AFR Readings

[andyf]

Those are good numbers and right in the range of how I program an EFI car. I typically start off with 13.5 idle, 12.8 WOT and 14.5 for cruise. Smooth those numbers at the boundaries and then take it for a drive. Works most of the time although a few engines are picky about the idle, especially with larger cams.

 

[Kern Dog]

Tuning by checking the spark plugs is a pisser if you have headers.
I have to let mine cool down before swapping plugs. I also have to use a series of u-joints, wobbly sockets and extensions and even a box end wrench to get the plugs in and out.
Nah....I'll use the gauge to guide me.
I don't drag race anyway. Street fun and road race type driving is what I like. A few WOT runs tells me what is needed. It is more involved than tuning with a laptop but I don't mind.

 

[RJRENTON]

When you have an AFR reading then you tune the carb circuit by circuit. Start with idle and work your way to power. Idle is the most critical and is fairly simple to do with the car at a warm idle with the choke off and the throttle blades in the correct position (transition slots square). If the idle mixture screws won't get the AFR in the right spot then you have to change the IFR which gets more involved.
Setting the main circuit is usually a little more involved. You have to drive the car on the freeway at cruise and note the AFR and then change the jets when you get back to the shop. Adjusting the power circuit is the most difficult since you need to have the engine at WOT. A chassis dyno is the safest method but it can also be done at the track. Carb has to be opened up to get to the PVCR. This all assumes a Holley carb. A carb with metering rods provides more tuning options.

As l mentiomed previously, I'm totally aware how a carb operates, its various circuits including the ones you mentioned, for Carter, Holley, Rochester, Motorcraft/Ford, Weber, etc.
When driving at highway speeds on cruise control (if equipped), one assumes the temperature and barometric pressure (Manifold Absolute Pressure which is the inverse of vacuum) are the same, as that effects air density and ultimately the A/F ratio the engine produces.
Probably, the most difficult circuit in a carb to "adjust" is the idle transition circuit, which functions ftom idle thru the main fuel feed system, as the are no true adjustable components involved.....omly fixed air bleeds, fixed fuel feed and the transfer slots in the primary throttle bores. The primary (and secondary) fuel emulsion tubes or channels also influence the mixture density and ultimately the A/F ratio becsuse of their effect on the burn rate of the fuel charge. Cam specs, compression ratio, spark advance characteristics (rate of change) also can infuence the A/F ratio as well. Static A/F ratios (single point) are nice to know but the best systems are the ones that electronically adjustable....i.e. fuel injection systems that use 02 trim to maintain the optimal A/F ratio for a given engine option.

 

[Moparfiend]

As l mentiomed previously, I'm totally aware how a carb operates, its various circuits including the ones you mentioned, for Carter, Holley, Rochester, Motorcraft/Ford, Weber, etc.
When driving at highway speeds on cruise control (if equipped), one assumes the temperature and barometric pressure (Manifold Absolute Pressure which is the inverse of vacuum) are the same, as that effects air density and ultimately the A/F ratio the engine produces.
Probably, the most difficult circuit in a carb to "adjust" is the idle transition circuit, which functions ftom idle thru the main fuel feed system, as the are no true adjustable components involved.....omly fixed air bleeds, fixed fuel feed and the transfer slots in the primary throttle bores. The primary (and secondary) fuel emulsion tubes or channels also influence the mixture density and ultimately the A/F ratio becsuse of their effect on the burn rate of the fuel charge. Cam specs, compression ratio, spark advance characteristics (rate of change) also can infuence the A/F ratio as well. Static A/F ratios (single point) are nice to know but the best systems are the ones that electronically adjustable....i.e. fuel injection systems that use 02 trim to maintain the optimal A/F ratio for a given engine option.

Were talking a simple Holley 4150 DP and low budget and me not an expert just trying to learn the basics and get reasonable performance. Not a closed loop control system by any means of the imagination.....

 

[andyf]

Were talking a simple Holley 4150 DP and low budget and me not an expert just trying to learn the basics and get reasonable performance. Not a closed loop control system by any means of the imagination.....

Depending on how old your Holley DP is it might not be very adjustable. The older carbs did not have screw in IFR and PVCR so you can't adjust those circuits. Basically just adjust the idle mixture screws and change the jets. If you have the correct carb for you application you should be able to get it close. If you don't have the correct carb then the older carbs can be very difficult to dial in. You'll just have to give it a try and see how it goes. You'll learn as you go.

 

[Rusty 72]

I may be the only one who'll "admit" this but I can't look at a spark plug and know what I'm looking at. I mean outside if it is black or white.
I have used a A/F gauge with success in tuning in addition to a vacuum gauge. An A/F is easy to check as I'm at cruising speed.
I just have one that I can swap in and out of other cars to check on things when necessary. When I'm not using it I just pipe plug the bung.

I know for me at least, when I was younger I had a tendency to tune everything rich and I wasn't even aware of it.

 

[Kern Dog]

Depending on how old your Holley DP is it might not be very adjustable. The older carbs did not have screw in IFR and PVCR so you can't adjust those circuits. Basically just adjust the idle mixture screws and change the jets. If you have the correct carb for you application you should be able to get it close. If you don't have the correct carb then the older carbs can be very difficult to dial in. You'll just have to give it a try and see how it goes. You'll learn as you go.

Quick Fuel sells metering blocks with replaceable orifices in the PVCR. I have one in the primary side of my Demon 850.

 

[andyf]

I may be the only one who'll "admit" this but I can't look at a spark plug and know what I'm looking at. I mean outside if it is black or white.
I have used a A/F gauge with success in tuning in addition to a vacuum gauge. An A/F is easy to check as I'm at cruising speed.
I just have one that I can swap in and out of other cars to check on things when necessary. When I'm not using it I just pipe plug the bung.

I know for me at least, when I was younger I had a tendency to tune everything rich and I wasn't even aware of it.

Don't feel bad, most people can't read plugs and the experts who claim to be able to read plugs will all argue with each other if you put them in the same room together. Hardly anyone knew how to tune a carb 30 or 40 years ago which is why most musclecar engines ate themselves to death. Washed rings, wiped bearings, etc. All the cars I worked on when I was younger were pig rich. We didn't know any better. There were a few guys who figured out how to stick wires in the idle feed restrictions on Holley carbs and those guys had engines that ran good but the rest of us just lived with slobbering Holley double pumpers that slowly destroyed our engines. Once the wideband technology showed up Holley had to start building carbs that could be tuned and now there are some good choices out there.

 

[andyf]

When I was in high school my buddy had a 1970 RoadRunner with a 440+6, four speed, Dana 60 car. We had to change the spark plugs every weekend before going cruising since that six pack setup would foul the plugs within a hundred miles of driving. We had no idea what to do with those three carbs. They were a complete mystery to us so we just changed plugs every week. Changing plugs on that car wasn't a picnic either.

 

[RJRENTON]

Everyone had a " buddy" who had a _____________ (fill in the blank...Ford, Chevy Mopar) that was a great funner/p-p runner. For those that did not understand how the Plymouth Six Barrel/Dodge Six Pack engine/carb set up operated and was tuned, the answer was simple.....take your head out of the sand and LEARN. As the old saying goes: IGNORANCE is NOT bliss. The Holley carbs are easy to understand and adjust and setup. Changing plugs is also easy....access them from underndith; just jack up the car, place the axle safety stands and have at it with a 13/16" combination wrench and a deep well socket.
I'm sure things have improved (or should have) since tten.
BOB RENTON

 

[Kern Dog]

Everyone had a " buddy" who had a _____________ (fill in the blank...Ford, Chevy Mopar) that was a great funner/p-p runner. Changing plugs is also easy....access them from underndith; just jack up the car, place the axle safety stands and have at it with a 13/16" combination wrench and a deep well socket.
I'm sure things have improved (or should have) since tten.
BOB RENTON

I have no idea what a funner/p-p runner is supposed to mean.
Also.....
Spark plug access may be okay for a car with stock exhaust manifolds but not with some headers!

 

[RJRENTON]

Sorry....
I fat fingered the incorrect key.....what i meant to say was...." a great runner or a piss poor runner"...
Since my primary interest is maintaining the origionality and appearance, yes, I'm using the original exhaust manifolds. I also understand that headers can make plug (and starter) access difficult....have you considered using the shorter plugs for better clearances. Headers are not a panacea....a trade off between "better" performance (?) or ease of maintenance.

 

[Kern Dog]

Headers make more power and I love the sound.

 

[Danny Tuel]

As l mentiomed previously, I'm totally aware how a carb operates, its various circuits including the ones you mentioned, for Carter, Holley, Rochester, Motorcraft/Ford, Weber, etc.
When driving at highway speeds on cruise control (if equipped), one assumes the temperature and barometric pressure (Manifold Absolute Pressure which is the inverse of vacuum) are the same, as that effects air density and ultimately the A/F ratio the engine produces.
Probably, the most difficult circuit in a carb to "adjust" is the idle transition circuit, which functions ftom idle thru the main fuel feed system, as the are no true adjustable components involved.....omly fixed air bleeds, fixed fuel feed and the transfer slots in the primary throttle bores. The primary (and secondary) fuel emulsion tubes or channels also influence the mixture density and ultimately the A/F ratio becsuse of their effect on the burn rate of the fuel charge. Cam specs, compression ratio, spark advance characteristics (rate of change) also can infuence the A/F ratio as well. Static A/F ratios (single point) are nice to know but the best systems are the ones that electronically adjustable....i.e. fuel injection systems that use 02 trim to maintain the optimal A/F ratio for a given engine option.

But the man is working on a carb so why do you keep going on about EFI ? It's of no help at all !!!