Stuttering issues with brand new 302

[whoohoo111]

Hey guys, it's been a while since I've been on here, but about a month ago I installed a brand new ATK 302.  Everything internal is completely stock.  I'm running a Weiand street demon intake and Holley performer 600 carb.  The problem I'm running into is my Bronco is stuttering and trying to die when I slam on the gas or hit high cruising speeds.  It actually sounds like it is fuel starved.   I'm running at 12 degree btdc, and 36 total with advance set in. 

What I've kind of figured out after going through my Holley book and trying to adjust it is that I'm pulling way too much vacuum and the carb can't keep up with fuel output.   At the manifold port, I'm pulling 25inhg~ or more idling at 650rpms.  I figured I should be running at 16-22inhg.   I called Holley, and I was told that I should get their largest pump accelerator, but even though, their formulas show that the pump accelerator should be half of the vacuum that the vehicle is pulling.  The problem is that their largest pump accelerator is only 10inhg.

Is there such a thing as too much vacuum?  I'm not sure if there is an underlying problem without everything being "sealed" too good.  Any ideas where I should go from here?

Thanks,

Rob

EDIT:  Fuel float levels are perfect after stalling out, so carb is definitely getting enough fuel from gas tank.

 

[Bully Bob]

"I slam on the gas"

Do you mean this literally..? (stomp on pedal) If so.., for what reason..?

There's nozzles in some Holley 4 brls. If memory serves, there's different sizes.

But ya.., it sounds line a fuel flow/feed issue.

Sorry, what's ATK 302.?

Is your ignition compatable..?

 

[whoohoo111]

"I slam on the gas"

Do you mean this literally..? (stomp on pedal) If so.., for what reason..?

There's nozzles in some Holley 4 brls. If memory serves, there's different sizes.

But ya.., it sounds line a fuel flow/feed issue.

Sorry, what's ATK 302.?

Is your ignition compatable..?

No!  I don't mean literally, but I give it enough to accelerate quickly. 

ATK is a brand of motor builders, and I purchased one of their rebuilt 302 blocks.

The ignition is HEI, and it's timed properly.

 

[Skitter302]

I would make sure that your vacuum advance is not causing the issue. I was working on a jeep earlier this year that acted this way and it was a vacuum issue. Just something I have experienced.

 

[whoohoo111]

Is there a specific way to check the advance?  I had plugged ported vacuum (advance vacuum) on carb at one point, and hooked the advance up to the full manifold vacuum, and it still did the same thing.  I'm assuming advance means nothing when plugged to manifold port?

 

[Bully Bob]

I'm thinking it's best at the carb. port --- for vacuum adv.

As I remember, it's metered a bit & is above (or below) the butterfly for a reason.

Check me on that.

 

[Rons beast]

I doubt the problem is related to vacuum.  A healthy engine will make only so much vacuum and the components ( carb, distributor, and any vac/ run accessory.) are made to be used with it. They could need adjustment.

I'm not familiar with a Weiand Street demon manifold.  Was it on your old engine?  Was the Holley on there too?  How did they work together?

I'm trying to determine if the problem is fuel or ignition and is the manifold/ carb suited for the engine and use.  

Try power braking the engine.  Does it miss or stutter then? (If so it's likely and ignition problem.)  Also try to cruise on the highway at 55 to 60 (legally) then rapidly accelerate ( safely , of course) and see if the engine stumbles.  If it does and continues it is most likely an ignition problem.  If it stumbles but smooths out it is probably fuel related. 

Have you replaced plugs and cap and wires and rotor.?  With a new engine the healthy compression can put a strain on the ignition system and any weakness will show itself.

Good Luck. 

 

[Bwonco]

You can try with the engine off, look straight down the venturies while opening the throttle to wide open position. You should see a healthy, long ,even squirt of fuel going down the throat of the carb aimed right at the middle of each primary barrel. if it dribbles or doesn't hit the center, you may need to find out why. These conditions can cause stumble when opening the throttle.

Also if your primary float bowl float level is set too low it could cause a leaning condition at higher rpms.

If you are pulling 25 inches of mercury @ idle, that is very healthy for an engine with new rings and a stock camshaft with little to no overlap on the valve timing. I dont this is the cause of any problem. try another vacuum gauge to be sure your gauge isn't lying to you.

Make sure your dist is connected to ported vacuum source, that slowly increases the higher the RPM's. If it's connected to any other vacuum source, your dist advance will be irratic when opening the throttle, and cause flat spots or lousy performance, and sometimes pinging in warm weather.

~BH~