Need help with Holley carburetor tune.

[338Raptor]

Need some opinions on what to try next.

To begin, I think my accelerator pumps (both 50cc, yellow and brown cams) and nozzles (both 40’s) are sized right. When I quickly smash the throttle 1/2 way and all the way my AFR stays at about 13 to 1.

At WOT my AFR is a bit rich at 12.2 to 1. I’m not concerned about correcting this until I get my tip in problem resolved (see below).

Details about the carb: 
780 double pumper
73 primary, 78 secondary jets
4.5 power valve.  But I’ve also tried 5.5 and 6.5 with similar results.
PVCR’s drilled to .0625”
10” vacuum at idle, transition slots are not uncovered. Idle = 1000rpm
Burning non-ethanol premium fuel
Float levels are set to mid window. I started with level at bottom of the window and raising it midway did help a bit.

Motor details:
10:1 compression.
Iron 427 SOHC 4V (500ish Horsepower)
Close to stock 1967 NOS cam specs. (Crane says operating range is 2500-7000 rpm)
2800 lbs street car

Here’s my problem:
The engine goes lean (16-17 to 1) when slowly increasing RPM at about 1/4 throttle and doesn’t richen up until I quickly press into the secondaries.  If I try to slowly increase throttle beyond 1/4 throttle the lean condition causes snapping and popping (bad noises).   
Anything less than 1/4 throttle (cruising) is a bit too rich (12-13 to 1).
When I go WOT I’m a bit too rich (12.2 to 1) I can easily fix this but I think I need to focus on the 1/4 throttle “too lean” issue first.

My first inclination is to increase primary jets and decrease secondary jets.  If I do this I’ll end up with close to the same size jets at all 4 corners with a power valve on the primary side only.  Does this sound normal? 

Should I be looking at increasing or decreasing emulsion jets?

[Joe-JDC]

First, this is just my opinion after working with Holley carbs for the better part of 50+ years.  A 780 cfm carb does not need 50 cc pumps for the street, nor does it need  40 squirters.  You are causing the jetting to be totally wrong by covering up the off throttle with the large squirters and pumps and not getting the carb metering blocks to work properly.  By running rich with the squirters and pumps you are having to lean the carb down too much, which is causing your lean condition at part throttle.  I would prefer to start with gong back to 30cc pumps, 31 squirters, and 72/82 jetting and work from there to find a good street tune.  Power valves should be a part of the final tune for street.  Joe-JDC

[fordman460]

Similar to Joe. Find the specs for the list number and get it back to std. Tune from there. Make sure your ignition timing is what it needs to be first

[Cyclone03]

Add a vacuum  gauge to your tuning kit,you’ll be amazed what you can learn about the carb circuits your running in.

If you want to play with some math, calculate the area of the PVCR and Main jet area and add them together. The total should be close to the area of the secondary jet.

That said your slow rolli in lean condition,I’m assuming your just rolling in here, that is working the Idle feed restrictor. To test richen the idle mixture and test again,it also is working the transition circuit .

Double pumpers struggle in this area when trying to tune for economy and day to day drivability, adding (dumping) fuel in this zone is the normal fix.

I would remove carb and clean/blow out every passage if the air bleeds are dirty that causes all kinds of weird problems,remove mixture screws too if they are ridged your going to have trouble.

You may end up with another carb to really tune that zone.

I read years ago that the last great tuned Holley carb was the OEM 780 on the 73 ‘Vette Tuned for drive ability ,not necessarily WOT. I once tuned a BG Demon 850 vacuum carb with the old 780 primary bleed and channel restrictions setting as a starting point and ended up with  433 FE that got 18mpg at 70-80mph 2400-2600rpm in my Mustang. My current FI set up gets 16 at best.

We’ve had good luck with Quick Fuel carbs but I fear Holley has bought them so things might have changed.

[Drew Pojedinec]

Lance, 780’s have different venturi size for the secondaries, so calculating area doesn’t really work for this particular carb.

[My427stang]

Lance, 780’s have different venturi size for the secondaries, so calculating area doesn’t really work for this particular carb.

Certainly can calculate the area versus the original specs though, given a LIST number and before and after PVCRs

That being said, agree with all above, no need for 50cc and sewer pipe squirters. There are a few 4150 780s, so if you post your LIST number we can give you original specs to start from. My gut says dual 30cc pumps on the street, blue cam on the front if you need some oomph and 25s for squrters, 5.5 or 6.5 PVsr

I will say this, although I think the carb needs some un-loving.  It doesn't seem that crazy that the engine would go backfire and popping lean.  Are you 100% sure no crossfire, crossed wires, vacuum leak, tight intake valve, etc?

Also, is it a backfire through the carb (lean), or bangs in the primary pipe (rich)

I think I would run the valves, look at all the wires and plug insulators, double and triple check firing order, and be very sure you don't have a big vacuum leak somewhere, because although the ports are big on a SOHC, your build shouldn't need a crutch like that. 

Another option would be to see if some buddies good running carb does the same thing without messing with anything other than idle speed

[338Raptor]

Lance, 780’s have different venturi size for the secondaries, so calculating area doesn’t really work for this particular carb.

Certainly can calculate the area versus the original specs though, given a LIST number and before and after PVCRs

That being said, agree with all above, no need for 50cc and sewer pipe squirters. There are a few 4150 780s, so if you post your LIST number we can give you original specs to start from. My gut says dual 30cc pumps on the street, blue cam on the front if you need some oomph and 25s for squrters, 5.5 or 6.5 PVsr

I will say this, although I think the carb needs some un-loving.  It doesn't seem that crazy that the engine would go backfire and popping lean.  Are you 100% sure no crossfire, crossed wires, vacuum leak, tight intake valve, etc?

Also, is it a backfire through the carb (lean), or bangs in the primary pipe (rich)

I think I would run the valves, look at all the wires and plug insulators, double and triple check firing order, and be very sure you don't have a big vacuum leak somewhere, because although the ports are big on a SOHC, your build shouldn't need a crutch like that. 

Another option would be to see if some buddies good running carb does the same thing without messing with anything other than idle speed

List: 14936
The guy I bought the car from said it’s a 780. That may be incorrect.  I thought List numbers started with an R, this one doesn’t.

1. No vacuum leaks unless the one vacuum port hooked to the PCV valve counts. 
2. Firing order is correct. It runs great otherwise.
3. Timing is at 29 total.  Maybe it’s coming in too early?
4. I know the 50cc pumps and 40 squirters are big but watching my wide band the AFR stays perfectly steady at approx 13 to 1 when quickly smashing the throttle.  It spiked 18 to 1 for 1/2 a second with the 30cc pump.
5. The snapping/popping noise occurs when AFR hits 18 or 20 to 1, not when rich. 
6. Carb has been disassembled, cleaned, holes blown out etc. I’ve done this 2x. 
7. Motor runs good at WOT and Cruising and smash throttle hard acceleration.  It runs super lean when slowly easing into the throttle under light load (when the accelerator pumps aren’t a factor).
8. I haven’t checked valve lash. I need to do that anyway. I’ll do that next.

[BattlestarGalactic]

29* total timing(that includes mechanical advance?).  Why only 29*? 

[428 GALAXIE]

29* total timing(that includes mechanical advance?).  Why only 29*?

Cammer?

[338Raptor]

Yes. Only 29.

1967 iron Cammer.

[BattlestarGalactic]

Oops, sorry, skimmed over the details a bit too fast I suppose.

[My427stang]

Lance, 780’s have different venturi size for the secondaries, so calculating area doesn’t really work for this particular carb.

Certainly can calculate the area versus the original specs though, given a LIST number and before and after PVCRs

That being said, agree with all above, no need for 50cc and sewer pipe squirters. There are a few 4150 780s, so if you post your LIST number we can give you original specs to start from. My gut says dual 30cc pumps on the street, blue cam on the front if you need some oomph and 25s for squrters, 5.5 or 6.5 PVsr

I will say this, although I think the carb needs some un-loving.  It doesn't seem that crazy that the engine would go backfire and popping lean.  Are you 100% sure no crossfire, crossed wires, vacuum leak, tight intake valve, etc?

Also, is it a backfire through the carb (lean), or bangs in the primary pipe (rich)

I think I would run the valves, look at all the wires and plug insulators, double and triple check firing order, and be very sure you don't have a big vacuum leak somewhere, because although the ports are big on a SOHC, your build shouldn't need a crutch like that. 

Another option would be to see if some buddies good running carb does the same thing without messing with anything other than idle speed

List: 14936
The guy I bought the car from said it’s a 780. That may be incorrect.  I thought List numbers started with an R, this one doesn’t.

1. No vacuum leaks unless the one vacuum port hooked to the PCV valve counts. 
2. Firing order is correct. It runs great otherwise.
3. Timing is at 29 total.  Maybe it’s coming in too early?
4. I know the 50cc pumps and 40 squirters are big but watching my wide band the AFR stays perfectly steady at approx 13 to 1 when quickly smashing the throttle.  It spiked 18 to 1 for 1/2 a second with the 30cc pump.
5. The snapping/popping noise occurs when AFR hits 18 or 20 to 1, not when rich. 
6. Carb has been disassembled, cleaned, holes blown out etc. I’ve done this 2x. 
7. Motor runs good at WOT and Cruising and smash throttle hard acceleration.  It runs super lean when slowly easing into the throttle under light load (when the accelerator pumps aren’t a factor).
8. I haven’t checked valve lash. I need to do that anyway. I’ll do that next.

That LIST number doesn't match anything in my books, even with or without an R

As far as those accel pumps, don't let yourself be talked into that they are helping.  Needing that much, you are likely not getting any transition fuel.  Maybe the wrong gasket on the base plate between it an the throttle body?  Could be blocking transition fuel.  The 50cc pump combined with big shooters are a symptom, not a solution.

In the end, when you get that carb right, the accel pumps just need to give a tickle, maybe 1/2 of what you are jamming down there now.  All your behavior seems to indicate an issue with the transition from idle to main jet.  Need to figure out why that is happening before you just bathe all those parts in fuel

[338Raptor]

Now that makes sense.  I’ll check my base gasket.
I don’t think my transition slots are providing fuel. 

[Cyclone03]

Like Ross said, the transition circuit seems to not be working.
Has this carb been drilled ?

If it has been “worked” you might save your sanity by punting that carb for a new one.

Having only touched a non running SOHC I don’t know what they want for a carb.

[Drew Pojedinec]

Certainly can calculate the area versus the original specs though, given a LIST number and before and after

Sorry, should have been more clear. Just mentioning that primary and secondary have different requirements.