Carburetor stumble update

[Aron Gleason]

So I had a chance to take the car out today. Cruising along and stab the throttle it still stumbles like a blubber but once it catches up. It gets up and goes. But if I'm cruising and dont stab the throttle, but do it in a way that opens the throttle somewhat quick it doesn't stumble. I have 37 squirters in front and rear. Just wondering if I'm lean on the secondary main jets and if I need to go back up on the power valve jets?

 

[Aron Gleason]

Took the car out for a nice cruise. The stumble is almost gone when I stab the throttle at anywhere from 35mph on up. If I'm at around 15mph and under, it still has a huge stumble. Squirters are 37's, pump cams are white. Any suggestions are greatly appreciated.

 

[hunt2elk]

It's always a trial and error with cam and squirter selection. Just moving from #1 to #2 hole on a cam will usually make a noticeable difference. I always play around with the cams first and then fine tune with the squirters.

 

[Geoff 2]

Specs needed again....I no longer mind read!

 

[Aron Gleason]

950 double pump. 75 primary jets. 84 secondary jets. 37 squirters. White pump cams both in #1 hole. 30cc accelerator pumps.

 

[Aron Gleason]

so the last time I took the car out it was performing a lot better. At 35 mph and above the stumble only lasts a second before it clears up. Anything below that it takes a little longer to clear up. I tried adjusting the primary pump arm only to realize there wasn't any way I was going to get it adjusted cause I had put the cam in #2 hole. It's now in #1 hole, both cams are white with 37 squirters. Should I try it like that or go with a different cam?

 

[RemCharger]

Pump arms are adjusted nice and tight?

 

[Aron Gleason]

Yes

 

[Budnicks]

Could be a lot of scenarios

Ther rear squirter/accel pump nozzle
is usually smaller than the front, by several sizes
rears maybe dumping way too much on tip in
(you said it didn't do bad when sort of eased into it)
& you got a lot of jet in the rear, 9 jet sizes is a bit much
when 6 is about normal

I haven't run a dbl pumper in a while, just 6bbls 2300's now
but that jetting seems a bit off to me
tuned my share of them on my cars over the decades

I've had decent luck when I did run Dbl pumpers
using the brown cam on #1 hole, if it needs more than go to #2
a bit quicker tip in more aggressive


maybe
look down the carb & see
take the air-cleaner off & work the throttle, see if the dribbling out
or a nice stream/atomization
may have crap in some of the orifices, todays crappy gas & additives
it happens a lot, if it sits for any real time

or
the float levels could be low 'or too high too'
(dribbling into/out of the venturis),
again shitty gas & sitting a lot not driving it doesn't help it
high or bad needle & seats, making it get fuel when it's not ready
(dribbling in),
check them also while running
adjust (or replace) as needed,

or
maybe low fuel pressure too, rear bowl isn't kept full
depends on what kind of pump (or style of a regulator) on what you can do there
(I usually adj. the floats, usually so they are flowing out, not just below to opening,
don't go too crazy thou, 1//8 of a turn like 1 flat on the needle & seats)

I'm lucky to get 3 - maybe 4 years out of a needle & seat lately,
good quality Holley Vitan
a lot of crap gas/corrosives or additives, winter & then summer formulas
I replace all mine at the same time

even bad gas can cause issue, as described

read the plugs, could be ignition timing aiding to it'

or
you can also use the air bleeds to fine tune it, if it had adj. air bleeds
'go bigger to lean out, smaller to richen' it's air you're controlling

or
the emulsion in the metering blocks, if they are adj. too
same as air bleeds deal
bigger leaner, smaller richer, your controlling 'the air' not fuel

or
it's a whole lot more than just power-valves & jets,
Generally rule of thumb, but not set in stone
use a power valve just above 1/2 (numerically) of the best vacuum you have
so if it's like a 12 in.'s, go to a 6.5 or a 7, power valve,
you don't need the hi-flow 4 port opening either
not for a street car, they can cause dumping issues too

Dave Vizard's book
'How to Supertune & Modify a Holley carburetor'
SA216 Car-tech books, is a great source

good luck

 

[Aron Gleason]

Could be a lot of scenarios

Ther rear squirter/accel pump nozzle
is usually smaller than the front, by several sizes
rears maybe dumping way too much on tip in
(you said it didn't do bad when sort of eased into it)
& you got a lot of jet in the rear, 9 jet sizes is a bit much
when 6 is about normal

I haven't run a dbl pumper in a while, just 6bbls 2300's now
but that jetting seems a bit off to me
tuned my share of them on my cars over the decades

I've had decent luck when I did run Dbl pumpers
using the brown cam on #1 hole, if it needs more than go to #2
a bit quicker tip in more aggressive


maybe
look down the carb & see
take the air-cleaner off & work the throttle, see if the dribbling out
or a nice stream/atomization
may have crap in some of the orifices, todays crappy gas & additives
it happens a lot, if it sits for any real time

or
the float levels could be low 'or too high too'
(dribbling into/out of the venturis),
again shitty gas & sitting a lot not driving it doesn't help it
high or bad needle & seats, making it get fuel when it's not ready
(dribbling in),
check them also while running
adjust (or replace) as needed,

or
maybe low fuel pressure too, rear bowl isn't kept full
depends on what kind of pump (or style of a regulator) on what you can do there
(I usually adj. the floats, usually so they are flowing out, not just below to opening,
don't go too crazy thou, 1//8 of a turn like 1 flat on the needle & seats)

I'm lucky to get 3 - maybe 4 years out of a needle & seat lately,
good quality Holley Vitan
a lot of crap gas/corrosives or additives, winter & then summer formulas
I replace all mine at the same time

even bad gas can cause issue, as described

read the plugs, could be ignition timing aiding to it'
View attachment 1675140

or
you can also use the air bleeds to fine tune it, if it had adj. air bleeds
'go bigger to lean out, smaller to richen' it's air you're controlling

or
the emulsion in the metering blocks, if they are adj. too
same as air bleeds deal
bigger leaner, smaller richer, your controlling 'the air' not fuel

or
it's a whole lot more than just power-valves & jets,
Generally rule of thumb, but not set in stone
use a power valve just above 1/2 (numerically) of the best vacuum you have
so if it's like a 12 in.'s, go to a 6.5 or a 7, power valve,
you don't need the hi-flow 4 port opening either
not for a street car, they can cause dumping issues too

Dave Vizard's book
'How to Supertune & Modify a Holley carburetor'
SA216 Car-tech books, is a great source

good luck

So you're saying to try a brown pump cam on the rear?

 

[Geoff 2]

I do not think there is a pump cam made that is going to solve this problem.......
What heat range are the spark plugs?

 

[Richard Cranium]

Specs needed

Here's 11 specs for you.

 

[Aron Gleason]

I do not think there is a pump cam made that is going to solve this problem.......
What heat range are the spark plugs?

Ngk 6

 

[Budnicks]

So you're saying to try a brown pump cam on the rear?

I'd do 1 change at a time & see what helps

I'd start with accel pump discharge nozzle/squirter sizes, smaller in the rear
maybe go down 2-3 jets sizes in the back
check float levels
check & see it it's dribbling or a good stream etc.
1 at a time, unless you find something obviously wrong

reading the plugs
do 1 thing at a time
can give you a good inclination what's going on, which way to go next
some ignition timing may help too
an extended tip plug, may help too, get the spark deeper into the void
(if you have room, it's a street car I'd assume it's not max compression)
& a NGK 6 is pretty hot already (bigger # colder on NGKs, like 7,8,9 are colder)
it' all depends on what's going on in the combustion chamber
read the plugs refer to the meme I posted, it works

'brown cam on the rear', no not necessarily'
you need to find what yours likes
(depending on spec.s or build)

like mentioned above there's no accel pump cam, that will cure it completely
it's a combo of issues, like listed

 

[Aron Gleason]

I swapped out the .035 ifrs to .033 ifrs. Now the stumble is worse. And my plugs still show it's running rich.

 

[Geoff 2]

I use NGK 4s. So does a friend with a 490 stroker, 10.5:1 CR. Our engines idle smoother & run better. Zero detonation.

 

[Geoff 2]

What size engine?

 

[Aron Gleason]

440

 

[Geoff 2]

Aron,

A 950 DP is never going to work stabbing the throttle at 15 mph. There is not enough air speed through the carb & the engine needs to be about 100 cu in bigger....

 

[Aron Gleason]

Funny you say that. Cause I have the stumble almost gone. Now it only happens for a split second.