off idle stumble

[Mike Savelle]

installed a Holley 3310 on our crate motor and it stumbles bad when staging , but when it gets to 2700 it smooths out and had our best 1/8 mile pass on this motor . Any help appreciated . Mike

[L.Fite]

Carb stumble/bog,
Things to check...
Accelerator pump adjustment.
Squirter size.
Vacuum secondary opening rate.

[Eman]

On my 3310 making sure the transition slots on the primary side aren't exposed too much. Flip the carb over and see how much of the transition slot is showing on the primary side. If it's more than square look at the secondary slots. If the secondaries are covering the transition slot open the blade adjustment enough to square the slot. Then you can lower the idle screw and go from there. Might still need to richen idle screws and possibly increase squirter size. Be sure to adjust the spring on the acc. pump lever and if needed put a diaphram and check valve in the pump.

[Ralph A Powell]

Not more than .060 of the slot showing. Does throttle blades have holes in them for more air at idle?

[Dyno]

Bump the squirter up to a 35 or 37. Use the purple spring in the vacuum diaphragm.

[6130]

The 3310 is an awfully forgiving carburetor- I've used them on everything from 302s to 460s, and they've always worked great, right out of the box...

[Bowtieguy1967]

Which specific 3310?--is there a suffix after the LIST 3310, i.e. 3310-6, 3310-7, etc? Is it a new or rebuilt carb?

What is the engine combination?

[Mike Savelle]

this a 3310-2 750 on a fast burn 350 crate motor , rebuilt carb .
there are no holes in the throttle blades and it idles fine
we had a 750 DP on and it seemed too rich , the 3310 feels sharper and runs great going down the track its just off idle when staging it runs rough.
I'm taking it apart today to check it out .

[Bowtieguy1967]

A decent baseline for that carb would be a 218 (white) cam in the #1 position with a .025" or .028" squirter.

If the accelerator pump lever is the early style that uses a slotted/hex screw on the top of the pump over-ride spring you should be fine, if it's the newer style with the long screw that comes up from the bottom with a nylock nut on the top, try adding a few thin machine washers underneath the over-ride spring. The later style override spring often compresses too much upon initial throttle opening which reduces the pump shot volume at the discharge nozzle, shimming the spring increases the tension (same as shimming a valve spring) which helps prevent this.

Make sure the part of the pump lever arm that contacts the pump cam isn't bent or "adjusted" by a previous builder. If the lever is bent upwards, it loses a lot of travel which again reduces the pump shot volume.

Lastly, --assuming the primary bowl is an early style which uses a pump well check ball and retainer strap in the pump well cavity--make sure there's only roughly .020" clearance max between the check ball and retaining strap. If the clearance is too great, the check ball will be too slow to respond to pump activation which allows fuel to bypass back through the feed hole and back into the float bowl instead of going through the discharge circuit. If the bowl is a later style unit with the red umbrella check valve, try replacing it--and make sure to trim the nipple after installing it so it doesn't interfere with float travel.

[6130]

Originally Posted by Mike Savelle

...we had a 750 DP on and it seemed too rich...

Was the 750 DP a 4779?

In what way did the 750 DP "seem" too rich? Did you do a plug chop and read the plugs? Did your wideband tell you it was fat?

If your 750 DP was rich, the correct fix for that would have been to lean it out with smaller main jets, not to swap to a different carburetor. Diagnosis follows a specific 4-step process, and guessing isn't one of those steps. Randomly swapping parts can add (and apparently has added) completely new problems, without ever solving the original one.