Stumble

[Aron Gleason]

Yes. From what I have learned by
asking many a question, jet sizes are
dependent on avaliable air not HP.
The higher the altitude, the smaller
the jet.

That makes alot of sense, as I remember doing a cruise in Klamath falls one year. It was a pig.

 

[1STMP]

Once pump arm to adjustment screw
clearance is verified, take your car out
test for the bog. This of course, setting
the carb back to what it came with,
including the power valve.
Fuel delivery to the secondaries seems
to be the problem here, so I would
concentrate the search to accelerator
pump and squirter sizes. This is where
things get a little time consuming
as the bog is created by too little,
or too much fuel being called into
action. Small squirters deliver the
charge quicker, where larger squirters
deliver the charge more drawn out.
I would try a squirter (only in
the secondaries) 3 sizes smaller, then,
if that doesn't fix it, go 3 sizes bigger.
Anything over a #37 squirter will
require a 50cc pump diaphram
assembly. And one over #40 will
require Holley's hollow screw to
mount the squirter.
Which ever side of the range you're
on should tell you if you're heading
in the right direction.
The secondary accelerator pump nozzle must supply fuel for a sufficient time, so that the secondary main nozzles can begin to deliver fuel to the engine, after the secondary throttle plates are open. Otherwise this will cause the engine to bog.
Sorry for the wall of text. There's no
short version to explain it.

 

[Aron Gleason]

Once pump arm to adjustment screw
clearance is verified, take your car out
test for the bog. This of course, setting
the carb back to what it came with,
including the power valve.
Fuel delivery to the secondaries seems
to be the problem here, so I would
concentrate the search to accelerator
pump and squirter sizes. This is where
things get a little time consuming
as the bog is created by too little,
or too much fuel being called into
action. Small squirters deliver the
charge quicker, where larger squirters
deliver the charge more drawn out.
I would try a squirter (only in
the secondaries) 3 sizes smaller, then,
if that doesn't fix it, go 3 sizes bigger.
Anything over a #37 squirter will
require a 50cc pump diaphram
assembly. And one over #40 will
require Holley's hollow screw to
mount the squirter.
Which ever side of the range you're
on should tell you if you're heading
in the right direction.
The secondary accelerator pump nozzle must supply fuel for a sufficient time, so that the secondary main nozzles can begin to deliver fuel to the engine, after the secondary throttle plates are open. Otherwise this will cause the engine to bog.
Sorry for the wall of text. There's no
short version to explain it.

Well I do appreciate your long winded explanation. And I'll definitely do all that you suggested.

 

[Geoff 2]

You can f**k around with squirters, jets, PVs etc but you will never have a carb that performs flawlessly at all rpms. Suddenly going to WOT at cruise speed with the large venturiis of a 950 cfm carb is the problem; the fix is a smaller car. See below. The air speed is too low with the large venturiis, not enough to create the depression at the booster reqd to get fuel flowing.

The factory engineers were not stupid & they knew this.
Here are some examples when less is more. A 1958 Plym Bel ran 118 mph with a 3.15 diff weighing 3700 lb; a 1961 Dodge Phoenix, also 3700 lb, ran a 17.2 quarter, with 3.31 diff.
These were heavy cars & had the cast iron T'flite trans.
What is phenomenal about these #s is this:
- all showroom stock
- open diffs, not LSD
- 318 Poly engines
- 9:1 CR & tiny 2 bbl carb [ lucky to be 180 cfm ]

 

[1STMP]

You can f**k around with squirters, jets, PVs etc but you will never have a carb that performs flawlessly at all rpms. Suddenly going to WOT at cruise speed with the large venturiis of a 950 cfm carb is the problem; the fix is a smaller car. See below. The air speed is too low with the large venturiis, not enough to create the depression at the booster reqd to get fuel flowing.

The factory engineers were not stupid & they knew this.
Here are some examples when less is more. A 1958 Plym Bel ran 118 mph with a 3.15 diff weighing 3700 lb; a 1961 Dodge Phoenix, also 3700 lb, ran a 17.2 quarter, with 3.31 diff.
These were heavy cars & had the cast iron T'flite trans.
What is phenomenal about these #s is this:
- all showroom stock
- open diffs, not LSD
- 318 Poly engines
- 9:1 CR & tiny 2 bbl carb [ lucky to be 180 cfm ]

View attachment 1520776

This from the OP-
"But if I stab it from a light in 1st gear, it goes like a rocket. No stumble"

The OP also stated that no matter
what size carb he's installed, the
symtoms are the same. Though we
don't know if anything was done on
those carbs to help alleviate the
problem.
I've got 750 cfm vac sec carb installed
on an AMC 360. Mild cam and slightly
over stock stall converter. It weighs
in at just under 6200lbs.(Jeep CJ7).
Average median altitude of 4200 ft.
It hits passing gear at 70 mph, easy,
with instant response, w/3.55 gears
front and rear. According to your
analysis, I should be able to get
the same response from a 500
cfm 2bbl.
The OP also has a carb builder of
Hollies telling him that this 950
will work on his particular setup.
I doubt they would put their
reputation on the line if they wish
to stay in business.
Would you be willing to take your
debate to them, and let us know
what their response is?
I'm sure you'd like to save anyone
here from suffering the same fate
as the OP by way of wasting his $$.
Instead of knocking what he's ended
up with, put your talents to work.
What's it gonna take to get his setup
to work besides changing to a 2 bbl?

 

[Aron Gleason]

This from the OP-
"But if I stab it from a light in 1st gear, it goes like a rocket. No stumble"

The OP also stated that no matter
what size carb he's installed, the
symtoms are the same. Though we
don't know if anything was done on
those carbs to help alleviate the
problem.
I've got 750 cfm vac sec carb installed
on an AMC 360. Mild cam and slightly
over stock stall converter. It weighs
in at just under 6200lbs.(Jeep CJ7).
Average median altitude of 4200 ft.
It hits passing gear at 70 mph, easy,
with instant response, w/3.55 gears
front and rear. According to your
analysis, I should be able to get
the same response from a 500
cfm 2bbl.
The OP also has a carb builder of
Hollies telling him that this 950
will work on his particular setup.
I doubt they would put their
reputation on the line if they wish
to stay in business.
Would you be willing to take your
debate to them, and let us know
what their response is?
I'm sure you'd like to save anyone
here from suffering the same fate
as the OP by way of wasting his $$.
Instead of knocking what he's ended
up with, put your talents to work.
What's it gonna take to get his setup
to work besides changing to a 2 bbl?

Yeah I'm not buying into that. My carburetor guy is well known and a trusted source for when it comes to performance carburetors.

 

[1STMP]

Yeah I'm not buying into that. My carburetor guy is well known and a trusted source for when it comes to performance carburetors.

How about a progress report?
Have you had a chance to work
on the carb some?

 

[Aron Gleason]

How about a progress report?
Have you had a chance to work
on the carb some?

I put it back to the way it was when I got it, but haven't had a chance to take it out yet.

 

[Aron Gleason]

Update on my stumble. After putting the carburetor back to where it was when I got it, the stumble has improved. It only stumbles less then 2 seconds and clears up. But cruising my afr's are low 11's. So I think I need to drop down from the .073 pvcr's to maybe .071 or .068. After taking the car out it's definitely evident I was running too lean of a primary main jet.

 

[1STMP]

Update on my stumble. After putting the carburetor back to where it was when I got it, the stumble has improved. It only stumbles less then 2 seconds and clears up. But cruising my afr's are low 11's. So I think I need to drop down from the .073 pvcr's to maybe .071 or .068. After taking the car out it's definitely evident I was running too lean of a primary main jet.

Sounds like you're pursuing this
problem in the right direction.
Please keep us informed.
Thanks, Aron.

 

[Aron Gleason]

Sounds like you're pursuing this
problem in the right direction.
Please keep us informed.
Thanks, Aron.

I have the .033 squirter in the rear. I'm wondering, should I leave it or go to a .035? And I'm thinking I need to go bigger on the idle air bleeds. Not smaller on pvcr's.

 

[1STMP]

I have the .033 squirter in the rear. I'm wondering, should I leave it or go to a .035? And I'm thinking I need to go bigger on the idle air bleeds. Not smaller on pvcr's.

I would take this one step at a time.
I'd try with the jump in squirter size
and see how that responds.

 

[Aron Gleason]

I would take this one step at a time.
I'd try with the jump in squirter size
and see how that responds.

The reason I'm needing to go bigger on the iab is it's running low 11's on the afr' at normal cruise.

 

[1STMP]

The reason I'm needing to go bigger on the iab is it's running low 11's on the afr' at normal cruise.

What you're describing are two different issues.
iWhat may help your afr is a flat
bottomed air cleaner base with a
generous radius transition from base
to carb kinda like a velocity stack.
Saw this a while back on Engine
Masters, though I can't remember
what their actual gains were, it did
increase their afr's at cruise.
Your bog issue stems from either
not enough/or too much fuel at
cruise when you stomp it to the
floor.

 

[Aron Gleason]

What you're describing are two different issues.
iWhat may help your afr is a flat
bottomed air cleaner base with a
generous radius transition from base
to carb kinda like a velocity stack.
Saw this a while back on Engine
Masters, though I can't remember
what their actual gains were, it did
increase their afr's at cruise.
Your bog issue stems from either
not enough/or too much fuel at
cruise when you stomp it to the
floor.

Not enough fuel going to transition, as the afrs go lean

 

[Geoff 2]

I would put away the AFR meter for now & just use your senses for now. Some engines need to be either side of the stoic A/F ratio for best operation.
Opening the throttle & getting a fuel related stumble is nearly always fuel related: too lean. More air, but not enough fuel with the extra air.

 

[1STMP]

I would put away the AFR meter for now & just use your senses for now. Some engines need to be either side of the stoic A/F ratio for best operation.
Opening the throttle & getting a fuel related stumble is nearly always fuel related: too lean. More air, but not enough fuel with the extra air.

Thanks Geof 2. Couldn't explain it as
you did.

 

[Aron Gleason]

I would put away the AFR meter for now & just use your senses for now. Some engines need to be either side of the stoic A/F ratio for best operation.
Opening the throttle & getting a fuel related stumble is nearly always fuel related: too lean. More air, but not enough fuel with the extra air.

I understand what you're saying. That's why I look at my plugs.

 

[Geoff 2]

Plugs take time to colour. Because this problem is transient, ie it only lasts a couple of seconds, plug reading will not help.

 

[Jonas Nordstrom]

Just curious, do you have a pic of your set-up? Intake, carb etc