I have a 600cfm holley street avenger on a mildly modified 350 (medium cam, headers, mild head work, rpm air gap intake). Problem is under acceleration with anything other than light to medium throttle. It stumbles terribly until I let of the gas some, then it "catches up" and resumes acceleration. From a stop it takes off good, eventhough I can't give much gas due to traction problems. It is not the timing, which I know because I have taken it from one extreme to the other. Am I starving this thing or what. The carb is stock out of the box. Please help me beat the mustang crowd again.

 

Need to put smaller jets in the back half of the carb.

 

Can you hear a backfire through the carb, when it stumbles? vs type carb? If it's a lean backfire, you may have to change the secondary spring rate and or pump cam on the primarys.

 

i would change power valves, get a vacume gauge and take a reading while the car is ideling and in gear, divide the number in half and add .5 to it, for example if you pull 12 inches of vacume then you need a 6.5 power valve. If the power valve is to stiff and theres not enough vacume to open it then, it won't let the fuel in fast enough to overcome the bog.

Check your fuel pressure to, if your running anything under 6psi then it will stummble at higher rpm's Get a good fuel pressure gauge and put it inline right infront of the carb, you want a steady 6 to 7psi at 3,000rpm.

 

Stock power valve should be just fine. Sounds like it's more about throttle position that rpm. Fuel pressure is always a good thing to check. Street avenger doesn't have rear jets (AFAIK). Get the secondary spring assortment if you don't already have it, and try some different springs. See if that helps any. Oh, and make sure the ignition system is all good.

 

From what you describe I believe that you don't have enough " Pump Shot " on the Primary side.

You have an RPM AirGap manifold. The Air Gap series have no manifold heat and the intake charge run considerably cooler than the " Non-Air Gap " intakes. That requires more pump shot than a manifold that is heated.

Do you have an Auto car or 5 speed. Auto cars usually need more pump shot than a stick car.

Raely will a Universal carb work " Out of the Box " 100%. They usaully need some tweaking. Read this Tech Info from Holley:

Holley Accelerator Pump System

Now my recommendations.

1) Check the float level. This is the first adjustment that should always be done. Fuel level with the engine running should be just slightly below the bottom of the slight plug. 1/16" below on Primary side.... 1\8" below on secondary side.

2 ) Get some #31 shooters. I prefer tube type.

3) Get an Acclerator pump Cam tuning pkg.

4 ) Install the Blue Cam in the #2 hole. Adjust as per the Tech article from Holley. There must be no freeplay between the Cam and lever at idle....yet a minimum of .015" extra travel available at WOT. This will give sufficient pump shot and duration to eliminate your stumble. SBC's seem to like a hard " Fat" shot of fuel when you mat the gas. The base setup with stock Holley's is very weak.

5) Make sure that your Vacuum advance is connected to " Manifold " vacuum....not ported Vaccum. Set initial Advance at approx. 10 to 12 deg BTDC with the Vacuum hose disconnected at 700 to 750 rpm.

6) GO KICK SOME FORD ***!!!

Once you've dialed out the initial stumble you may be able to go down on Shooter size. Stock Secondary Vacuum spring should be fine for now.

 

Thanks Chicken, I'm in there.