Holley Tuning Help Please

[btwick]

Hi,

Fighting an off-idle bog and hoping for advice on next steps.

• Engine: 383 sbc, Comp XR276HR cam with 1.6 rockers
  Car: 71 Corvette, 336 rear gear, Th400, 2400 stall
  Electronics: HEI dist with MDS 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.
  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 in 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 or so stumble improves, but still there. Sometimes dips into 11s (gauge 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!

[steve cowan]

Hi,

Fighting an off-idle bog and hoping for advice on next steps.

• Engine: 383 sbc, Comp XR276HR cam with 1.6 rockers
  Car: 71 Corvette, 336 rear gear, Th400, 2400 stall
  Electronics: HEI dist with MDS 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.
  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 in 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 or so stumble improves, but still there. Sometimes dips into 11s (gauge 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!

You should not really need bigger than 31 squirter for your application, there is one thing I have found over the years is alot of carb parts especially squirters are under size in specification, I have had 31 squirter that measured 0.022" so I pin gauge everything,.
I would try more initial timing first.
With your vacuum advance connected do you have 26 degrees at idle??
When you hit the gas,manifold vacuum is reduced so will your timing and I don't think 16 degrees is enough.
The factory carb settings really won't be out that much in my opinion.
Use your afr gauge in conjunction with looking at spark plugs, if they are rich that leads to further problems as in oil contamination etc.
If the timing does not fix or help you can look at putting adjustable transfer slot jets in the main body and possibly moving IFR in metering blocks but that's a different conversation.

[skinny z]

I can agree with Steve on the squirters usually not being the culprit.
Good question regarding how the vacuum advance is plumbed in.
Torque sag is not unusual when connected to full manifold vacuum.
I've had similar engines and they easily handle 20+ degrees of idle timing. Manifold vacuum advance (in my case) added another 14-16 at idle for a 34 or so total. I had to severely limit the available mechanical spark lead so as to keep WOT timing less than 34.
Smooth as glass at tip in or anywhere else in the rev range.
Mostly street at that time with weekend drag racing duty.
All that timing gives a boost to the MPG as well.
I've learned also that tuning with an AFR gauge for idle mixture typically doesn't yield the results you might think. I've not managed a stoich idle that way. I tune for highest vacuum and the AFRs are always on the rich side.

[rebelrouser]

On the squirters I find on a lot of new carbs the spring on the pump arm is too weak. With the engine off smack the throttle quick and hold open, watch the lever arm, you should see the spring compress and then expand as the diaphragm empties. I usually fix them by adding a couple small washers under the spring. You can't get it too stiff or you may bend the lever arm. You also will notice how much larger the stream is at the squirter with out a bigger nozzle, as well. I always just drilled them, a little extra pump shot does not affect performance, .038 is where I usually start.

[HQM383]

What model Holley?

What is the IFR size?

Going up to .075” iab got idle screws out more but leaned transition slot and slightly reduced how far transition slot carries. Like Steve said . 031” squirter should be enough - even .028” when tuned right. I have 650 through to 1000cfm on a 383 and none need bigger than .035” squirter or orange cam. Also hook up a vac gauge and see what the vacuum does off idle. You may need another timing strategy.

[bobmc]

use the blue pump cam on the primary

[BobbyB]

I agree with Steve & Skinny… start with timing first. I have a 650 Brawler vac secondary on a 347 ford with 14 in vacuum in park, 12 in vacuum in gear, 2800 rpm stall…so, same as yours.
I ended up with limiting centrifugal advance limited to 10 degrees, 22 degrees initial, and vacuum advance connected to ported on the carb, tuned for no spark knock on hills. (Try vac advance both ways, it’s too easy not to on the carb you have). Many people have strong opinions about which is best…I say try both and use what you like best.
I would not change much on the carb until I got very close on the timing.
My AFR reading are:
Idle at 650rpm in gear 12ish to 1
Cruise 15.5 to 16 to 1
9 power valve open 13ish to 1
WOT 12.5 to 13 to 1
Good Luck

[skinny z]

My AFR reading are:
Idle at 650rpm in gear 12ish to 1
Cruise 15.5 to 16 to 1
9 power valve open 13ish to 1
WOT 12.5 to 13 to 1

Those numbers look very familiar.
With that 12:1 idle AFR, I'm curious as to your cam overlap. (Seat to seat if it's a hydraulic.)

[F-BIRD'88]

Diz curve:. This cam;. Wants 22 to 26 ° base initial timing.
Shorten mech curve to 10° . Eg: 24 base 34 max WOT.

Use ported vaccuum on the carb.
Carb IAB likely too big.
(off idle lean spot as yiu drive low speed.
Solder up Pri IAB and redrill .062" 1/16".
Now it will be a bit too rich shift off idle as you open the throttle (AFR gauge)
So hone the IAB hole a bit bigger by hand with the drill bit and test... Idle afr set to best idle (12:1 ish afr)
The part throttle shift as you open the throttle should not drift leaner than say 14.5:1 afr.
IAB final custom size .064" to .068". approx ±

[RCJ]

I would put a .067 iab in and if it helped I would go smaller. Wouldn't worry about afr.

[btwick]

D

So hone the IAB hole a bit bigger by hand with the drill bit and test... Idle afr set to best idle (12:1 ish afr)
The part throttle shift as you open the throttle should not drift leaner than say 14.5:1 afr.
IAB final custom size .064" to .068". approx ±

Thanks to everyone for the help. Will be fiddling with it this week and report back.

BTW.. F-Bird, my assumption for afr was idle should be around 14:1 (close to stoich), light throttle crusing (out of idle circuit and in primaries) should be 13-14, and heavy throttle (fully into secondaries) should be around 12.5 for max torque.

Isn't idle at 12:1 or so a bit fat?

[F-BIRD'88]

No, idle on a carbed engine will always be richer than 14.7:1.. 12:1 ish is typical.. Always adjust curb idle scews for best idle quality and vacuum..
not a specific AFR.
But you do use the AFR guage to check the off idle transition afr shift (IAB). .

Remember at idle allways give it what it wants.

You have to limit the mech advance travel stop.
10° ish travel.
Any all exhaust leaks will throw off the AFR guage.

[btwick]

But you do use the AFR guage to check the off idle transition afr shift (IAB). .

Thanks F-Bird.. Did notice car likes 12.5 at idle for best response, which was confusing and why I asked. But, when driving and at that lightest throttle cruising/coasting it does go very lean.. before more pedal gets it into the primary jets. So that's the idle air bleeds? I did take it from the stock 71s airbleeds to 75s as my fuel screws were only 3/4 turn out, and thought more air would help get me to 1-2 turns out. Would that cause the lean transition on the afr gauge? Thx.

[F-BIRD'88]

Yes the pri IAB need custom tuning now.
Will be smaller than .070". start at .062" hone to final IAB size. Final drive test with air cleaner on... It matters.

[1980RS]

Look up Thompson power plate, that little piece of brass plate that goes under the squirter really works well at getting rid of stumbles. The tow I got were on a QF 750 and that's why I bought the carb when I spotted those plates on there. Put one on my street car that had a hesitation off idle, installed the plate, stumble completely gone for me.