V8Astro Captain

iTrader: (1)

I have 73/80 jets in my 750 Demon now. My cruise AFR is ~12.5:1 and my WOT AFR is ~14:1. I've tuned the carb according to the sticky at the top, which is how I arrived at a 7.5 PV and 73 primary jets. I know I need more fuel up top but do I jet up the secondary or is there something I'm missing? What about the air bleeds? I'm not so sure what these are supposed to do.

Sorry for my newbness...this is my first real carb tuning experience...EFI is so much easier

sofakingdom

iTrader: (1)

Sounds like you need more fuel in the secondaries....

You need to get the AFR down to about 12.8 or so at WOT. That means you need to change it by 14 - 12.8, or 1.2. So, you need that change divided by the current value, or about 1.2/14, or about 8-9%, more fuel. Since a little over half your fuel comes from the secondaries at WOT, and you don't want to mess up your satisfactory primaries, that means you need to increase your secondary jets by somewhat less than twice that, or, somewhere around 13-14% more fuel through those. That means you need about 6% more jet diameter. So, look at your secondary jets, and whatever they are, multiply them by 1.06, and put that in there, and see where that takes you. I.e. if they're 72s now, you want 1.06 * 72 which is 76.5, try 76 or 77 and see what happens. Should get you close.

There is no "largest acceptable" jet change. However, a change becomes UNacceptable, when it goes too far.

IOW, just give the engine what it wants, and don't worry about the size of the change.

xpndbl3

iTrader: (5)

I'd pop in 88 jets in the secondaries and see where that gets you. Also the primaries "could" be leaned out as well by 2-4 sizes I'd imagine to get the cruise AFR into the upper 13s. But right now until it's dialed in more this isn't a choice. A datalog of rpm vs A/F ratio would be awesome if you have a LM-1 wideband. First jump up on the secondary size, I've done 74/90 before on a 306 mustang and that was what the motor wanted, but the standard deviation is 10 jets typically.

sofakingdom

iTrader: (1)

1 jet size is imperceptible
2 sizes is a small step (fine adjustment)
3 sizes is a "standard" step
4 sizes is a large step (coarse adjustment)
5 sizes is a HUGE step
6 sizes is GIGANTIC, enough to make up for taking out the PV and replacing it with a plug
10 sizes would be .... unthinkable, in most situations

To put it into perspective, going from say 70 to say 80 jets, is a 31% change in jet area. That would be enough to take the AFR from 17:1 (super lean emissions economy cruise mode) all the way to 13:1 (WOT race track mode), in one single step, if it was the only venturi feeding the motor.

lukn4trbl

Is this for the Impala motor in your signature?

If so, based on your components and power valve reading, I'm going to guess
the engine is reading about 10 in./hg. of vacuum at idle?

With a 12.5:1 cruise fuel ratio and a 73 primary jet, you will want to delay the fuel curve of the primary circuit. With that amount of vacuum, and the cam specs for the HR cam you list, it's highly likely that you need a bigger primary idle air bleed (or it is partially clogged).

With a bigger primary circuit bleed (on the primary side), the fuel curve will
go fatter later. By doing this, when the engine goes wide open
the jetting and power valve stay in the window for best power.

12.8 is about right for an efficient motor, but you may need to go fatter
depending on your combo. The dyno, or trap speed will be your best measure.

I don't agree with an 88 jet on a 350 cube motor and 750 CFM of carb. With
that much of a spread, there will be poor fuel distribution to the rear cylinders feeding from the rear barrels. 82 -84 is more than enough.

If you're needing more jet, that means the air bleeds are dirty, or incorrect
size. Air bleeds and jets are like brother and sister. They are tuned together
to flow properly.

Think that:
The jet is the "water valve", and the bleed (along with the venturi size) sets the
fuel curve. The smaller the orifice, the sooner it will respond to pressure change.

Just for an example, a 500 HP , 355 CID engine with a 700 CFM carb requires
about 86-88 jetting per barrel.

That said, there is either something very out of tune with the primary side
(fuel pressure, float level, faulty power valve), or the high speed air bleeds
on the secondary side are far too big.

If I were you, I'd start with a good basic tune up of the float levels,
check the power valve function (with a vacuum pump), and set the idle
mix. No more than 8 PSI of fuel pressure at the carb inlet.

Fuel filter could be causing a lean condition so check that as well.

You'll need to know what the fuel pressure reads when you are receiving
a lean reading while going wide open.

It is very odd that your cruise ratio is fat, and then leans out so rapidly
with that jetting combo.

Work on getting the idle and cruise ratio into spec, and then look to the
secondary side.