Carbureted vortec heads

[JPetroJr]

I have a 94 350 5.7 with summit vortec heads. 750 cfm holley. What should the plugs be gapped at. Been told 45. Been told 60. 45 doesn’t run to good…bogs and stumbles.

 

[LC2NLS6]

I'd verify fuel pressure first. Run a wideband O2 guage? It runs, so there is some spark and some fuel and air and compression? Open the hood at night in darkness, are there sparks from plug wires arching out?

 

[SAATR]

I have a 94 350 5.7 with summit vortec heads. 750 cfm holley. What should the plugs be gapped at. Been told 45. Been told 60. 45 doesn’t run to good…bogs and stumbles.

That's like retorquing your lugnuts because the tires don't grip well. The nuts are fine; your tires are worn out!

Bogs and stumbles are usually fuel related. Too rich, too lean.

With a vac secondary 750, if it's a tip-in stumble, it could be a vacuum leak, accelerator pump shot (cam or squirter), bad or improperly sized power valve, low vacuum (big cam or that leak we talked about), secondaries trying to come in too early, float level or needle/seat issue, and on and on. Download Holley's tuning manual and start working it out.

 

[Hipster]

The carb needs to be tuned. Get the manual mentioned. First you need to dial in/verify initial and total timing values. If you haven't checked you don't know and you could be pulling in 50+ degrees of advance under a load. I would be nice to know what distributor you're running in this set-up. Most holley 750's I've dealt with had too much pump shot too soon out of the box for a mild build. Buying the carb is the beginning, off memory most 750's come with a 31 squirter when you really need a 28 and/or different pump cams as well.

Holleys like at max about 5.5psi fuel pressure or you can overpower the seat. Where's your fuel pressure at?

I've run .045 on a lot of stuff including high output ignition systems. It's not your problem. Plug gap and indexing is usually far down the list of things to tamper with when trying to find that last ounce of power.

 

[JPetroJr]

Fuel pressure is 7. HEI in cap coil new. Accel 8 mm wires new. All check out good. AC DELCO plugs new gapped at 45. Is 45 correct for vortec heads? Summit says 60…

 

[Supercharged111]

Fuel pressure is 7. HEI in cap coil new. Accel 8 mm wires new. All check out good. AC DELCO plugs new gapped at 45. Is 45 correct for vortec heads? Summit says 60…

60 is factory, I run 45 with the blowers. Again, you're wasting your time with plug gap right now. What is your initial timing? What is your total timing? When is it all in? Have you even touched the carb? 7psi is too high. Everything you need to check and respond to was posted already but you haven't acknowledged it.

 

[JPetroJr]

Sure thing buddy. I will start going over everything when I get to the shop in the morning. Thanks for the help

 

[Schurkey]

WHAT PLUGS? Plugs intended for a .060 gap may be unhappy if the gap is crushed down to .045. Plugs intended for a .045 gap may be unhappy when sprung open to .060.

As said--the gap alone (if it's the correct gap for the plugs used) is not the issue.

 

[Erik the Awful]

If you're pushing too much fuel pressure, it'll typically try to die at idle and run fine at WOT. I had a needle and seat that would hang open in my RX-7 and I had to constantly blip the throttle at idle to keep it from dying. Still, you might try backing the fuel pressure off to 5.5 psi.

I would suspect carb tuning, but double check your timing first. I'd shoot for 15-20° for a ballpark setting and work from there.

I haven't dorked around with Holleys, but here's a rule of thumb for carburetor tuning. Jets affect the mixture from idle to 5000 rpm. Air bleeds/emulsion tubes affect the mixture from 4000 rpm on up. A bigger jet will richen the mixture, while a bigger bleed will lean the mixture, each in their respective range.

If the bogs and stumbles are on gas pedal application, check your accelerator pump.

Carb Science Series — Accelerator Pump Tuning for Holley Carburetors

Getting the accelerator pump tuning right can be a challenge. But once it's dialed-in, your engine will pull like a herd of oxen.

www.enginelabs.com

 

[Schurkey]

Holley main jets don't affect idle. They affect the main fuel system, and could cause it to begin delivering fuel a little early, but it does not affect idle or idle-transition otherwise.

The "big mistake" folks make with Holley main jets is changing jet size to increase WFO fuel delivery. This kills part-throttle economy and throttle response. The proper way is to increase the power-valve channel restrictions (and the power valve as needed) for WFO delivery, while using the main jets for part-throttle fuel.