Tuning the Holley for WOT using AFR gauge

[Paul G]

Is a PITA! Sorry for this being so long but there are a lot of details.

I am playing around with the 750 I had sitting around. It is a 3310 vacuum secondary 4160 which I converted to 4150 with a Holley 34-13 secondary metering block kit. Converting to 4150 was in an effort to lean out the way too rich WOT condition. I have a tuned SA670 which works well but doesnt pull as hard as the 750, even with the 750 too rich.

This is on a mild built 360. Stock stroke, RPM air gap intake, iron heads with mild porting, long tube headers, 2 1/2" exhaust with x pipe through turbo mufflers. No specs on the cam or CR. A518 auto with 3.91 gears. With the 670 it ran a best run of 14.9 @ 91 MPH last spring in Las Vegas, 4200 # car.

What I have so far with the 750;

Idle and cruise are dialed in pretty well. Idles in the mid 12's AFR, cruises in the mid 13's to low 14's AFR. No stumbles or lean surges. I jetted the primary down to 70 jets from the stock 72 jets to get the cruise AFR's in the mid 13's to low 14's. I tried 68 jets in the primary and AFR's at cruise went in to the 14's, there was some stumble on acceleration, and the car didnt pull as hard like that. I want this carb for performance. The 670 is dialed in for economy.  

WOT ran in the 10.9 AFR range. This is a problem. I am shooting for AFR's in the high 12's to low 13's and having a very hard time getting it there. I have the secondary side jetted down to 66 jets from the stock 75. (75 jets are the equivalent to a 21 secondary plate) The AFR's at WOT didnt change, stayed around 10.9. I changed the secondary vacuum spring from the stock plain one to the purple one, one size lighter. It seemed to pull a bit harder but did not change the AFR's much at all.

I dont have any smaller jets to try in the secondary but it seems like a futile effort anyway if coming down 9 jets sizes haven't made any difference. Something tells me the fire hose amount of additional fuel is coming from the PV circuit.

I tried a 5.5 PV, stock is a 6.5. What I am seeing at WOT is the AFR's holding in the 13's a little higher in the RPM range than with the 6.5 PV. I assume it is because the 5.5 PV is holding out a little longer before it opens up, dumps fuel and the AFR's drop in to the 10's. I took a good look at the PVCR holes in the primary metering block. They are smooth and look un touched. Dont look to be drilled or hacked by a previous owner or tuner.

Could Holley have really intended for this carb to run this rich on purpose?

To try and restrict the fuel flow from the  PV circuit I have a small piece of wire in the PVCR holes. Found a a paper clip, cut off two short pieces and bent a 90 on the end of each so they cant get too far up the channel, have one in each of the PVCR holes.

Took the car out for more WOT tests and the WOT AFR's are now in the upper 11's, 11.9 is lean as it gets after a few seconds at WOT.  The paper clip wire is taking up quite a bit of the PVCR holes. Am I going in the right direction?  

Next thing to do is try to find a larger diameter wire to put in the PVCR holes, or...

Get a kit to make the PVCR holes adjustable by drilling them out and tapping them for air bleeds, or.....

Going to Ace Hardware and get some small 6/32 set screws, drilling a small hole in the set screws, tap the PVCR's for 6/32 and put the drilled set screws in the PVCR holes. Then open the hole in the set screws till WOT AFR's get to about 14, then stepping up the secondary jets and dialing it in to the mid 13's AFR in that way.

What am I doing wrong here? Any one else go through this?  

[BSB67]

Generally, something does not sound right.  Keep thinking and looking.  Changing the PV opening vacuum level should not change the AF ratio.  Make sure they isn't something afoul with the PV gasket and proper seal.

Maybe put a plug in the secondary instead of a PV.  If it had a plate before, and now you added a block with a PV and PVCR, I would assume it would be too rich with comparable jetting.  Plugs are commonly used in the secondary's.

Please stop believing magazines on what the right WOT AF ratio is.   The meter gives you a measurement, not an answer.

[c00nhunterjoe]

I also agree with the plug on the secondary circuit. Does this carb have extensions on it? Possible fuel slosh since you said it clears up after a few seconds wide open.

[Paul G]

There is no PV in the secondary, it has changeable jets in the secondary, just no PV.

Changing the 6.5 PV out for a 5.5 did not change the AFR, it only moved the point when it gets very rich higher in the rpm range. To be expected with a lower number pv right? Because the richness moved as expected with the pv change, I decided to try leaning out the PV circuit. Putting some restriction in the PVCR channel did lean out the WOT AFR, a full point, 10.9 prior to the restriction to 11.9 after the restriction.

To test the PV, run the idle mix screws all the way in and the engine dies. Good PV and seal right?

Coonhunter,

As for extensions, are you referring to jet extensions? if so then no, no extensions. 



The testing method I am using to determine if power is going up or not has been very helpful along with reading the AFR. I go out on the same back road near the house, same pavement, same grade every time and no traffic. I come to a stop, then nail the gas. With the 750 in stock form it would lay a nice patch in first, shift to second and loose power. After the changes I have made it will now easily spin first with a quick shift to second, easily spin in second, shift to third and spin in third before it looses power and downshifts back to second, then hooks and goes.

Huge improvement just jetting down the primary, lower number pv, and massively jetting down the secondary.   

I have read that a good AFR for WOT is in the 12.5 to 13.5 range. Is that a good point to shoot for? I am not a racer, have never messed with a carb like this before, I am learning, and it is fun.

[c00nhunterjoe]

I missunderstood. I thought you were running valves in both blocks. In that case, based on your rich condition regardless of substantial jet changes with no results, i suspect you have an internal fuel leak. 1st thought is your test method. The burnout method is not what i would call accurate for tuning. Among other things, the load on the engine is changing as you fry the tires and shift, which could throw your afr meter off. Before you go pulling out your hair, i would suggest making some pulls in the rpm range your meter says it runs rich and immediately shut down the engine and pull a few plugs to either verify or deny the meter.

[BSB67]

Still seems like something is still not correct. 

When you drop the hammer the PV opens irrespective of its opening value/rating, as the vacuum drops to nearly zero instantly. The PV does not gradually open at WOT.  Changing the vacuum rating is only for tuning part throttle actuation.

And like stated, you should not be evaluating AF during tire spinning.  WOT from 3000 or 3500 to redline in 2nd or 3rd gear, no tire spin.

Maybe lower your fuel level.

[Paul G]

Understood. The burnout has shown that the engine has become stronger with leaning out the carb. The AFR gauge has shown that I am not too lean at cruise and definitely not too lean at wot.

With the gauge I can see afr at idle, cruise using very light throttle which is just off idle, the transition from idle to the main jets, and afr when on the main jets during light to moderate acceleration, and most importantly watching the PV open.

I have had the carb apart many times. Put a kit in it a few years back. Using the blue gaskets which still look good. Using carb cleaner I have sprayed through all the ports and specifically the emulsion tubes to make sure they are clear.

I am most concerned that fuel is bypassing, leaking, or somehow getting past the metering section causing the rich condition when the PV opens, or the secondary system comes in to play. I just cant find it.

[BSB67]

Disconnect the secondary's and do a WOT run.  This way you will be able to tell if it is in the front or back for sure.

[Paul G]

Disconnect the secondary's and do a WOT run.  This way you will be able to tell if it is in the front or back for sure.

Tied the sec closed and drove the car last night. Very little change other than wot performance is gone, which was to be expected since the carb is basically a two barrel without the secondary's. When the pv opens which is easy to see on the gauge, it would read from 11.5 to 12 AFR. When just running on the main jets, like before AFR is in the mid 13's. No real change in AFR disabling the secondary's.  

So I can assume that the richness is not coming from the secondary side of the carb since it never opened right? It is coming from the pv circuit. I can try to double the size of the wire restriction in the PVCR hole before I get out the drill and tap. Maybe try to get two pieces of wire in each hole as a test.

Some history;
When I bought this car in 2010 this 3310 750 was on the car. When it started to leak fuel I bought a kit and rebuilt it. I found that the person before me appeared to have intentionally blocked the PV closed (it was jammed and had no spring action, would not open), and he had 76 jets in the primary instead of the stock 72 jets. Without an operating PV he had to up-size the primary jets a lot, right? It still had the 21 plate in the secondary, which is equal to 75 jets. I put the kit in the carb, new PV and needle and seat. Then it was a dog. Running way to rich with 76 jets in the primary and a 6.5 PV. I went back to the stock 72 jets with the working PV, the car ran better, but was not as strong as before I rebuilt it.  I guess I undid his custom tune (hack).  

Before the carb was rebuilt the mileage was horrible for a small block. Like 8 MPG around town.

[c00nhunterjoe]

When the pv opens, it should go rich. Thats the point of it. Now, from what i have gathered without viewing the afr meter and tach myself through your rpm range, i think you have a jet problem and not a pv problem.
   It sounds to me like under wot accel from a low rpm that you are seeing a rich condition until you get the rpm up higher. Is this correct? If so, what rpm does the mixture level off? I see a few possibilities to try here. Drop 2 jet sizes and see what it does without your wire in the pv circuit, or drop a 3.5 pv in and see what the readings are. Have you checked your float level yet? What is your fuel pressure during wot runs?

[Paul G]

When the pv opens, it should go rich. Thats the point of it. Now, from what i have gathered without viewing the afr meter and tach myself through your rpm range, i think you have a jet problem and not a pv problem.
   It sounds to me like under wot accel from a low rpm that you are seeing a rich condition until you get the rpm up higher. Is this correct? If so, what rpm does the mixture level off? I see a few possibilities to try here. Drop 2 jet sizes and see what it does without your wire in the pv circuit, or drop a 3.5 pv in and see what the readings are. Have you checked your float level yet? What is your fuel pressure during wot runs?

It sounds to me like under wot accel from a low rpm that you are seeing a rich condition until you get the rpm up higher. Is this correct?

I will try to explain, before I put the wire in the PVCR's, still using the 5.5 PV, in third gear (automatic) rolling at a steady 45 mph around 2200 rpm, cruising in the high to mid 13 AFR's, slowly push the pedal to the floor without kicking down a gear, it will first go lean in to the 15 AFR's for a quick second, then drop to the mid 13 range for a short time, then goes very rich to mid 10 AFR range, as the rpm's climb it will lean to the upper 10 AFR range but no leaner even at 5500 rpm and wot.


Drop 2 jet sizes and see what it does without your wire in the pv circuit

Before I put wire in the PVCR I tried 68's in the front with 67 jets in the rear, had a lean surge at cruise and slight stumble on acceleration. AFRs at cruise go pretty lean also, high 15 AFR range. WOT AFR's were in the high 10's low 11's.


drop a 3.5 pv in and see what the readings are. Have you checked your float level yet? What is your fuel pressure during wot runs?

I could try a 3.5 PV. But wouldnt that just bring the rich condition on a little higher in the RPM range when accelerating slowly? When the hammer drops and vacuum goes to zero the PV would just open and go rich very quickly, which is what it does now.

Float level front and rear are, fuel just dribbling out the hole.

No fuel pressure gauge, it would be nice, I am running a stock replacement mechanical fuel pump, no regulator.

Thanks for all the help guys. With what I have done so far I have seen a noticeable improvement in the power of the 360 already. I feel that there might still be a little more in there. If I can get the WOT AFR's in to the low 13 range I might see that extra power.

   

[c00nhunterjoe]

Ok, it sounds like you make make it run rich for quite a while so i will suggest doing that, then immediattly shut it down and coast to the shoulder and pull a few plugs to verify the overly rich condition before going any further.

[Paul G]

http://www.musclecardiy.com/performance/holley-carburetor-full-power-circuit-calibration-guide/

This is probably the most informative read I have found so far. My Holley is does not have the fully adjustable metering block like he is describing in detail in this article. He does explain a lot about what is happening with the air bleeds and emulsion tubes at various engine speeds, and how he adjusts them for maximum power using the AFR.

I am going to make one more effort to fully clean out the emulsion tubes and see if there is old crud restricting the passages. There are plugs in the top of the metering block that seal the emulsion tubes. Has anyone ever tried to pull the plug and see what is in there and clean them out really well? Are replacement plugs available? 

[johnnycharger]

I have no info on this but this link may help
http://www.bob2000.com/carb.htm

[Paul G]

I have been keeping a spreadsheet on what changes I have been making to the 750, what each change does to the AFR, and how the car responds. What I am seeing is much like you guys have stated, the numbers by themselves dont mean much, the engine likes what it likes.

Playing with the PVCR's using wire restrictions and making basic jet changes I have been able to achieve WOT AFR"s in the high 12's like the articles I read have said is optimum for performance. That may be true on paper but my engine likes to be much richer than that. My engine seems to like WOT AFR's in the mid 10's to mid 11's. It does the longest burn out when AFR's are in that range, and pulls to 100 MPH much easier, that 100 MPH pull is just SOTP on back roads though. No track time.

I have found that idle mixture effects mainly idle smoothness and cruise economy.

Primary jetting effects performance from light to medium throttle acceleration along with WOT AFR's. This is the most critical of the jetting.

The point when the PV opens is critical to economy and overall performance.

Secondary jetting is fine tuning, less critical than the others.

I know weather conditions have an effect on AFR and performance. Now that we are in to winter and the temps are cooler I suspect my tuning is all screwed up? Going from 100+° during the summer to 60° and 70° now is going to have an impact. 

So what I am still getting wrong?

[c00nhunterjoe]

Main thing i still see wrong is judging your progress on how it does a burnout. Otherwise, good info and research learning your car.

[1974dodgecharger]

I run a wideband also, but my thinking is slightly different than yours paul unless im wrong about carbs.

when you hard accelerate aren't you running the nozzles those are in full use correct?

Then under cruise your now in the jets circuit mainly and nothing is coming out of the nozzles?

I have a 60 to 40 ratio arm on my double pumper and my front nozzles are 31 and 27 rears and under WOT I get down to 12 to 1, but if I increase my nozzles under WOT I get down to 11 to 1.  increase as in size 35 to 37 nozzles and keep my rears the same. 

HOlleys have a lot of adjustability as we all know, but that's how I have my adjusted based on my wideband. 


I ran 37s front nozzles before and it made it pig rich so much your eyes would slightly ever so burn.

I kept my jets the same in all instances here because when im cruising im getting 13 to 1 no pressing on gas peddle.  I have run the jets higher for experiment and have gotten 11 to 1 on a cruise and then accelerate hard with my nozzles mentioned above and lets just say the person behind me their eyes are burning im beyond 10 to 1 that my wideband cant read it 

[XH29N0G]

My understanding is that the (2 or 4) idle mixture screws are to set the mixture when the transfer slots are partly exposed, but that the idle circuit (air bleeds) still control A/F up into the 2000-2500 RPM cruise range because the blades are open enough to fully expose the transfer slots, but there is not enough flow to make the main jets and (downleg, straight, annular) boosters pass gas. For higher cruise we get partly into the main circuit (when the main jets and boosters flow) and to keep that down, a power valve is installed so smaller jets are used and less flow happens.    

The jets (and power valve) are for WOT conditions.  Once the car settles in to WOT condition, we are on the main circuit and using the jets.  The power valve opens at WOT to add a little more gas to make up for the slightly reduction in jet size that is there to keep the cruise a little more lean.  The main air bleeds are to set whether the A/F is flat, rises or drops during WOT.

The accelerator pump cams and pump nozzles (those at 28, 35, 37) are there to squirt fuel in when the butterflies just start to open and there is not yet enough air flow to have the boosters flow gas from the main jets so that there is not a lean spot at throttle tip in.  

See if others correct this.  

[Paul G]

From what I have learned about Holley type carbs,

Cruise; the fuel mixture/AF Ratio at cruise is coming mostly from the idle mixture screws. The throttle blades are just barely open at cruise, the transition slot is just coming in to play. The engine can run pretty lean here for fuel economy, light load.  

Light acceleration; the main jets are coming in to play. The squirters and accel pump and cams all play a part in transitioning from the idle circuit to the main circuit. The throttle opens past the transition slot and starts drawing fuel through the squirters boosters from the main jets. Fuel comes through this path as long as the throttle is open past the transition slot. This is the main tuning circuit of the carb. There are other things like air bleeds, squirter nozzle size, accelerator pump and cam selection that have a big effect on tuning this circuit. Nozzle size and pump cams are more for transitioning from idle in to the main circuit. You can tune out lean or rich (bogs) areas with nozzle sizing and accelerator pump and cams. Air bleeds fine tune this circuit.

Power valve; As engine load increases, the main throttle is opening a lot, engine vacuum decreases, the main jets can no longer feed the engine under this much load, that is where the power valve comes in to play. When engine vacuum drops to the rating on the PV, it opens, allowing a shit ton of fuel to pass through the Power Valve Channel Restricions and enter the fuel mixture. This happens on vacuum drop, because the engine is under extreme load and needs this extra fuel. The PV circuit is not tunable on most street carbs without using trickery, like drilling out the PVCR holes, or dropping restrictions in them.

Secondary circuit; the primary throttle is opening fully, the engine is under maximum load, the main circuits are maxed out in fuel delivery, the secondary throttle plate is now opening allowing additional air and fuel in to the engine. On vacuum secondary carbs air moving through the primary venturi creates a vacuum on the secondary diaphram (nothing to do with manifold vacuum at all) pulling it open which opens the secondary throttle. The vacuum created in the venturi is determined by the needs of the engine. Secondary jetting and springs in the diaphram can be changed to tune this circuit. On double pumpers, the secondary throttle opens via linkage. There is a PV in the secondary as well. Secondary tuning is much like primary tuning with the PV circuit and accelerator pumps, cams, and nozzles.        

That is all I know so far.

Ignition timing has a play here as well. When AF ratios are adjusted, engine timing can use an adjustment as well. AF ratios in the 12's, optimum fuel mixture for maximum power, burn the fastest, need less spark lead. Richer, and leaner, than the optimum AF mix needs more spark advance. Or so I have read. Something else to play with now.

[c00nhunterjoe]

The squirters have nothing to do with the jet sizes. The accel pump drives the fuel through them based on throttle movement via the cams. Which is another thing to look at when you are going wide open and see that initial change on your gauge. That initial change is decided by the squirter size and cam profile, not just the power valve. And the squirters will change it more then the power valve channel size in the metering block will.

[Paul G]

The squirters have nothing to do with the jet sizes. The accel pump drives the fuel through them based on throttle movement via the cams. Which is another thing to look at when you are going wide open and see that initial change on your gauge. That initial change is decided by the squirter size and cam profile, not just the power valve. And the squirters will change it more then the power valve channel size in the metering block will.

You are correct. Thats what happens when I think too hard too early in the morning. I lined out the word squirters in reply 18 and replaced it with boosters. I have to say that figuring out how the carburetor works has been a real blast. My car sure feels like it has gained power. I am using less than scientific means of judging power, but the length of the burn out has improved, along with the ease that the car pulls to 6000 RPM in third gear. It used to labor to get to that RPM, which is about 115 MPH with 3.91 gears.

I think that the notion of jetting up gains power is misleading, "let er eat", "more fuel goes faster". etc. When in my experience it has been the opposite, "lean is mean". Holley sends the carbs out on the rich side. Safer that way for them, less chance of engine damage running a little rich, liability issues in my opinion. Only exception I have experienced is with the Street Avenger 670 I have. It was set up on the lean side. AFR's are in the 14 to 15 range at cruise and light throttle, 11's WOT.  I would think that the engine it is sitting on would have a determination in AFR's as well? Stock to mild would have different needs than highly modified?   

[c00nhunterjoe]

Stock, mild and wild all get tuned the same way- maximum combustion efficiency. The stocker means it will cruise nice and idle for hours without loading up. The mild will pull hard and not foul plugs and burn your eyes when the windows are down. And the wild.... well we all know how the wild ones run. Lol

At this point, it sounds like you are splitting hairs attempting to achieve perfect stoicheometric efficiency at all rpm ranges under all loads. I hate to break it to you, but even the best fuel injection on modern cars cant do that. Drive your car and enjoy it. Your model carb is not designed to do what you want. The next step would be a race style metering block or better yet a full race carb that has adjustable bleeds as well. But on a street car, thats a lot of money to throw at it when by your data, its running pretty darn good.

[1974dodgecharger]

Stock, mild and wild all get tuned the same way- maximum combustion efficiency. The stocker means it will cruise nice and idle for hours without loading up. The mild will pull hard and not foul plugs and burn your eyes when the windows are down. And the wild.... well we all know how the wild ones run. Lol

At this point, it sounds like you are splitting hairs attempting to achieve perfect stoicheometric efficiency at all rpm ranges under all loads. I hate to break it to you, but even the best fuel injection on modern cars cant do that. Drive your car and enjoy it. Your model carb is not designed to do what you want. The next step would be a race style metering block or better yet a full race carb that has adjustable bleeds as well. But on a street car, thats a lot of money to throw at it when by your data, its running pretty darn good.