Doing some fine tuning now that car on road .....439BB with 250/261 @,050 solid roller 9hg of vacuum at idle 4.5 PV idles 950-1000rpm Have FAST Wide band AF meter .....what AF should I shoot for for clean idle ???

Thanks Bill

Stock jetting? 78's square? What does it show now?

Stock jetting? 78's square? What does it show now?

Sori, currently jetted 79/81 A/F 15.5-16.2

I've never used an A/F meter. I've never had a carburetor as good as the 950HP either. I'll be watching this post to see what the experts say. Mine was jetted 78's square on my 505 til last week, I removed the rear pv and jetted up to 86's. So far so good.

I've never used an A/F meter. I've never had a carburetor as good as the 950HP either. I'll be watching this post to see what the experts say. Mine was jetted 78's square on my 505 til last week, I removed the rear pv and jetted up to 86's. So far so good.

Me neither ......the jetting was from break in session on Dyno .now have the meter and just want to get a clean idle .runs fine just alot of raw gas smell at idle ... rear PV plugged

Bump ......still need some feedback on A/F at idle

I was getting about 13-14 afr at idle w/my 770 avenger. All my double pumpers were in the 12 range...little stinky.

AFR in the 15-16 range sounds lean...are you sure its accuarate? Where's your afr at cruise and wot?

I was getting about 13-14 afr at idle w/my 770 avenger. All my double pumpers were in the 12 range...little stinky.

AFR in the 15-16 range sounds lean...are you sure its accuarate? Where's your afr at cruise and wot?


I agree I thought lean and yet it very stinky ......have not logged the A/F's at cruise etc .car only has 32 miles on it so just starting out with tuning ......will ensure I using the Meter correctly ....... what I really looking for is the A/F at idle I should try for while still having good drivability

Yea...drive it for awhile. My LM-1 was to be calibrated often. If your showing 15-16 while driving, the meter has to be off. When my afr got to around 14-15, I would start to get a stumble/lean surge.

Yea...drive it for awhile. My LM-1 was to be calibrated often. If your showing 15-16 while driving, the meter has to be off. When my afr got to around 14-15, I would start to get a stumble/lean surge.


OK this was checked at idle in my driveway .......blipping the throttle did bring it up to 13-12 at certain times .....the FAST unit supposedly does not need to be calibrated like the LM-1 ....... should I assume idle A/F should be around 13-14 or should it be richer ? My PV is a 4.5 so no enrichment til accleration

I think with 250/261 @.050 you will always have a stinky idle. I'm in the same situation with 253/253 @.050, and while I have cut alot of the stink down and got the plugs looking fairly good, it still stinks. If you do lean it out too much it will surge/run lean, like Vince mentioned.

Before my car would run you out of the garage within the first few minutes of starting it. It would burn the hell out of your eyes and make them water up like crazy. Now it doesn't run me out of the garage, but it still stinks and smells my clothes up. If you wear anything around that car while it's running it better be washed.

When you say stinky, what exactly do you mean and how bad?

Have you tried tuning the transfer slot relationship and the idle mixture screws?




Hows it going Vince??

Rat ..stinky is as exactly as you describe and yes it may be as good as it gets with that duration cam .have done idle mix screws to highest vacuum but not the transfer slot .......... Thanks

I wouldn't place a ton of credence on the wideband AT IDLE with that cam. There's a lot of overlap.

Agree you might always have a "stinky idle", somewhat no matter what.

My advice is lean it out until it gets rough and richen it up till it idles best.

I have a slightly bigger cam in my enigne and have it set at 14:1 closed loop with the EFI at idle.

Remember your idle mixture screws affect your entire fuel curve as well.

Thanks guys appreciate the feedback ....

I think with 250/261 @.050 you will always have a stinky idle. I'm in the same situation with 253/253 @.050, and while I have cut alot of the stink down and got the plugs looking fairly good, it still stinks. If you do lean it out too much it will surge/run lean, like Vince mentioned.

Before my car would run you out of the garage within the first few minutes of starting it. It would burn the hell out of your eyes and make them water up like crazy. Now it doesn't run me out of the garage, but it still stinks and smells my clothes up. If you wear anything around that car while it's running it better be washed.

When you say stinky, what exactly do you mean and how bad?

Have you tried tuning the transfer slot relationship and the idle mixture screws?




Hows it going Vince??
My 406 is doing the same teary eyes in the garage idle,and I have just rejetted to 74's and swapped to a 4.5 PV.I can't get a decent idle below 950 or so. Is that normal for a 254@.050 cam,or do I need to drill my throttle plates to get a lower idle. I pull 7-8 inches at 950 or so,but if I try to slow it down it loads up and eventually stalls.

I don't idle much anyway,but if I can fix this it'd be great.

MoneyPit .with that size cam I would say that the most you would get is 9"hg and 900 rpm is about right ......my 950Hp has drilled throttle plates I can idle it slower but not much I was just hoping to get the idle mixture cleaner but maybe not possible with a cam this size ...just styarting to use my A/F meter for idle and cruise and was looking for some A/F numbers to shoot for ........used it on Dyno to get the big HP numbers but now want to clean up the idle / cruise ,if possible hoping to get the idle to 13.5 with no stumble or detonation WOT is 12.0

First you need to really understand how your carb works.

There is absolutely no point in even mentioning what your jets are when talking about idle. The jets are part of the main fuel circuit and have no bearing whatsoever during idle operation. Idle operation is solely the domain of the idle mixture screws, transfer slot, idle restriction, and idle air bleeds. There is a book about carbs and intake design by David Vizard with some really good info in it that I highly recommend. Much better than the stuff by Dave Emmanuel.

Next you need to make sure the ignition is setup correctly. If you run a cam with significant overlap and anything close to a stock amount of initial timing advance, it is not going to work well. You will get way off into the weeds if you try to compensate with the carb. You *will* need more initial timing advance, possibly a *lot* more. For example I run a Crane HR296 236/242 @ 050 in my 402 and idle much better at 30* than I ever did at 16-18*, so much better that vacuum went from 7-8" to 11-12".

Theoretically, AFR-wise, what you want is to run that sucker as lean as you can under idle and cruise conditions without it surging. This will produce best economy and least stink and again, theoretically, is an AFR of 14.7. With that cam you will probably always have some stink at idle and potentially a bit of inaccurracy with the AFR reading (O2 sensors can be confused by raw fuel in the exhaust stream). Note that a good wide band O2 can tell you when the idle mixture screws are balanced: there will be less AFR bounce when you have the same mixture strength on both sides.

Of course as any experienced tuner will tell you, you do not want to run this lean under full throttle acceleration. Many will claim you *never* want to run this lean, but cruise pressures and temperatures are much lower than power pressures and temperatures. So you can get away with a leaner fuel AFR. In the power mode you want an AFR closer to 12.5, which is the max power AFR in theory (in some cases you may want an even richer AFR as you can use extra fuel for cooling the engine).

To best tune a Holley, start by insuring the throttle blades are indexed to the transfer slots at idle. There should be no more than a square of transfer slot exposed under the throttle blades. Ever. If you need more for idle then start looking for issues in other places like the ignition setup. If you have more slot exposed your idle mixture screws will stop working and you will idle excessively rich. If you just absolutely cannot get idle with out opening up the throttle blades, then you can consider the old trick of drilling bleed holes in the throttle blades.

Next you can adjust the idle mixture by closing the mixture screw on each side until engine speed decreases and then opening up until just before engine speed maximizes. Go back and forth doing each side by itself until they stabilize at the same setup each time. If you have four corner idle... then you just have to go around and around and around doing the same basic thing.

Now you can start looking at the jets on the primary bore. Here you want to jet up/down to achieve an optimal cruise AFR. There may be a relationship between vacuum and AFR: as vacuum drops AFR increases (lean). This will limit just how close to 14.7 you want to go because at it's leanest you want to be 14.7 or less.

Next you can look at the jets on the secondary bores. Jet up/down to achieve an optimal power AFR or in the 12.5-13.0 range. Do not change the jets in the primary bores during this tuning stage.

Now you may have some stumbles here and there and tuning them will require a vacuum gauge along with the wide band O2. With both you can start to detect trends like "when vacuum falls to 6" it goes lean and stumbles". This is where you start tuning with power valves. These provide extra fuel over and above the regular main jet fuel.

If you get really advanced you can start logging vacuum and AFR and start to adjust fuel curves and air bleeds. But that definitely takes a bit of experience and a lot of understanding.

Hey Steve ............thats what I was looking for Thanks

"Theoretically, AFR-wise, what you want is to run that sucker as lean as you can under idle and cruise conditions without it surging. This will produce best economy and least stink and again, theoretically, is an AFR of 14.7. With that cam you will probably always have some stink at idle and potentially a bit of inaccurracy with the AFR reading (O2 sensors can be confused by raw fuel in the exhaust stream). Note that a good wide band O2 can tell you when the idle mixture screws are balanced: there will be less AFR bounce when you have the same mixture strength on both sides."

I understand what controls idle and also the effect of proper ignition advance/curve I think I have that all covered ......but was looking for an A/F to shoot for .....also good info on how to use the meter to set Idle mix ...... must admit I'm not confident i got the 4 corner idle set properly

Thanks again I will reset the idle mix withthe meter and a vacuum guage and check my transfer slot position

Sorry if that came across poorly... I hate to see people get all turned the wrong direction and there seems to be a lot of misinformation (or at least exceptional/strange information) out there. You happened to hit upon a topic a friend and I were discussing very recently with the tuning process for Holley carbs.

I guess one of the real points I buried in all that was not to rely solely upon specific numbers on the AFR readout in tuning at idle. I know personally that it is an easy trap to fall into and I still tend to do it myself more than I should. My setup can get up to around 15.5 AFR before the butt dyno detects a stumble. My setup also seems to change readings depending on how close the O2 sensor wiring gets to the headers. So there can be some variability in specific details between different instruments, sensors, vehicles, and even days.

I have been driving my El Camino for a year or two with a Tech Edge WBO2 and a vacuum guage velcroed to the transmission tunnel. I have added in RPM and vacuum/MAP sensor inputs as well. I have played around with computer logging. It has been very educational to say the least, and I have progressively gotten the tune and setup better as I learned more about how it all works. I have found the WBO2 very very useful in all of this even with it's limitations.

One think I have not done is hook up thermo-couple inputs to the WBO2. That could also indicate balance on multiple circuit idle adjustments. With headers and one of the cheap hobby store IR temperature readers you could probably achieve the same result.

No problem Steve ...you did not come across poorly I just wanted to eliminate some variables to get info I looking for .......this is first time i really used a WBO2 on a carb car ,especially with long duration cam etc ......I learned ton when i got involved with a FI supercharged OBD11 car and a LM-1 wide band and I agree a great tool once you understand how to use it I appeciate the info on the mixture setting using the WBO2