krochus

iTrader: (1)

Alright I've read and read and read on tuning these things so much so that my brain is overloaded with useless details that aren't helping me with the big picture of getting this carb tuned to where it needs to be.

This is me asking you guys for your help on this, please let's take this step by step throughout the RPM throttle range one item at a time in a methodical if not dumbed down manner that may be of use to others.

My specs

Aluminum 5.3
241 heads
Performer rpm intake
2.5" exhaust, mid length headers
2900lb car 3.27 gears
Th400 2700rpm stall
Comp 219-233 @109lsa cam
AEM UEGO a/f gauge
Running temp 180 degrees

Carb 670 CFM vac secondary Holley street avenger. Stock configuration 65 primary 71 secondary 6.5 powervalve, transfer slot squared, idle AF 13-13.5 in gear or out.

Idle is OK so the first order of business is to deal with low RPM very light throttle RICH condition. For example cruising on flat and level ground @ 30mph or so the A/F ratio is a pig rich 11.5-11.8

I suspect this discrepancy is a prime factor in the car being an absolute gas hog well beyond the bad mileage you'd expect from a setup like mine.

Pop N Wood

iTrader: (1)

Is the carb new, ever had it apart? A leaking power valve or clogged air bleed can make it go rich.

What is your idle and cruise vac reading?

Are you running a MAP sensor and what is your timing curve?

krochus

iTrader: (1)

The carb for all intents and purposes is new and unmolested.

Timing is

21 at 1000 to 2600 by 1700 then 30 on out

There's 10 degrees of vac advance coming on right where my part throttle cruise vac is @ roughly 7 psia

krochus

iTrader: (1)

Possibly related I'm noticing that if I go from light low speed cruise to light acceleration the a/f goes from rich to full lean almost dropping off the scale at 17

krochus

iTrader: (1)

And here is that timing curve as it currently stands

ZONES89RS

iTrader: (25)

More aggressive timing will help as is. Put it at 20 at 0 rpm and ramp to 28 at 1000 and up to 34 all in at 2000 and lock it out from there and see how things feel. The torque should improve the sluggishness unless the cam is simply fighting that tight converter.

forcd ind

iTrader: (7)

what RPM do you cruise at-as said bring in more advance at cruise
I am not fam. with the avenger carb, prob. not replaceable air bleeds
you could jet it down a little (pri. only) if you are past the trans. circuit, see how it works
if you can get the AFR's into the 14's, you should see a big jump in mileage

krochus

iTrader: (1)

My optimum cruise is around 2700 rpm at 55 where the AF ratio rapidly becomes ideal as the main jets come into play above 2500 rpm. Below that say 50mph and 2500rpm < it's pig rich. I was running more aggressive timing @32 on out above 1600 but it had a tremendous tip in stumble set this way. The stumble isn't gone but it's a bit more under control with the timing shown above

As it is at high vacuum part throttle cruising I have nearly 40 degrees total. Are y'all sure adding MORE is the way to go?

Pop N Wood

iTrader: (1)

7 psia means your around 15" of vac so the power valve should be a good starting point.

I would start by putting a small piece of wire in the two idle feed restrictors. I used a piece of stainless I got off a wire brush, bent it into a V and pushed it down into the orifice. Here is a pic of what I mean, don't worry about the numbers in the pic, you will need to trial and error it.



Here is an OK car craft article

http://www.carcraft.com/techarticles..._tuning_guide/

BigEd_72455

iTrader: (7)

Good question there..... Trash inside the bowl can affect about anything.

krochus

iTrader: (1)

Anything is possible but the carb appears to be squeaky clean in and out. I have tried a couple jet changes previously but I strove to keep things clean. It's now in the stock configuration jet/PV wize. I'm running dual filtration in the form of the stock foxbody style fuel filter in back and a plastic "see thu" up by the carb.

When I get home I'll try restricting the IFR's as pop-n-wood suggests and report any changes

BigEd_72455

iTrader: (7)

Best of luck with everything. I am hoping I don't have too many changes to make to the carb I got for this 5.3.

krochus

iTrader: (1)

I put two pieces of .017' wire bent into a small 90 into the IFR ports.

Now even though I'm 5 turns out on the mixture screws the idle mixture is full lean with NO ability to idle on its own.

I'm gonna go ahead and drive just to see what the AFR is now

krochus

iTrader: (1)

Waiting on a train.... It appears as though the .017" wire is simply too big as the part throttle cruise at low speed AF is now super lean as well. I'm gonna pick up a new metering blk gasket and try some .010 wire

In spite of being so lean throttle response is noticeably dare I say vastly improved

krochus

iTrader: (1)

.010 wire is looking like a real winner. 35-40ish mph ratios are mid 13's to low 14's now. Losing the extra fuel on bottom has leaned things up above 2500rpm somewhat with things running 15ish at 50-55

About the only fly in the ointment down low now is the idle AF ratio seems perpetually lean in the upper 15's even with the mixture screws out FIVE turns.

ETA with the surplus fuel on bottom gone the primary jetting in now a bit too lean especially as the temp drops so a jet change is in order.

Even so those two tiny pieces of wire have TRANSFORMED the driving experience. Now at part throttle around town and through the country side it drives like a 2800lb car with nearly 400hp should!!!!

89gmcs15

iTrader: (7)

At idle you could have a bad o2 reading at the sensor from a large cam with lots of overlap or even a exhaust leak? 5 turns out is huge. Does it run and sound ok at idle with the lean readings?

krochus

iTrader: (1)

It idles. "Ok" but it's quite evident that I'm now lean from being cold natured, hard to start and at very very light throttle low low speed (Parking lot speed) I now have a lean surge/stumble condition that wasn't present before.


ETA is is idling very lean. And will not idle on its own in gear without added throttle input.

Any suggestions?

89gmcs15

iTrader: (7)

Does that carb have adjustable air bleeds? Your bleeds might be causing a early transition from idle to primary. When I went extremely far off from normal with my bleeds I experienced this...

krochus

iTrader: (1)

In a word, NO Or at least not that can be made smaller. Bigger is however an option

I just came in from doing some idle speed/mixture tweaking. By in large the carb was unresponsive.

The more I read on this carb the more I'm believing that tuning it to match my application is an exercise in futility.

89gmcs15

iTrader: (7)

Any ability to drill and tap the bleeds or other ports? Im not saying that is the issue but I know when I had some issues the bleed tuning helped out too...