urban_cowboy

What do you think it means when the tune on a Holley double-pumper/HP carb results in the primary side jetting being higher/richer than the secondary jet in order to achieve a low 14 cruise and a mid 12 WOT? This has been the case with two different 750cfm carbs I have run and it just does not make much sense to me. Secondaries should need to be a few sizes richer usually. FYI, I am running a Edelbrock Air Gap intake, 6.5PV, and an MSD ignition. I measure the AFR by wide band O2 and confirm with plug reading. The idle vacuum is a little low (usually 11-12"). I can find little to no info on this phenomenon. I have wondered if it was due to the carb being too large, but a 383 at 7000rpm needs 750cfm. Any ideas?

67mustang302

Are you sure the secondaries are opening all the way? If they are, I'd suspect something is wrong, way too big of an air bleed up front or clogged metering circuits.

kalli

that indeed is very odd. the carb size seems bang on.

it can't be the PV in my understanding as if it kicks in at cruise already you would have the opposite effect (you'd end up with too small primaries). and not having them on in cruise is exactly what you want.
the power valve restrictor could be "too big", so maybe when you floor it the powervalve would dump so much fuel that with a lean back it ends up ok.

However 67m302 and cpr preached that changing the spread inthe carb is something you shouldn't do. One ofthe reasons being that if the front is too rich and the rear to lean, chances are very high that all rear cylinders are running leaner than the front although the AFR is bang on.

if the issue is sporadic it might be just accellerator pump, but I guess you measure in a long nasty pull :-)

i have no other ideas than to bring change the spread back to stock and move up/down from there until WOT is ok. this will end up with a lean cruise, but maybe you can sort that with the air bleeds? this on my side is now guessing. Curious what the others have to say

forgot to say: re-check float levels
maybe check as well if you can spot a difference on the plug (#1 cylinder vs #4), but that has to be immediately after a WOT run

urban_cowboy

The phenomenon happened with both my double pumper 750 and my 750HP. It is a mechanical secondary, so they are opening fine. The double pumper did not have air bleeds but the HP, I have the primary air bleed pretty rich, so the jets really cannot be any lower and still have below a mid 14 cruise AFR. The secondaries HSAB are about in the middle so I could go a little leaner but they will only allow me to raise the secondary jets maybe one number...basically to the same jet as the primary.

Maybe a characteristic of the intake and cam?

cprstreetmachines

Some info from a "know it all"

http://image.carcraft.com/f/9209479+...sfer_slots.jpg

This is how your primaries should look. The slot slot is covered just enough to be a square.
Anymore than that and you will start to draw on the jets at cruise, and you shouldn't be. Most of the fuel at a light cruise should come from the idle circuit. Put the primaries like that and if you need to raise or lower the idle, you need to open or close the back butterflies.

the PV should only open under a hard load or WOT.So you cruise are in the idle circuit, as you lean on it harder it goes into jetting, then the PV opens up.

Spend a weekend and do this for me. Put it all back as it was. Sounds like your idle circuit may be a little lean for you, but your trying to compensate with jets.

Set your idle. Check your light cruise If it's OK, lean on the cruise a little harder. Let it be 13.5, that's fine if it's not stumbling. If this is OK, then move to WOT. Remember the jetting you had in the front, that will still be your final jetting.
Go WOT and if it's lean, and jet front and rear equally.
Once that's done, your rear jetting will stay the same, return your front jetting to what it was. Now is when you drill your PVCR so you have as much fuel as you did with the bigger jets that worked for WOT, but you run the smaller leaner jets for putt-ing around.

urban_cowboy

Out of the box, I opened the secondary throttle plates a tad to keep from exposing too much of the idle transfer slot on the primaries (maybe 1/2 turn more open on the secondary throttle plate). Right now, the front and back throttle plates at idle have just a tad of the slot showing, so they are basically the same with relation to the slot. I will try closing the rear secondary throttle plate some and opening the front a little more like you suggested. My LSAB are fairly rich. I am running around a 75 or so front and back. With all four corners set about 7/8-1 full turn from seated, I have about a 14:1 idle. The PV basically does not open unless I open the throttle enough to get into the secondaries as the cruise vacuum stays around 15-17". Really the AFR is pretty consistent except for a crazy mismatch at WOT that I have not figured out (passenger side runs about 12.2:1 and the driveside shows 14:1)...only at WOT. Oh, and the fuel bowls are just a tad below the opening at idle.

I am trying to find what the 0-82751 Street HP cam with jetting wise out of the box. I have it written down somewhere. I think it was 72 primary and 84 secondary with the primary HSAB being like 27 and the secondary HSAB being like 38. If I remember correctly, I am running a 68 primary jet and 67 secondary with my current setup. I would have expected to run more like a 67/70, but with those jets, the cruise stumbles with being 15:1.

I will try closing the secondary throttle plate a little and see what that does.