A/F Ratio???

[61ragtop]

Ok so I have hooked up an autometer wideband to my car and have a question.

At idle I am right at about 14:1 whick I think is good?

At normal acceleration (like driving a daily not like we like to drive, lol) It drops to between 10:1 and 12:1, where should I start to lean this out, primary jets?

I have not had a chance to test out a WOT cause it is to hard to watch the gauge and road as i punch it . But plan to talk my wife into holdin on tight and going for a ride.

FYI It is a 750 speed demon Vac sec carb on a 383ci sbc. Here are my specs....

NGINE
-383 stroker
-Chevy 350 bored 30 over and stroked to 383ci
-Scat crank 3.75 Stroke Cast 9000 series
-Scat 5.7 forged I beam rods with 7/16 arp cap screws
-Forged Flat top Probe Sportsman Race Series Pistons
-Comp Cams Xtreme Energy™ XR282HR Duration-Intake 230deg Exhaust 236deg @ .050 / Lift Int .510 Exh .520 and 110 lobe separation
-Air Flow Research 195cc Heads 74cc chambers
-9.7:1 compression
-Headman Block Hugger Headers
-Holley 110 gph Fuel Pump
-Speed Demon 750cfm Carb Vacume secondaries
-Polished Air gap Intake
-Edelbrock aluminium water pump
-MSD Plug Wires
-All Bolts Are ARP
-Edelbrock double roller timing set
-Comp cams roller lifters
-Crane cams energizer roller 1.5 rockers
-Full Mandrel bent Flowmaster American Thunder exhaust With 50 series deltaflow mufflers
-700r4 tranny
-Edge Racing 9" 3200 rpm converter with lock up
-9 inch ford rear end
-Center Section With Motive 3.89 Gears And Auburn Pro series limited slip

And the engine dyno specs....

Intake center line 105 1/2
jets 76 front 85 rear
vac sec spring short yellow
timing 35 deg
RPM----HP---TQ---A/F--Vol Eff%

3900 - 328 - 442 - 13.3 - 97
4000 - 340 - 447 - 14.0 - 97
4100 - 356 - 456 - 14.2 - 97.3
4200 - 370 - 462 - 14.0 - 97.5
4300 - 381 - 465 - 13.9 - 97.5
4400 - 390 - 465 - 13.9 - 97.4
4500 - 397 - 463 - 13.8 - 97.4
4600 - 404 - 461 - 13.7 - 97.2
4700 - 410 - 458 - 13.8 - 97.1
4800 - 416 - 465 - 13.8 - 96.9
4900 - 422 - 452 - 13.8 - 96.7
5000 - 427 - 448 - 13.8 - 96.3
5100 - 430 - 443 - 13.8 - 95.8
5200 - 433 - 437 - 13.9 - 95.2
5300 - 435 - 431 - 13.9 - 94.9
5400 - 436 - 424 - 13.8 - 94.6
5500 - 437 - 418 - 13.9 - 94.3
5600 - 439 - 411 - 14.0 - 94.0
5700 - 440 - 406 - 13.9 - 93.5
5800 - 442 - 400 - 13.9 - 92.8
5900 - 442 - 393 - 14.0 - 92.1
6000 - 440 - 385 - 14.0 - 91.5

I have since upgraded to the New AFR Eliminator 195cc heads due to an issue I had and had them milled to 72cc chambers from 75cc to bump from 9.7 to 10.2:1 compression. They claim the head change with out the compression bump is worth a solid 60 HP and 60 TQ plus 10 HP+TQ with the compression bump. Added to my dyno #'s thats 510 HP and 530TQ it should be putting out now.

[67ragtp]

Its quite difficult to dial in cruise A/fs with a carb, if you start changing main jets you will effect wide open throttle A/f. I tried with an lm1 and data logged but it was tough, problem is the same circuits in the carb feed a wide range of loads. Your engine dyno A/fs look a bit on the lean side, becareful leaning it down more on the primary main jet, especially since you changed the heads. Wide open throttle ussualy see's the A/fs in the 12.7 to 13.2 range. Id throw more fuel at that motor, Id bet it would make more power.

Keep an eye on your header tubes, if they start glowing a bit of red your to lean, I had a smallblock that at idle was so sensitive to the four corner idle adjustments when adjustining it above 13.8 the tubes would just start to glow, pissed me off since it spotted the jet hot coating.

Rich

[61ragtop]

Thanks for the info I have been told the dyno A/F looks lean and my dyno guy told me it was cause on the dyno it was open headers and a couple other reasons I dont remember and that it would run richer in the car.

Thats why I decided to get the wideband gauge set up to confirm especially with the new compression and heads.

[61ragtop]

I might try to lean the primaries and just keep an eye on the A/F ratio gauge. or maybe the vac secondaries are opening too soon? or power valve too high of a rating?

Not sure where to start and at 1.52/L at the pump as close as I can get it to run at optimum ratio for cruise and light throttle the better!! WOT is a different story but 10:1 for cruise seems rich to me.

[sweetstang68347]

You need to start by setting your af at wot. Usually you don't want more than 8 jet diff between the pri and sec. The power valve on the pri side makes up the diff. With good aluminum heads like you have you want between 12.5 to 13.2 depending on the motor combo.

Once that is set you can adjust the idle mix. The fuel that flows through your idle circuits is present at all times and has the biggest impact on your part throttle af. You want it as close to 14.7 as you can get it. That being said, now completely disregard it. Due to the performance nature of our engines that is not usually the case. Especially with a carb. In reality you will probably wind up closer to 13.5 or maybe 13.7 would be my guess.

Having done that see how the drivability is at low speeds to cruise. You may have to richen it up if it has flat spots. A tip in stumble can be cured by changing out the squirters. More on that in a min.

Next I would work on the transition from part to full throttle. Remember wot af should be good because you already set it. The transition can be smoothed out by changing when the power valve opens. To lean means you need it to open sooner. To rich means delay it a little. Ideally when you punch the gas from a stop you want it to go to 12.7 to 13.2 and stay as flat as possible through all rpms.

If you have a lean spot when you punch it now is the time to adjust the squirters to the same as the wot af. To rich and you use unneeded fuel and loose some power. To lean it stumbles and that's no good.
Once you do all of that it should run pretty good and get you the best mileage power compromise.

I have the same carb and got it to run great doing that. It took a lot of jet swaps, squirters changes, etc. If your serious buy some gaskets, a jet kit, maybe a squirter kit, and a data logger. I used the lm1 and recorded passes then read them on a pc. Takes notes and make one change at time. Don't forget to keep the jet spread equal and take your time playing with the idle adjustment. I will upload some graphs when I get back on Sunday.
Have fun! I love this demon carbs and you can get them really dialed in. My model didn't come with air bleeds but that would be even better. Good luck.

[Ruslow]

to really check a/f you need a wide band style sensor.you can lean and richen the cruise and low speed circuits by changinging the air bleeds next to the ventris.I know the oriiginal BG had air bleeds that were removable,also you can drill out the power valve jets[for wot a/f].in the holley they are at 9 and 3,but be careful when drilling you can remove metal but not replace it.cruise should be 14.7 or there abouts and power should b e 12.8-13 depending on c/r and timing curve.Stan

[61ragtop]

Well I dropped the primaries to 74's and now the light throttle has leaned out to 11-12 and the WOT still seems good to at 12. I might drop them down to 73's and the secondaries down one to 84's to get me around 12-13 light throttle and closer to 12-13 WOT. cruise is around 13 and idle still 13-14. All drivability seems good too no tip in issues or flat spots.

[Tweaked Zed]

I have never had a holley before, and ended up with a 750 Ultra double pumper. Actually, I've been EFI for so long I figured I wanted to go full old school in the tuning too, so just read plugs to dial it in. Learnt alot, and still have a couple things to play with before checking with the wideband.

Ended up with 68 primary 80 secondary mains, 8.5 power valve pri, 6.5 pv sec, 37 squirter pri, 28 squirter sec, pink pump cam, and a 50cc primary accel pump.

This is to show how much you can change to dial a carb in, or maybe how much you have to. I'm no means an expert, but it IS trail and error with every engine. I get 21mpg with 412ci at 1800rpm and 75mph. So I think I'm close to optimum for my setup. I'm still going to try to drop to 66 in the front with a green cam to see what happens. I am getting to the point of wanting to check with a wideband though!