So I just recently rebuilt my 400 with a whole bunch of new parts. Before it ran just terrible and leaked everywhere so I went through with the total rebuild.

Machined out to .060 over, new comp cam, new high compression pistons, new valves, springs, headers, everything. I topped it off with a Holley 4150 650cfm double pumper.

The motor screams and makes great power but I cannot for the life of me get this carb tuned. When you step on it off idle, it stumbles so bad that if you don’t let off, the engine will die. Even if you slow rev it to 2000, then step on it, it still stumbles before it catches up and starts to rev normally.

As anyone else would try first, I checked the timing, and for vacuum leaks. Timing is set to 34°-35° total and I can’t find any vacuum leaks that would be an issue. So initially I though it was a rich condition. Giving the engine too much fuel because typically when an engine is lean, it’ll backfire through the carb, an issue I had previously before I rebuilt everything and had a Edelbrock 1406 on it.

So I made some adjustments to the accelerator pump lever, made sure it was set correctly and then I added some smaller nozzles. From size 28 to 25. No change. So I’m starting to question what the actual tuning issue is. I’ve messed with the nozzles, the accelerator pump cams and the pump arm to no avail.

Any help would be much appreciated. My best friend has a 78’ K10 and my truck has always been faster. Not sure if it’s the case anymore but I need this thing in ship shape if I’m gonna keep up with his new 383 + FiTech.

Thanks is Advance!

 

Try going larger on the accelerator pump shooter. Maybe a 35 and see what that does. This will give it a larger amount of the shot sooner.

Take the carburetor off and look at the position of the throttle plates when it is on the curb idle screw. How much transition slot is showing beneath the primary throttle plates? The slot should look like a square hole where what is exposed is about as tall as the width of the slot. If a lot more is showing you need to do something to get the throttles closed more. Either open the secondary throttles a little bit or drill a hole through each primary throttle plate. I'd start off small at about .062 and go larger if it needs more.

What might be happening is that you're starting out too far up on the accelerator pump cam and not getting a full shot and or you're using up all of your slot before the main can get started and making a big lean hole.

 

Auto or 4spd ? vac advance on ported or manifold ?

What is your vacuum reading and what power valves are you running ?

 

New or used carburetor??

With the engine OFF have you physically looked down the carburetor and opened the throttle and verified that gas is squirting out o the nozzle??

The float bowl gasket in the above picture can be put on either way.
The arrows are pointing to the accelerator pump fuel passage to the metering block and then to the nozzles.
Some carburetor kits have a float bowl gasket that only has one of those holes and only goes on one way.
If you have the gasket with only one and its installed incorrectly then no fuel will be discharged into the venturi when you open the throttle.

Adjust the gap on the accelerator pump lever as per the instructions.
From a closed throttle position gas she spray out the nozzle as soo as you start to open the throttle.

"it’ll backfire through the carb", we referred to that as a "flash back".