Carbureted vortec heads

[Hipster]

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.

Bingo. All of the above.

 

[L31MaxExpress]

Plugs need to be the corect length. Seen alot of people put short plugs in a Vortec head and they run like garbage.

 

[Supercharged111]

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.

The one time I had my hands on a Holley I had a real hard time getting a satisfactory AFR swing to deliver stoich or leaner cruise with 12.5-13:1 WOT. I seem to recall the difference the power valve options made just not being big enough thus forcing a compromise. I didn't have to compromise with my Edelbrock, but it vapor locked like crazy and I hated that.

 

[Schurkey]

The one time I had my hands on a Holley I had a real hard time getting a satisfactory AFR swing to deliver stoich or leaner cruise with 12.5-13:1 WOT. I seem to recall the difference the power valve options made just not being big enough thus forcing a compromise.

The power valve controls the vacuum level (Load level) that the power circuit engages at.

The actual amount of fuel delivered is metered by the Power Valve Channel Restrictions--PVCR. You'd screw-around with jetting to get the part-throttle mixture "perfect", and then get the WFO fuel mixture where you want it by drilling the PVCR using tiny drill bits and a pin vice. But no-one is going to do custom, difficult-to-reverse mods on a "dyno mule" carburetor, so the dyno jockeys and the magazine writers slap in different jets instead. Yeah, that'll fatten-up the WFO metering, but it fattens-up the cruise metering, too.

And then the magazine writers tell ten thousand readers that the carb had to be "jetted up" for full power on the dyno.

Newer, more-expensive Holley and Holley-style carbs have screw-in replaceable PVCR just like they have screw-in air bleeds. But that leads to the second problem with the Holley design--the power valve itself is On/Off; the better design is a power piston linked to tapered metering rods that provide a gradual, progressive enrichment as vacuum drops/load increases.

Or EFI, which does all of that with tremendous precision; and uses feedback from the O2 sensor to fine-tune the fuel mix a jillion times a minute.

 

[letitsnow]

Or EFI, which does all of that with tremendous precision; and uses feedback from the O2 sensor to fine-tune the fuel mix a jillion times a minute.

Just to be fair for any rookies who might read this - the EFI does this if all sensors are working perfectly, there isn't anything rattling to fool the knock sensors, the wiring is in good shape, all connections are perfect, the gas is perfect, fuel pump is perfect, and the ecm is tuned perfectly.

As these trucks age, this is something to think about.

 

[Dirty Jim]

As stated above, carb tuning. 750cfm can be a lot for a 350 depending on other things. You don't mention what intake or cam you run. More info needed if carb adjustment doesn't pan out.

 

[SAATR]

He'll be back, once the smoke from the parts cannon clears.