Slight stumble
Thread Starter
Le Mans Master
Joined: Jan 2001
Posts: 6,938
Likes: 1
From: San Diego CA
Slight stumble
I have a slight stumble when I first give it gas. It just started. No problems after the stumble. Stumble occurs about 800-900 RPM. Yes timing is dead on, carb was professionally set at dyno, VAC is good, plugs and wires check okay. Performance is flawless except for this stumble off idle and it's not even apparent unless your revving the motor while sitting at a light. I'd say it was due to such a big cam and Torquer II intake, so no bottom end response, but it didn't do it before.
Instructor
Joined: Jul 2003
Posts: 172
Likes: 0
From: Richmond Virginia
Stumble
Not sure what Carb you have but in any case check to make sure the accelerator pump in the carburetor is working properly and squirting gas down into the throttle bore when you crack the throttle.
Thread Starter
Le Mans Master
Joined: Jan 2001
Posts: 6,938
Likes: 1
From: San Diego CA
The carb is a Eldebrock 600cfm with larger jets, accelerator pump is working fine. No engine changes since last tuneup aside from new headers and chrome ignition shielding. I'll have to check those damn wires for a cut one more time.
Thread Starter
Le Mans Master
Joined: Jan 2001
Posts: 6,938
Likes: 1
From: San Diego CA
I checked all the wires, again, and all the plugs. Everything is firing correctly so ignition problems are out. Have to assume it's a carb problem. It's not so much a problem since no one drives at 800RPM but it is annoying.
Any suggestions?
Any suggestions?
Racer
Joined: Dec 2002
Posts: 421
Likes: 2
From: Long Grove, IL
Originally Posted by Cali,68,L-79
I checked all the wires, again, and all the plugs. Everything is firing correctly so ignition problems are out. Have to assume it's a carb problem. It's not so much a problem since no one drives at 800RPM but it is annoying.
Any suggestions?
Any suggestions?
Jay
Pro
Joined: Dec 2002
Posts: 560
Likes: 1
From: Mooresville NC
Your accelerator pump may be working fine, but with a larger cam more air is being introduced into the cylinder. A slight off idle stumble, with everything else checking out OK seems to me to be that the accelerator pump is not discharging enough gas to cover the increased air flow from the larger cam. This is especially true with open plenum manifolds. You may have to explore either the pump cam or discharge nozzles.
Good Luck with another suggestion.
Good Luck with another suggestion.
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Team Owner
Joined: Jun 2004
Posts: 20,216
Likes: 4
From: Smile, it's just the internet.
If I understand Holley carbs correctly, the accel pump doesn't discharge during slow throttle movements. It bleeds off into the fuel bowl, instead. If the Edelbrock is the same, I'd rule out that carb system. Again, my only experience is with Holley, so my initial thought was the transfer slots being hosed up... I guess I agree with Griffths, check vacuum.
Advanced
Joined: Jun 2004
Posts: 63
Likes: 0
From: North Canton Ohio
I agree with MILO and Missileman. I had this exact problem on a '69 with a 600cfm Holley. I drilled out the accelerator pump discharge nozzle holes using small wire gage drill bits in a hand vice (you can also buy larger size discharge nozzles). This cured the problem. I don't know if the Edelbrock is similar to the Holley, but I would look there.
Drifting
Joined: Nov 2002
Posts: 1,665
Likes: 1
From: Ontario
Maybe try a different power piston spring or adjust your power piston lower stop. Engine vacuum, this spring and the lower stop determine the relationship of how your primary rods move in and out of the primary jets.
If your stumble is caused by a lean condition perhaps your primary rods are staying down in the jets. In this case you would need a stiffer spring to push the rods out when engine vacuum decreases. It could also be that your lower stop is set too low so that even when the engine vacuum decreases to allow the rods up they simply don't come up enough.
Conversely if your stumble is caused by a rich condition perhaps your primary rods are not being pulled into the jets. In this instance you would need a lighter spring to allow the rods to seat in the jets.
My advice to you is based on my knowledge of the Rochester QJet so I am assuming your carb is very similar.
Good Luck
If your stumble is caused by a lean condition perhaps your primary rods are staying down in the jets. In this case you would need a stiffer spring to push the rods out when engine vacuum decreases. It could also be that your lower stop is set too low so that even when the engine vacuum decreases to allow the rods up they simply don't come up enough.
Conversely if your stumble is caused by a rich condition perhaps your primary rods are not being pulled into the jets. In this instance you would need a lighter spring to allow the rods to seat in the jets.
My advice to you is based on my knowledge of the Rochester QJet so I am assuming your carb is very similar.
Good Luck
Thread Starter
Le Mans Master
Joined: Jan 2001
Posts: 6,938
Likes: 1
From: San Diego CA
I'm assuming the jetting done by the dyno would have included drivability issues but if not for performance alone I'd say they'd be on the rich side. While I can change the springs the actual jet settings I don't want to touch becuase they really found her sweet spot. Before 2000RPM there is nothing and then you creep over 2000 and it hits hard like a ton of bricks (real impressive for a SB).