opinion on primary rod / jet change

[kilowattkid]

I guess to start things off, here is a thread giving the background on the vehicle / engine combo and the carburetor that Cliff helped me get set up, which worked great.

https://cliffshighperformance.com/simplemachinesforum/index.php/topic,3400.0.html

Fast forward some and I decided to send out a carb to get replated.

So I sent out a #7044224 unit that I had for my 74 Nova and it came back looking, well, amazing. Upon inspection, I'd seen a hole in the primary butterfies, which was good, I had those in my existing carb. Of course the instructions said that I could not open the carb up, it would void my warranty, so I coudn't check the bypass passages. OK, whatever. I did notice that it did not have an APT like I had in my current one and they assured me that I probably didn't need it. It turns out that I didn't, but initially I thought I did. More on that later.

So I put the carb on and gave her a try. Accellerator pump pumped,choke worked fine and I had full control of the idle screws. Great start! Road testing was ok, some part throttle areas gave me some concern and a plug reading showed me a lean reading at the tip, with some graying deep down in the cone. Took it out for another drive and decided to give her a rip. A lot of popping and let off, knowing it was lean.

So I pulled the secondary rods out and seen it had an "H" hanger with DA rods. I mic'ed out the hanger and it seemed fine and proceeded to put in a set of AX rods in that I had from my "good" carb. Took it out for a test drive and normal driving was as before but a good WOT rip showed some improvement, but still a lot of lean popping.

I sent an email to the replater / rebuilder telling them about my cruising issue and WOT issue as well as secondary rod change. It was during this conversation that I learned that it had 75 jets and 43B rods. The decision was made to put 77 jets in and leave the rods alone. This made a vast improvement on casual driving and a much better plug reading (cruising reading, not a WOT shut it down reading).

After some nice cruising, I decided to give it a WOT rip, and it was decidedly better, but still had some lean popping, better than before, but still some. Before I get into my question, keep in mind that this has been a slow process on my end, I received this back from them in February of this year (2024) and I've been slowly working out some other bugs as well as this issue,
all while enjoying the car as well. I also want to reiterate, the carb looks amazing!

 So, here I come to you now with my question on a rod / jet change, but before I do that I want to mention that I have no issue with the work that was performed on this carb, it's just a matter of dialing it in. So here it goes-

 I know that there are richer secondary rods than the "AX" rod, But the "AX" is the richest "long tip" rod and seems to be the go to for performance rides. I also know that the primary rods / jets do a big part in WOT performance as well. So I now have a carb that performs well in a cruising mode, but  lean in a WOT scenario. Here is my question / thoughts-

If I am tapped out on secondary rods, and increasing primary rod /  jet combos can improve WOT, with 77 jets and 43 rods (3.204 area cruising and 4.125 area WOT), couldn't I change those to 79 jets and 46 or 47 rods? Couldn't this give me the same drivability cruising but more WOT fuel? 

Thoughts?

[Kenth]

I´d say go for it. With .120" upper and lower main airbleeds it´s not uncommon to see #79 jets (measuring .081-.082") combined with 45B-49B primary rods.

[Cliff Ruggles]

Two completely separate issues.

Right to start it the carb should NOT need smaller secondary metering rods.

If you are having issues at WOT, popping, going "flat" at high RPM's, etc, it's NOT going to be a secondary metering rod problem with AX or DA rods in it.  Most likely you are NOT keeping the carb completely full on a hard run.

To address that issue take a look at the fuel inlet seat diameter and see what size was installed?

Being a tad lean at light throttle doesn't require a jet change if it pulls fine at heavy part throttle.  Test heavy part throttle W/O the secondaries and if it's good to go there, which it should be, employ the APT system in the baseplate and/or change to smaller primary metering rods.  I much prefer to employ the APT system instead as it makes tuning in that range just a turn of the screwdriver vs pulling the top off the carb and changing parts.

I'd also be interested in who did the work on your carburetor?......

[kilowattkid]

Thanks Cliff. Sent you a PM

[kilowattkid]

I will check the needle inlet size, but the companies info states that a high flow inlet was used. As stated in the OP, it performs well in normal driving and even under medium to hard accelleration W/O the secondaries. And a very noticable improvement was made going from 75 to 77's. This improvement was even on the secondaries, it still had some pop, but lesser than with the 75's.

I think my thought process was that since it working so well on just the primaries, I was looking at a jet / rod combo with a similar area for cruising range and less area when WOT. Since it doesn't have an APT installed, I figured rods and jets were my only option. I don't think I want a smaller rod in the fat (cruising) area, just more in the WOT area. At least that is how my pea brain was looking at it.

On a side note, the timing is set at 18* initial, 32* all in at 33-3400rpm. With a cobverter that flashes to 3500, I'm fine with that all in RPM. I also have a vac advance that is manifold vac and adds 14*. All of this was set up on a distibutor machine and double checked when in the vehicle. It's very conservative and I'm very certain that advanced timing is not causing any of the popping or any detonation. The Pro-filer heads have what looks like a fast burn type chamber and 32* was about all I wanted to throw at it.

[77cruiser]

How much compression do you have? I run mine with Profiler heads @ 38 deg. 9.8-1 CR.

[kilowattkid]

10.5

[kilowattkid]

How much compression do you have? I run mine with Profiler heads @ 38 deg. 9.8-1 CR.

Do you run yours locked out at 38 degrees? Are you running a MSD box. This video shows a guy testing a locked out distributor with a CDI and shows a loss of like 10 degrees of timing at 5000 rpm.

https://www.youtube.com/watch?v=XN8NBCeKTFs

[77cruiser]

No 18 initial & 20 mech. in around 2500, HEI.

[kilowattkid]

Still trying to wrap my head around this before I go tearing into it, which is probably going to be after Christmas.

I get that a regular inlet seat could cause low flow issues and if that was the case I'd think that going from 75 jets to 77's would make the problem worse. It actually improved with going to 77's and the lean pop was lessened.

Digging deeper, this carb was for a 1974 SBC truck rated at 185HP and came factory with 75 jets and 43 rods. My 385 SBC I'd certainly think that it is at least double that and should be tickling the 400HP range.

From the paperwork provided they stated a high flow inlet seat and also a different primary power spring to match my cam. I can only assume that is correct. I just can't wrap my head around a 1974 stock rod / jet combo will work with a more performance oriented engine. Adding to my confusion is that my other carb I was running had 71 jets and 36b rods which is less area for fuel to get through.

BTW, this is the chart I was using looking at jet / rod combos - http://www.corvettefaq.com/c3/QJetTuningPaper.doc

[Cliff Ruggles]

Couple more things to add here.  We do NOT tune WOT fuel with primary jets.  77 jets are PLENTY for what you are doing as are AX or DA secondary metering rods.  Look elsewhere for your issue going to and at WOT.  Fuel inlet seat diameter, fuel delivery to the carburetor, ignition issues, etc.

I'd also take a look at the choke pull-off release time if you are experiencing this problem the moment you get into the secondaries.  Also look at the hanger to make sure the arms aren't bent up or down or both. 

This changing the main jets is NOT going to be the cure here.  Main jet size is adequate if you have good power and performance at heavy part throttle W/O the secondaries.  The later Chevy carbs with the huge MAB's do well with jets in the .075-.077" range for most applications.  I find however that employing the APT system in the baseplate and/or going to smaller primary rods really helps them out.

I'd also add here that we DO NOT tune a carb richer for a larger or more powerful engine.  Matter of fact all of these engines operate pretty much in the same range far as A/F goes.  You may have to add some idle fuel and idle bypass air for big camshafts, but richening up the carb clear across the load speed range isn't typically needed.  Matter of fact the better job you do on choosing the parameters for your new engine the LESS fuel it's going to need far as A/F goes and more efficient it will be.  These choices include the compression ratio, squish distance, squish area, cam events, head flow and combustion chamber shapes........