More carb tuning problems.

[Zoom This]

Jake...there's no need to balance your idle speed screws for the pri and sec. throttle blades. You just need to open the sec. side just a tick to allow fuel to draw from the fuel bowl. If your plugs still look too lean after a good drive at all engine speeds, bump you jetting to 75's on the primary side. No need to bump up both pri and sec., since most of your driving is on the primary side. To sort this all out, I recommend some chassis dyno time and focus on your part throttle and full throttle A/F ratios. Ya gotta get that right first, HP and torque will follow a good A/F ratio. If you can get somewhere around 13.5-14.0 A/F at part throttle on the chassis dyno, and 12.5 - 13.0 at WOT, your in the ballpark. These two measurements will tell you if you need to bump up the primary and/or secondary jets too.

[Excaliber]

I was able to cure this problem by using more BASE TIMING advance than I really wanted to. Like 18 degress. I had to recurve the distributor so my TOTAL advance was not to high at cruise rpm.

[niles]

Jake;
The first and wrong thing to do is start messing with the carb; especially when you said it was running great before the hardware change.
Ref; foursecsflat.com.???spelling ("don't touch the carb before getting the ignition RIGHT!!!)
You have been silent on that.
I would suggest you put the carb back exactly like you had it when it was running good and get the ignition right.
You need inital around 14-18 deg. total about 35 and all it before 2500 rpm.
Go to the tech section and plug reading and get you ignition right; only then start messing with carb. If your ignition is'nt right for your eng/hardware you will chase your tail for years. After 2 yrs of "bad carbs" 3 different one etc, almost all my problems disappeared after boosting the intial up to 18 deg. The whole motor changed personality and the carb would adjust like the books said.
Been there and screwed it up
gn

[Humbucker]

Originally Posted by niles

Ref; foursecsflat.com.???spelling ("don't touch the carb before getting the ignition RIGHT!!!)
You have been silent on that.

Yes, I should have mentioned the timing. Sorry 'bout that. It's an MSD ignition with the two small silver springs and black bushing. My total timing is 32 deg with initial being about 14. It's all in by 2400rpm. We made sure the timing was right before ever messing with the carb.

[scottj]

Spark plug readings may fool you into thinking you're lean when you really aren't. With an MSD or other high-energy ignition it takes much longer to develope any color to the plugs. I can run on the track for a whole weekend and the plugs come out bone-white when I'm jetted 1/2 a jet rich. To properly read a plug you have to cut the threaded part off exposing the entire porcelain and "read" the very base of the porcelain.

[JAM1775]

I differ with some of the opinions especially that involving timing change or even to the long end changing the entire curve. . Since all you changed has to do with the air fuel charge you should be able to get your motor running crisp after ensuring your timing is exactly where it was before. Of course, once you iron out the kinks the better heads might like a degree or two of less timing on the dyno for that extra 6-8-10rwhp.

I still think 73F/78R is small jetting for your 392 and although you may get a crisper thottle response from the 650 you can tune the 750. For all we know your 750 may have larger than normal annular booster which would enhance fuel signal snap. I run a 11:1 351W with a roller cam at .576 lift , 750DP custom built. The throttle anywhere above 2800rpm is neck snapping , it idles steady even when at 130 degrees heating up and never stumbles on deccel.
As Holley would explain the nomrla spread in arounf8. therefore, I'd see no problem bumping up your rear jets to 81R....I think you'll see it rise up and roar at top end.

I'd want to get back where you were with this carb and this distributor where they are. Can you try bring all four corners out one turn and trying the idle screw for adjustment ? 1/2 turn on the rears seems too lean. The holes in your plates are okay, they let air in even when your plate is almost or completely closed. Yes...they can lean you out BUT you had it purring before with less motor ! Another trick is easy. Call the Holley tech line for direction first. the number is on their website. But.....you have 8 air bleeds which look like little jets all around your carb body. do not touch the 4 insides as they are for top end. You should not mess with those w/o and AF reading on a dyno. But...the 4 outer air bleeds are for idling. They can easily be removed . Go 2 sizes larger all around and you let MORE air in at idle. these air bleeds bypass the normal fuel pulling circuit thru the main body. By go larger you will intro more air at idle "under" the throttle body , leaning out the idle mixture allowing you to turn out your 4 corner screws adding fuel that way while allowing have more fine tune capability with the 4 corner screws. These 4 outside idle Air Bleeds do not effect by front or rear main jetting. They do however effect idle air/fuel mix and slighty lesser as rpm climbs near 2K.

But first ? Verify timing, PUT A VACUUM GAUGE ON YOUR MOTOR.
See where you you pull vac at idle , bring out your 4 corners to one turn each. Try to start and use the idle screw to get close to best idle.
Using tach and vac gaugeught to get you where you fatten or lean out until you see your highest vac pull, using the tach to keep your rpm near where you want it. 4 corner adjustments should be doen at 1/16th to 1/8 turn increments all around, rev the throttle , let it settle and examine the results. If you're going 1/2 turn you're going too fast ! Good luck and I hope one of us gets you to settle that bad boy down !

[Frank Messina]

Jake,
It appears you're closer to sorting this thing out than you might think. I'm not going to muddy the water but just clarify a couple points you raised. Drilling the throttle blades: This is usually only done when the engine has a really big cam (relatively speaking) and you have trouble getting it to idle properly without opening the throttle blades so far that they are now uncovering the transition slots which results in a huge off idle stumble. Drilling the throttle blades introduces air into the engine and allows the throttle blades to remain in their correct position. I'm not saying it's hurting you, just what it does.
Idle mixture screws: No they don't have to be the same all the way around. Starting off 1.25 turns out on the primary and .5 on the secondary is fine. Give the motor what it wants.
No Power Valve: If you run no power valve on the secondaries you need to jet up 9 -10 jet sizes to compensate for the lack of PV enrichment. So if you're 73 on the front you'll need to be somewhere around 83 on the rear, maybe more. I'm not going to make a prononcement here and say you should leave it out or put it back in as this is up to you. I've run my dragstrip cars launching at 7500 RPM's with, without and everything in between and you can make a case either way. For the street though, I prefer to run with a secondary power valve.
Jets: I might be off one number here but you're carb came from Holley with #72 or # 73 jets all the way around. That's you're base. You won't stray to far from there. HTH
Frank