 rpm's matched my tach I was able to proceed with the testing. A bit of a pain but it worked. Doing either of their calibration techniques did not help. It wasn't calibration, it was noise.

[Tom Vaught]

Skip, Here are some conversions for you:

14.696 psi = 29.92 " Hg or 1 atmosphere

1" Hg = .49116 psi

1 psi = 2.036" Hg

Hope that helps.

I found my chart.

Tom V.

[Skip Fix]

I don't know why my timingn light can get a good one! Too much moise. we tired it on the dyno and rpm was all over there also.They recommend hard wiring from tach plug, I guess I'll have to do that. Their MAP reads in PSI or BAR I think. I think a 3BAR limit.

[Junkyard Dog]

Wow, OlGoat, you are right, your settings would not be the same for any other car, so I will just drop my car off so you can do the same procedure to mine (and then my settings will be for my car)!

[Tom Vaught]

One BAR, Skip is 14.5 psi.

Everyone assumes that a BAR means the same thing as ONE ATMOSPHERE (which it is not). 14.696 psi is one Atmosphere.

Tom V.

[Skip Fix]

It reads 0-14.7 as I have the default, has a setting for boost also, I assumed wrong then I looked on the input screen. How do I convert 8si to vacuum"?

[Cliff Ruggles]

Skip, there are other variables that will effect AFR. Float level, fuel pressure and most important main airbleed sizes. If the carb is equipped with APT or an adjustable main airbleed system, you can usually dial them in exactly for the application.

Even more important that tuning exactly for the AF meter would be tuning for best throttle response and fuel economy. Lean settings do NOT mean less fuel consumption. Matter of fact, many combinations will respond better and use less fuel with richer settings. It depends on the application. Richer setting often end up with a lower throttle angle and the power piston may spend more time in the down position to get the work done. This will almost always result in less fuel consumption if the vehicle is driven over the same distance, especially for stop/start driving.

Keep in mind that the laws of physics will require a specific amount of BTU's to get a certain amount of work accomplished.

Many years ago when I was still doing a lot of fuel economy testing I used to drive exactly 600 miles back and forth from Virginia to Ohio when I was stationed in Yorktown. I used three different GM vehicles, a 1970 GMC 3/4 ton truck, 1979 Blazer and 1967 Impala SS. All used q-jets and had very well prepared engines. I would often install smaller metering rods or lower the APT in attempts to get better fuel milage for the run. In not one single instance did a setting in the "lean" range make any improvements. This was in part due to to having to go through a portion of the Blue Ridge mountains and more throttle needed to be applied climbing long grades.

For the distributor "guru's", running a LOT of spark advance via the vacuum advance didn't help either. With all of the applications mentioned I ended up between about 42-46 degrees BTDC at cruise using combinations of mechanical and vacuum advance (ported or manifold source made no difference either).

We're a bit smarter than those days, but we still see the same results when fuel economy testing is performed.....Cliff

[fiedlerh]

It reads 0-14.7 as I have the default, has a setting for boost also, I assumed wrong then I looked on the input screen. How do I convert 8si to vacuum"?

If it's a 'gauge reference' gauge (reads zero wtih nothing hooked up and 14.7 with full vacuum) then 8 psi = 16.29 inHg.

If it's an absolute reference gage (reads atmospheric pressure with nothing hooked up and zero at full vacuum) then 8 psi = 13.64 inHg.

[Kenth]

Cliff, no wonder fuel milage woldn´t get better with smaller rods.
You must have got a lot smarter than those days :-)

BTW, spent a couple of days tuning a Buick #7042240 with 1968 Pontiac top and throttle plate, yes i plugged the idle air bleeds in the body, when i tried a #72 jet i had to go to a #42 rod and still somewhat lean.
That puzzled me, since i was quite pleased with #73 jet and #46 rod.
Later i discovered my new set Delco #73 jets actually was #76 size.

Cliff, i didn´t think this was possible, is this a trick, or can´t Delco be trusted when it comes to jet stamped sizes.
I always thought that the stamped # reflected the actual drill size?
Guess taking measurements on parts used isn´t such a bad idea?

Anyway, now i have a couple of days and a tankfull to evaluate a correctly sized #73 jet. This is the fun part :-)

[Skip Fix]

14.7=0" on the scale on the meter.

Yes I have some 41 rods that are bigger than these 42s but no matched pairs Kenth. I was suprised how far off these were, most of my others are pretty close.

[Cliff Ruggles]

Kenth, I've learned to measure everything. Many factory metering rods are .001-.002" smaller than the number stamped on them. Jets can be all over the place, some are worn out from the rods sliding in them for many years.

The "smaller" rods was a missprint, should have read "leaner" rods. I've never found in one single instance once we got the carb running right, or as lean as practical to get the best throttle response and light throttle power, that going any leaner helped anywhere. Actually, going a bit richer will typically result in better overall vehicle performance and much lower throttle opening(s) to get the work done, hence better overal fuel economy. I still remember on one of those 600 mile test runs that I went to a metering rod that I knew was .001" smaller than I should have. The engine ran OK, just a tad "sluggish", but seemed good enough at part throttle to make the trip. I was driving the GMC 3/4 ton truck at the time. It typically would get about 14.5-15.0 mpg. The leaner rods netted the first ever tank that got just over 15 mpg. It was on the second half of the run AFTER getting through the mountains. It actually used a bit more fuel for the first part of the trip, about 13.7 mpg. Despite the very slight increase in fuel economy for the one tank I had to switch back to the richer rods. The carb had a lean "tip in" that was quite annoying for "normal" driving and it was just just too darned cold blooded until the engine reached operating temperature.

The 240 carb will like more jet and the rod staggered a few numbers smaller than is typical for a similiar Pontiac q-jet. I've had a few of those carbs as big as .074" jet with a .041 rod to get them running right.....Cliff