65tripleblack

That was then and this is now. That number is no longer valid with E10, and is misleading to anyone reading this who is trying to dial in a Rochester FI unit using a multiplier with Lambda.

Of course not. As you already found out, the stoich constant that must be used as a multiplier for Lambda is different for gasoline versus E10. In college, I learned that number to be 14.7:1 for 100% gasoline, which means that every O2 molecule is used up in complete combustion with 100% efficiency (zero CO out the tailpipe). Since E10 contains more free O2 molecules, then the stoich number for E10 will be a lower ratio of fuel to air.

I'm referring to this, where the answer lies in the * "note" in post #1.:

https://www.corvetteforum.com/forums...xperiment.html

jim lockwood

And your point would be .... what?

If you use Innovate AFR measuring equipment, you can tune your Rochester FI unit for an indicated air/fuel ratio of 15.5 and be right on the money, regardless of the actual fuel you use.

65tripleblack

The point is, that depending on the stoich multiplier you use, the calculated A/F ratio will be different, which is misleading.

Stoich for gas is 14.7 AFR. Stoich for Ethanol is ~9.00AFR. That means that Stoich for E10 would be (14.7 x 0.90) + (9.00 x 0.10) = 14.13AFR.

Assuming your fuel injected engine runs 4% lean at sea level, then the true A/F ratio using E10 is:

1.04 x 14.13 = 14.69:1

when using race gas, or pure gasoline that ratio would be:

1.04 x 14.7 = 15.29:1

So although you can make your innovator widget come up with any number you'd like to see and still achieve a perfect state of tune, the number you originally gave for ideal A/F ratios with Rochester is misleading because it is based on Stoichiometric for 100% gasoline.

AZDoug

Couple notes:

Dyno tuning will usually be too lean for the street, you may need to bump your jets a couple numbers up from what the dyno says is best.

You can't make a power valve determination with out a vacuum gauge, with someone reading it, while you stomp the throttle.

You can get more needed fuel during a bog situation by upping the squinter size, (not the accel pump). It seems like you tried this already. Note I only have exp on double pumper Holleys, I think they come out of the box with the biggest accel pump cam installed, to my recollection. Your 4053 may be different.

And lastly, this is why i LOVE my EFI systems. No muss, no fuss, an hour on the chassis dyno and your motor is dialed right it, both AFR and timing. Need to change something? Plug in your lap top.

Doug

PAmotorman

the idle transfer slot is to take up the time it takes for the fuel from the squirter to get to the bottom of the carb and if the slot is open too far to get the idle speed you will have a bog as the passage is low or empty of fuel. set this opening at .020 and fix the idle speed other ways.

tbarb

Now that we have imposed our great wisdom on the OP, I hope he did not go awal.

I think he should think about the above statement and go back to baseline settings including primary throttle blade position and see if that helps the lean hole toward the end of the transfer slot.

jim lockwood

All of this has already been noted in the three year old thread you referenced previously.

In any event, these statements....

..... are still true. If you don't like the 14.7 stoich multiplier which produced the numbers, you are free to use a 14.1 multiplier and come up with numbers which please you more.

AZDoug

What intake manifold are you using, and what headers?

I ran into a weird part throttle reversion of some sort with the Vac JR manifold, on two completely different motors (327 and 427 SB), that required a lot more fuel to be programmed in around 2300 RPM, than one would normally expect, when the AFR was ideal, above and below that RPM range.

Doug

XCSP700

What was the indicated mixture when you say it was rich or lean?

And did you try running your wideband on both sides of the motor?

The fuel distribution on a carb manifold (Especially a dual plane) can vary significantly from cylinder to cylinder, I've had plenty of cars I've had to set up slightly rich at cruise because of this, its possible that you're leaned out on a couple of cylinders. Remember the gauge is just giving you an average of the 4 cylinders on that side.

Very rarely have I seen carbed cars that would cruise well in the high 14s most need to run around 13.5-14.2.

-Rich

65tripleblack

Correct. Look at post #8.

MikeM

So Clem and Smokey were "deleted" early on? Now you have the rest of the story.

PAmotorman

I think you guys are spitting into the wind with all this talk about air fuel ratio as a carb is a crude device the can not compensate for all the variables out there. if you are really concerned convert to electronic FI.

MikeM

As seen by an oxygen sensor, I'd think the Rochester FI could run leaner than a carb due to better, more consistent fuel distribution.

Faslane

I would say it is a pump/cam issue, orange or green? What does the AFR say when hesitates? seems like a lean issue. you can get the hesitation out without touching the air bleeds, but if you want a nice AFR throughout you will have to do the adjustable air bleed, I know it is a big hassle but worth it in the long run. If you tried smaller squirters but did you try larger?
Lane

aaronz28

hey guys - damn - for some reason, i did not get any notice that anyone replied to my OP and i've been wrenching away on 3 other cars as well - (had to replace the wiper motor for the 3rd time on my 79 Coupe De Ville- the remans just do not hold up)
and started the front end tear down on my 70 Monte... anyhow

OK back to the OP

the motor is a 421 small block, dart iron eagle 180 heads, Z28/LT1 intake, 4 speed, 3.23 gears - stock exhaust with log type manifolds.... the car is setup to look 100% OE - - - and i'm running the good ole #178 - LT1 camshaft...

i spent some "time" with the advance curves and after considering the long seat duration with lower compression 9.5:1 and not a ton of gear, I decided to lock out the advance - i also went back to 70 jets up front, a slightly larger power valve (8.5, instead of 6.5 thinking that with 15"HG i was seeing at idle, it might need a little higher number)

i did put a 50cc pump on, which might have been a waste - and now have a 31 squirter with the pink pump cam on screw #2 - the stumble is all but gone... the car feels great, although the visible black smoke under WOT tells me i still need to get my AF meter back on and see whats going on.

normally i wouldn't have done so many changes at once, but i know the timing was wacky - and she really likes the full locked out mechanical advance - and i have vacuum advance pulling another 14-15 degrees.

its literally snowing right now, so the car is probably done for the year - although with temps in the 40s - if I get sunny, dry days I may take it out again.... either way - the car is now drivable and enjoyable - even if i have tuning more work to do...

the 4053 is a vac sec Holley - the original carb for a 68-9 Z28, and L78 396 Camaros - again, i want the stock look - although i could conceivably sneak something like a HP Holley or AED/CarbShop hot rodded Holley that has adjustable air bleeds - but that wouldn't happen until the spring!

I'll save this post and review/read the recommended books this winter so that next spring, i can get it dialed in...

Thanks
Aaron

Barry's70LT1

The above statement is so true. I've had a 750 DP on mine for over 40 years now. It took some "tuning" to get it right for the street.
Rich or lean at cruise always came down to the Idle Feed Restriction", which feeds fuel to the idle and transfer slots. Since the 750 DP was designed for rich running, the absolutely best way to tune for street/cruise driving was to restrict the idle feed circuit.

I have different diameter wires that I insert into the feed circuit and this will tune your off idle/cruise mixture. A jet change has little effect on cruising mixture.

Many years ago a friend with a '66 327 (Holley 3310) was rich at cruise. I inserted some wires in the idle circuit and his exhaust went from black at cruise to light grey. Nothing else was changed.

tbarb

That's why it's important to understand the circuits on a carburetor and only make one change at a time. You can always go back if the change does not go the direction you want.

On my 67 holley 3810 I removed the pressed in idle feed restrictions and tapped for 6-32 brass set screws in the factory 4743 metering block. I also removed the pressed in idle air bleeds and tapped for 8-32 brass set screws. The idle circuit is completely tuneable and you would never know I was there. All brass set screws available from McMaster Carr.

Nice learning experience but be careful.

DansYellow66

That's the same procedure the article in Hemmings described performing.

aaronz28

ok so removing the plugs and drilling/tapping etc is not beyond my ability - but i'd be curious to know an order of approach... i.e. go for ideal WOT a/f first, then part throttle a/f then idle?

or the other way around?

i have dart heads on the car which according to everything i've read, like 32-35 degrees wot - i've always lived with 37-40 wot with the old heads/SHP cams... the stumble goes away with more timing - but then it looses top end....

and again - now its 35 degrees so i'm not sure what i can do until spring.

DansYellow66

I don't think the order in which you proceed matters too much. There are 5 or 6 different steps in a Holley's operation from idle to wide open throttle with all 4 throats open. I think the big problem is figuring out which steps in your carbs operation are not optimized or at least within reasonable calibration. Then you can concentrate on those. Easier said than done without a dyno, computer control or an exhaust analyzer.