btwick

Hi,

Fighting the off-idle dead spot, and hoping for advice on next steps.

• Engine: 383 sbc, Comp XR276HR cam with 1.6 rockers
• Car: 71 conv, 336 rear gear, Th400, 2400 stall
• Electronics: HEI dist with MSD 6al box, VC302 vac adv can (providing 10* advance at 8-10” vacuum), Moroso HEI recurve kit.
  • Currently timed at 16* at initial, 35* total at 3200 or so. Recurve giving me ~19* centrifugal adv.
  • Showing 14” manifold vacuum in N, 12” in Drive.
  • Tried to follow Lars’ timing info as close as possible
• Intake: Edelbrock EPS, Quick Fuel Brawler 650 dp.

Carb started life with:

• 70 primary jets, 74 secondary jets
• 6.5 power valve
• 31 squirter nozzles
• 70 outer air bleeds (idle), 28 inner air bleeds (secondaries)
• Pink acc pump cams in #1 position
• 30cc acc pumps

Early tuning had me at ¾ turns out with all 4 fuel screws at idle, indicating rich, but always had stumble off idle. Richer I went with screws, the stumble reduced, leaner was more pronounced.


What I’ve done:

• Checked floats at just below halfway on glass
• Adjusted the acc pump arms to remove play
• Checked transfer slots, look appropriately “square” - .020” showing
• Put in 75 idle air bleeds to get fuel screws to be more 1-2 turns out
• Changed squirters from 31 to 35
• Primary jets stayed at 70
• Secondary jets from 74 to 75

Current situation with new AFR gauge:

• Bigger nozzles have not helped much
• When adjust idle to around 14:1 afr with fuel screws, stumble very pronounced
• When richen idle up to 12.0-12.5 with fuel screws, stumble improves. Sometimes dips into 11s (gauges bounce around a lot)
• Fuel screws with the new larger air bleeds are now at 1.25 turns out vs 3/4
• Mid throttle afr is 12.5 – 13.0 - which is my target
• Full throttle was 13.0 – 13.5, which is leaner than target. Will be swapping 74 secondary jets to 75s or 76s to get closer to 12.8 or so under heavy throttle

Next steps:

• Have ordered 37 nozzles
• With 37s, will need hollow nozzle bolts?
• With 37s, will need a 50cc accelerator pump kit? If so, both sides?
• Should I swap out my pink cams for something else? Know screw position 2 on the pinks delays the squirt, which sounds the opposite of what’s needed here
• If 37s don’t work, just keep going to 40s, etc.?
• Something else I need to check?

Thanks in advance! Hoping @Lars stumbles on this post.

Jebbysan

Add three to the primary jet.


Jebby

pspicci

i believe the #37 nozzle is the biggest you can go without the hollow bolts. the 30cc accelerator pump should be all you need.

dan1495

I fought that battle with my 72 LT-1 for years. 70 primary jets, 31 squirter and the blue accelerator pump cam solved my issues.

HeadsU.P.

The big 50 cc ACC pump will need a spacer under the carb. Not always easy with C3 hoods.
Not all 50 cc or 30 cc are used in one shot of full throttle. Only a portion is actually discharged.

Are you using the extended squirters or standard? Some people swear by the longer tube placing the fuel shot dead center of the venturi.
Was a steel washer installed under the squirters? And, there are check valves under the squirters we assume?
I believe the hollow squirter bolts come into play at size 35 and up. Not sure. Check Holley's web site.

I suspect the all-important gap between ACC pump arm and linkage is off a little.
Holley and others recommend 0.015. You may try 0.012 feeler gauge. Just don't go too tight and risk a rupture of the diaphragm.

They say a instant bog is lean fuel condition. A bog, then quickly picks up speed is too rich.

Go to YouTube and enter Holley carb stumble. Holley and Summit tech both have good vids on the subject.

resdoggie

Went through the same thing but what I suggested above worked for me. YMMV especially if you go bigger on the primary jets but it might need bigger jets all around. Your timing curve looks ok so do play with the carb tuning. One step at a time beginning with idle then hesitation then cruise then wot. Good to see you have the afr gauge to be able to dial in the carb.

kossuth

I would change the pump cam from the pink to either the orange or green and go from there. 35 on your squirters is too much I think. As another poster said stay 31 for now. Readjust your accelerator pump lever and I think it will run alot better. Also check your timing.
https://www.crankshaftcoalition.com/...p_Cams4777.jpg

ctmccloskey

I would listen to Jebby as He is DA MAN! (and he knows carburetors)....

btwick

Thanks to everyone who offered help. Fixed the problem. Did jet up the primaries per Jebby, but the real problem, and embarrassed to admit this, was the arm.

On this carb, when you get rid of slack between the pump and arm at closed throttle, it is pretty difficult to get a .015 gauge between them at WOT. I can, but really tight. Was worried about tearing the diaphragm at WOT, and left just enough play to get the stumble.

Watched some videos, and it does appear to be a tight fit. Fixed it and no more stumble.

Thanks again.

btwick

Realizing the issue of the pump arm being tight enough with no play at closed throttle but too tight with the arm at WOT is the shape of the cam. So, will be going to orange or green to see if it has more of a true .015 gap at WOT.

But have never found any info online about if they should match on primary and secondary side. You get one per color in a kit (~$40), so to get both the same color is a bit pricey.

Assuming no issues running an orange cam on the primary side and green on the secondary side?

resdoggie

You try one cam change at a time. Put the orange cam on the primary and leave the pink on secondary. Go wot and see see how it runs. I still have the pink on the secondary. Remember, only one adjustment at a time and evaluate. I would go back to the 70 primaries and see what the cruise afr is. You should be able to get 14:1 and maybe higher. Accelerator pumps and squirters are not at play at cruise and changing jet size to elimnate a hesitation may work but it's not the way to do it. The cams and squirters are for that. Primary jet size is for cruise fuel economy.

HeadsU.P.

btwick,
I am pretty sure both pump cam colors should match. I see no reason why the secondaries would-should-could have a different profile on a double pumper.

The next time you have it idling with the air cleaner off, take note of the shooters to see if they dribble fuel in the venturi. Sometimes with radical cams the vibration is enough to trigger that event. Hence, part of the reason for the 0.012 - 0.015 arm clearance.

At WOT, if you can still move the pump arm (0.012) then you are good to go w/o risk of damage to diaphragm.

resdoggie

Pump cams don't need to match. The jets don't need to be the same all around either. It's whatever combo works best but it will take a bit of trial and error to dial the carb in and get it close to running like an efi.

scottyp99

On a double-pumper, the secondaries go from all the way closed to all the way open at a faster rate than the primaries, so what would make you think that the accelerator pump shot should be the same as for the primaries?

Scotty

Jebbysan

This is true....
I am not sure how Holley is getting away with these carb models that have a 4 jet spread......like the Brawler...and the Street Avenger......the power valve is 8 jet sizes....so the jet spread front to rear should be close to that.
Every classic Holley has an 8 jet spread.....

To the OP....good job finding the issue.....

Jebby

HeadsU.P.

On a double pumper the primaries are open at 40 degrees before the secondaries open. The linkage is called "progressive" linkage".
This is street able set-up.
Using the other hole on the throttle linkage is called 1 : 1 ratio and is for drag racing only. Both throttles open simultaneously.
The secondaries do not open at a way faster rate.

For what its worth, if want just one color cam and don't want to spend $35 for another set, Ebay has single cams for $7.99 free shpg.

scottyp99

So, the secondaries open 90 degrees in the same amount of time that the primaries open 50 degrees. That's almost twice as fast. Twice as fast isn't way faster?

Scotty

HeadsU.P.

I don't think you understand progressive linkage. Which is what Holleys come with.

scottyp99

What is there to understand? When the primaries are 40 degrees open, the secondaries are closed. When the primaries are 90 degrees open, the secondaries are also 90 degrees open. How can that happen unless the secondaries open at a faster rate? Magic?

Scotty

HeadsU.P.

I am referring to over-all throttle action. From idle to WOT. Primaries open then LATER the secondaries. So to me, that is not a faster rate. That is a delay.
What you are referring to is ratio, not rate.