Carb Help hesitation
Thread Starter
Drifting
Joined: May 2004
Posts: 1,258
Likes: 0
From: Huntsville Alabama
Carb Help hesitation
Alright got my new year one creat motor up and running. Car runs great except when you go to take off from idle then it hesitates acts like it wants to quit then goes. Specs on the motor are 9.5 compression vortec heads rpm air gap intake, Holley 4160 750 vacuum secondary carb. Cam 218/228 520 lift. Need some help. My buddy said it might be the pump cam. I bought a pump cam assortment and chaned from the orange one that came on the carb to a black one and it still has the hesitaion.
Tech Contributor
Joined: Aug 1999
Posts: 14,373
Likes: 6,371
From: At my Bar drinking and wrenching in Lafayette Colorado
Initial timing and total timing, along with how you have your vacuum advance hooked up, are the biggest issues regarding stumbles. You need 36 degrees total timing with about 18 initial.Once you have that dialed in, make sure you have no more than .020" of your carb's primary side transition slots exposed below the throttle blades. If you do, you need to open up your secondary idle speed screw to get your primary throttle blades closed down. Too much transition slot exposure is a huge cause of off-idle stumbles.
Be sure float levels are set right, and set idle mixture initially to 1 turn out and tune from there. Depending on the carb and your exhaust system, you may want to richen the jetting up about 2 jet sizes if the above adjustments don't do the trick.
Do all this, and your stumble will be gone without messing with accel pump circuits.
Lars
Thread Starter
Drifting
Joined: May 2004
Posts: 1,258
Likes: 0
From: Huntsville Alabama
CARBURETOR 80508
Primary Jet 70
Secondary Plate 21
Pump Discharge Nozzel .031
Power Valve 65
Secondary Spring Black
Car is an auto with th400 and stock gear. Have 2200 stall.
Exhaust is headers into 2.5 inch pipe to dynomax super turbo mufflers. it is dual exhaust
Primary Jet 70
Secondary Plate 21
Pump Discharge Nozzel .031
Power Valve 65
Secondary Spring Black
Car is an auto with th400 and stock gear. Have 2200 stall.
Exhaust is headers into 2.5 inch pipe to dynomax super turbo mufflers. it is dual exhaust
Last edited by Silver00V6Camaro; Nov 16, 2006 at 02:51 PM.
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Tech Contributor
Joined: Aug 1999
Posts: 14,373
Likes: 6,371
From: At my Bar drinking and wrenching in Lafayette Colorado
Do all the other things before you change the jets - the jet change is the last thing you want to do if all the other setup changes do not fix the hesitation. Never tune the carb before you verify and set up your timing and timing advance, and always set the carb up with the correct transition slot exposure, float levels and mixture settings before ever thinking about a jet change.
Drifting
Joined: Sep 2006
Posts: 1,986
Likes: 21
From: Columbus MN
Same problem - bigger squirter
I had the same problem with my Holley 80457. I replaced the pink cam with a green and the throttle response was MUCH better. I had to change to a bigger squirter to really fix the problem. That's easy to do from the top of the carb, but be careful you don't drop one of the gaskets down the tube.
Drifting
Joined: Sep 2004
Posts: 1,959
Likes: 1
From: North Bend Ohio
Yes, vacuum advance disconnected. Lars likes to see it all in by 2500 to 2600 RPM. I always figured all in by 2800 RPM is still way better than stock and won't stress some of these old engines quite so much.
BigBlockk
Later.....
BigBlockk
Later.....
Drifting
Joined: May 2006
Posts: 1,425
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From: Seattle Wash.
At our TFB we recently had, Lars recommended that you should use a non adjustable vac. advance with 14" of vacuum. In his opinion, adjustable vac. advance is not as reliable as a non-adjustable unit.
Originally Posted by Silver00V6Camaro
Ok got it timed to 36 degress at about 2600-2800. Next question is the timing ok around 48-50 with the vacuum advance hooked up. I am also using an adjustable vac advance.
Drifting
Joined: Sep 2004
Posts: 1,959
Likes: 1
From: North Bend Ohio
Originally Posted by Silver00V6Camaro
Ok got it timed to 36 degress at about 2600-2800. Next question is the timing ok around 48-50 with the vacuum advance hooked up. I am also using an adjustable vac advance.
BigBlockk
Later.....
Thread Starter
Drifting
Joined: May 2004
Posts: 1,258
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From: Huntsville Alabama
Originally Posted by BigBlockk
It sounds like you got it set up OK now.
BigBlockk
Later.....
BigBlockk
Later.....
Drifting
Joined: Sep 2006
Posts: 1,986
Likes: 21
From: Columbus MN
Originally Posted by Silver00V6Camaro
Yeah but i still think im gonna have to jet it the hesitation is still there when i hit it at idle once i get the tranny leak fixed I will drive it and found out if i do need to jet it or not.
My Holley 4160 had a pink accelerator pump cam on the throttle shaft - low, flat, and short. I replaced it with a green - a little bump at the front end, steeper, and longer. There is just one screw holding the cam on. You just unscrew it, push the old cam off, push the new cam on, and replace the screw.