General AFR Guidance (Holley Tuning)

[mopowers]

From my experience with mine, I still expect you'll find the IFRs too small. You can test your idle circuit by temporarily putting small wires in the idle air bleeds, which will richen your transition circuit. ***Make sure you secure the wire so it doesn't get sucked into the carb*** If it runs better, then you know you need a richer idle circuit.

If it's just off idle that you have your problem, a new power valve probably won't help, nor will a different pump cam (this is only from my experience: your results may vary).

I do think the IFR needs to be larger in order to richen the transition circuit a bit. That would mean the idle mixture screws would need to be tightened as well, right? I'm already only at 3/4 turn out from bottomed out on all four idle mixture screws.

 

[Mattax]

You can test your idle circuit by temporarily putting small wires in the idle air bleeds, which will richen your transition circuit. ***Make sure you secure the wire so it doesn't get sucked into the carb*** If it runs better, then you know you need a richer idle circuit.

Wires are a good way test.

If it's just off idle that you have your problem, a new power valve probably won't help, nor will a different pump cam (this is only from my experience: your results may vary).

Your experience is backed by physics. See post here for link to video. The main jets are usually much larger than the IFRs so have little effect on flow. This assumes a submerged IFR. A good modification to start with if its high. See http://racingfuelsystems.myfunforum.org/viewtopic.php?f=25&t=1338

I do think the IFR needs to be larger in order to richen the transition circuit a bit. That would mean the idle mixture screws would need to be tightened as well, right? I'm already only at 3/4 turn out from bottomed out on all four idle mixture screws.

Not enough info yet to decide whether to change the IFR or the IAB. Yes, with 4 corner idle the fine trim screws might only be 3/4 out.
First you should make sure (idle circuit) needs to be richer by doing the super slow opening test I described earlier. If it doesn't need it, then its just extra fuel washing things down..
If it does need to be richer in some of the range, you will then have to experiment with IAB and IFR combinations. Does it need to be just richer or also carry higher into the rpm range? Changing the combo will effect both richness and when it stops flowing.

Keep noting the turns out of the idle mix screws. It's another clue as to what is going on with each change.

 

[mopowers]

Wires are a good way test.

If it's just off idle that you have your problem, a new power valve probably won't help, nor will a different pump cam (this is only from my experience: your results may vary).
Your experience is backed by physics. See post here for link to video. The main jets are usually much larger than the IFRs so have little effect on flow. This assumes a submerged IFR. A good modification to start with if its high. See http://racingfuelsystems.myfunforum.org/viewtopic.php?f=25&t=1338



Not enough info yet to decide whether to change the IFR or the IAB. Yes, with 4 corner idle the fine trim screws might only be 3/4 out.
First you should make sure (idle circuit) needs to be richer by doing the super slow opening test I described earlier. If it doesn't need it, then its just extra fuel washing things down..
If it does need to be richer in some of the range, you will then have to experiment with IAB and IFR combinations. Does it need to be just richer or also carry higher into the rpm range? Changing the combo will effect both richness and when it stops flowing.

Keep noting the turns out of the idle mix screws. It's another clue as to what is going on with each change.

Wow, I'm definitely learning. Thanks a lot for the reply and the education. I've done all the tests you've suggested except for putting wires down the IABs. Do you have any suggestions how I could do this safely? What type of wire should I use? I have .023 welding wire, but I'm assuming that'd be too big. I don't get a stumble when opening the throttle slowly from idle. As you said, that indicates an issue with the pump shot, right?

 

[Mattax]

Well if it didn't stumble its probably ballpark. How close depends on how much load and whether you were able to test it up to the point the primaries take over. But yes then if stumbling on a normal accel from stop light to 30 mph you want to try a more initial pump shot.

Wires for testing. For short term, anything that is handy will work. I've used electric wire strands as well as music wire. Obviously stiffer wires are a little harder to shape. Shape it so one end is in the air bleed, run it over the choke tower if there is one, then under the air horn. I like to make mine so both bleeds are on one wire. Either way, trap the wire under the air cleaner gasket and air horn.

.023 might be fine to start with. See what happens, then try two or smaller based on the results. You will probably have to readjust the idle mix screws.

At some point, its helpful to map out all the bleeds and restrictions on the carb.
How big are the air bleeds?
.023/2 = radius. radius squared x PI will equal area.
Do the same calc for the idle air bleed. It's probably around .070"

Now when you stick the .023 wires in there you'll know about how much change in area occured. You can then figure the equivalent air bleed size. Neat huh!

With the carb mapped out, you can then do some better comparison with other peoples experiences on racingfuelsystems forum. There's a limited range where these restrictions operate predictably. For why, go over to Racing Fuel Systems forum andlook at the threads on Emulsion Tuning for "two-phase flow" and the one on submerged idle restriction.

 

[mderoy340]

Do not use the AFR gauge to set the idle mixture. I'm assuming you have an automatic tranny. Gas motors need rich fuel mixtures at high load low rpm (coming off idle to get the car moving) You can set the idle mixture in N to smoothest idle and drop it in D. Set the idle mixture to the smallest rpm drop went put in gear. You need to fatten up the idle mixture and see if that helps. Also make sure the motor is at full operating temp when you dial in the idle mixture. Don't be surprised if the AFR gauge reads 12.0 - 12.5 when your done.

 

[mopowers]

Well if it didn't stumble its probably ballpark. How close depends on how much load and whether you were able to test it up to the point the primaries take over. But yes then if stumbling on a normal accel from stop light to 30 mph you want to try a more initial pump shot.

Wires for testing. For short term, anything that is handy will work. I've used electric wire strands as well as music wire. Obviously stiffer wires are a little harder to shape. Shape it so one end is in the air bleed, run it over the choke tower if there is one, then under the air horn. I like to make mine so both bleeds are on one wire. Either way, trap the wire under the air cleaner gasket and air horn.

.023 might be fine to start with. See what happens, then try two or smaller based on the results. You will probably have to readjust the idle mix screws.

At some point, its helpful to map out all the bleeds and restrictions on the carb.
How big are the air bleeds?
.023/2 = radius. radius squared x PI will equal area.
Do the same calc for the idle air bleed. It's probably around .070"

Now when you stick the .023 wires in there you'll know about how much change in area occured. You can then figure the equivalent air bleed size. Neat huh!

With the carb mapped out, you can then do some better comparison with other peoples experiences on racingfuelsystems forum. There's a limited range where these restrictions operate predictably. For why, go over to Racing Fuel Systems forum andlook at the threads on Emulsion Tuning for "two-phase flow" and the one on submerged idle restriction.

Thanks for the math refresher. I'm assuming smaller IABs richen the idle mixture? If it comes down to drilling and tapping the stock IABs, I'm just gonna sell this carb as I've got a proform 650 dp and holley 4779 sitting on the bench. I was just hoping to get this 670 street avenger running well enough to do some track testing against the other carbs.

Do not use the AFR gauge to set the idle mixture. I'm assuming you have an automatic tranny. Gas motors need rich fuel mixtures at high load low rpm (coming off idle to get the car moving) You can set the idle mixture in N to smoothest idle and drop it in D. Set the idle mixture to the smallest rpm drop went put in gear. You need to fatten up the idle mixture and see if that helps. Also make sure the motor is at full operating temp when you dial in the idle mixture. Don't be surprised if the AFR gauge reads 12.0 - 12.5 when your done.

That's an interesting thought. I'll fatten up the idle even more than it is and see what happens. I initially set the mixture screws by using a vacuum gauge. The RPM hardly drops at when put in gear. It's got a 9.5" converter though too, so that may be why. Unfortunately, between work and the weather, I'll likely have to wait for the weekend to do some testing. Thanks again.