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Old 08-17-2008, 10:01 PM
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Edelbrock stumble

Hey, folks! I have to run this by everyone here because I'm stumped. I have a ZZ4 with the GM HOT cam, performer RPM manifold and an Ede 750. Runs strong as can be, except for when I just blip the throttle off idle I get a quick stumble. It's not much and just barely there, but is getting annoying because I can feel it.
When driving normally in city or highway I can nail it and there is no bog or hesitation. I have it calibrated perfectly and my plugs are running a light tan. I've tried all the accelerator pump stroke holes and it kinda seems the least stroke is the best.
Initial is set at 14*, total is 36* all in by 2800 rpm, 8* of vac on full manifold vac. Any ideas? I've never had this problem out of the gobs of hot rods and AFBs that I've run.

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Old 08-17-2008, 10:14 PM
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http://www.edelbrock.com/automotive_...ers_manual.pdf
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Old 08-18-2008, 06:10 AM
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Originally Posted by techinspector1

Got one with the carb. Doesnt address my issue.
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Old 08-18-2008, 06:40 AM
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the trouble shooting chart for your symptoms is on page 22
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Old 08-18-2008, 08:05 AM
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Quote:
Originally Posted by red65mustang
the trouble shooting chart for your symptoms is on page 22

Read it, followed it, no results. Been battling this for 4 years.
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Old 08-18-2008, 08:41 AM
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vac advance not dropping out fast enough?

linkage tipping into the primaries?

timing starting to soon with to much?

sticking and/or worn advance?

sticking, slow reacting rods vacuum plungers?
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Old 08-18-2008, 09:04 AM
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(forgot one)

rods springs are too weak (going rich to soon at higher Hg) or too strong (not going rich quick enough)
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Old 08-18-2008, 10:41 AM
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Quote:
Originally Posted by red65mustang
vac advance not dropping out fast enough?

linkage tipping into the primaries?

timing starting to soon with to much?

sticking and/or worn advance?

sticking, slow reacting rods vacuum plungers?
Thats why I'm stumped. I've done all of this. Primaries and secondaries are dead nuts, distributor has been gone thru and lubricated, although it is only 4 years old, vac unit is a new crane adjustable set to 8* to be all in at 12". Mechanical starts at 700+ or - RPM.
Rods are new and free, springs are the 7" vac flavor.
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Old 08-19-2008, 10:17 AM
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Greg,
plug the vac adv into the ported nipple and just do a quicky/crude idle screws adjust for a steady Hg idle needle and then blip the throttle....

post back if better/worse/no change
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Old 08-19-2008, 10:55 AM
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Quote:
Originally Posted by red65mustang
Greg,
plug the vac adv into the ported nipple and just do a quicky/crude idle screws adjust for a steady Hg idle needle and then blip the throttle....

post back if better/worse/no change

Been there done that. Makes it a tad worse, but not appeciably.
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Old 08-19-2008, 06:35 PM
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Sounds like the front float is a tad too low. BTDT

I bet the runner velocity is a tad slow at idle and you are getting a little bit of fuel pooling. The richer float might help it a little.

Sometimes setting the idle up another 100 rpm helps the velocity enough to stop the pooling.

The other thing I would try is a 4 hole tapered spacer to smooth the transition of the atomized fuel from the carb base to the runner/plenum volume thus eliminating the sharp corners stops the fuel "droplets" from slinging out of the airstream onto the side of the plenum.

I'd bet money it wouldn't do it with a 600 because of the smaller bores.

Does it seem that the intake air temp makes a difference?

Last edited by ScoTFrenzel; 08-19-2008 at 06:47 PM.
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Old 08-19-2008, 10:41 PM
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Quote:
Originally Posted by ScoTFrenzel
Sounds like the front float is a tad too low. BTDT

I bet the runner velocity is a tad slow at idle and you are getting a little bit of fuel pooling. The richer float might help it a little.

Sometimes setting the idle up another 100 rpm helps the velocity enough to stop the pooling.

The other thing I would try is a 4 hole tapered spacer to smooth the transition of the atomized fuel from the carb base to the runner/plenum volume thus eliminating the sharp corners stops the fuel "droplets" from slinging out of the airstream onto the side of the plenum.

I'd bet money it wouldn't do it with a 600 because of the smaller bores.

Does it seem that the intake air temp makes a difference?

Here's what i did tonight. I took a #28 squirter out of a 600 and replaced the #35 that came with the 1407, put the pump on the leanest stroke. Rsults, much much better, but could use less yet. 28 is the smallest they install but I'm going to check out the site for accessories to see if I can grab a smaller one yet. It is getting too much shot.

BTW, cant use a spacer. I have exactly 1/4" hood clearance.
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Old 08-20-2008, 08:48 AM
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Greg,
my $.02:
your squirter change does make sense....
(you have the reverse of a common radical cam low Hg problem, the normal squirter shot is "used up" before the hg recovers enough for the primaries to work)

the Edelbrock idle gas flow circuit is independent of the primaries so there is always a transition "hump" as the primaries take over
your cam (215/225?CRS) is small enough that there is plenty of low rpm Hg signal for the primaries work during the transition with no load on the motor....

because your cent timing is starting virtually at idle (700rpm), which does add Hg,,,,with no load on the motor, there is no 0Hg moment during the idle to primaries transition when the primaries don't work and you would need a big plunger shot....

the standard squirter sizes are on a chart towards the front of the manual (?CRS)

my next move would be to change the springs tabs so that the cent doesn't start till 1,000-1,200 (which is more typical for your cam) and test that with the present smaller squirter...
with no load on the motor and very light springs, the cent weights inertia often adds to much timing to soon when you "blip" the throttle....

I didn't list squirters with the "probables" because the RPM tall intake and bigger than needed carb would typically need a normal squirt.....

(dang CRS!)
doesn't the ZZ motor have "fast burn heads"?
if yes then 32-34* is likely a better WOT TT

edit add:
with only a 1/4" hood clearance(!) I'd add shim to the driver side motor mount or use a chain so the motor movement when you launch is restricted....so the air cleaner edge can't scratch/rub the hood....

Last edited by red65mustang; 08-20-2008 at 09:03 AM.
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Old 08-20-2008, 10:35 AM
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Quote:
Originally Posted by red65mustang
Greg,
my $.02:
your squirter change does make sense....
(you have the reverse of a common radical cam low Hg problem, the normal squirter shot is "used up" before the hg recovers enough for the primaries to work)

the Edelbrock idle gas flow circuit is independent of the primaries so there is always a transition "hump" as the primaries take over
your cam (215/225?CRS) is small enough that there is plenty of low rpm Hg signal for the primaries work during the transition with no load on the motor....

because your cent timing is starting virtually at idle (700rpm), which does add Hg,,,,with no load on the motor, there is no 0Hg moment during the idle to primaries transition when the primaries don't work and you would need a big plunger shot....

the standard squirter sizes are on a chart towards the front of the manual (?CRS)

my next move would be to change the springs tabs so that the cent doesn't start till 1,000-1,200 (which is more typical for your cam) and test that with the present smaller squirter...
with no load on the motor and very light springs, the cent weights inertia often adds to much timing to soon when you "blip" the throttle....

I didn't list squirters with the "probables" because the RPM tall intake and bigger than needed carb would typically need a normal squirt.....

(dang CRS!)
doesn't the ZZ motor have "fast burn heads"?
if yes then 32-34* is likely a better WOT TT

edit add:
with only a 1/4" hood clearance(!) I'd add shim to the driver side motor mount or use a chain so the motor movement when you launch is restricted....so the air cleaner edge can't scratch/rub the hood....
The cam is 218/228 @ .050 and .525 lift on 112 lobe sep. Vacuum is actually 14" at 650 rpm. I found a squirter kit and it pains me to pay 26 bux for three squirters when I only need one. I think the 24 will do it.

No fast burns. It has the L98 Corvette heads. 194/150 with 58cc chambers and 10:1 static. I have super stiff urethane mounts to keep that mill rock solid. She hasnt touched yet.
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Old 08-20-2008, 11:25 AM
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thanks for the memory jog hearty chuckle!!!....

I shared your "price pain" on my last build....

had to buy and pay shipping on a full set of specific size valve spring shims to get the 2 additional shims I needed....none of the local engine shops had that size

lol...what my wife "never" knows can't hurt me right?

do post if the even smaller squirters work, may help somebody later on
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