Holley 750 CFM Ultra Double Pumper or Quick Fuel Technology SS-750-AN 750 CFM Street?

[vector1]

To me the plugs just indicate you are not burning oil. You could be cruising lean, accelerating lean, lean at idle.... you don't really know. I'd guess from your carb card you are pig rich anywhere the lean spot is not between your idle and main circuit, hence the clean porcelain and tip and carboned threads. Without the a/f gauge your guess is good as mine.

tried to edit my above post--if it wasn't evident, the secondary PV on the OEM Holley that came off the 400M was plugged. I used 74 main jets all around on the HP 750. You can do either. I'd also prefer the single plenum intake over the dual.

Also just an fyi, the Holley Boss 302 carb main jets were usually down around 70, and maybe they used a 780 cfm.

[blykins]

Did you talk to Scott?

The plugs are showing it's lean. The porcelain looks extremely white and bright.

The ground strap is what indicates if the timing is close or not. By the looks of plug #5, timing is pretty close to where it needs to be.

To me, it looks like the idle is a hair rich and then everything after that is lean.

I think the plug may be a little too hot as well.

Next time you throw plugs in it, just use a drop of oil on the threads. The copper/nickel seize makes it very hard to read the plugs. That stuff gets everywhere....

[jhv48]

I’ve run both of those vacuum secondary carbs on two of my engines and wasn’t pleased with either one of them. Vacuum secondary carbs are designed for cars that are much heavier than ours.
You are running lean. The plugs tell the tale.
You’ll be happy with either the Holley or QFT double pumper. Right out of the box they’ll be a bit rich, due to your altitude. Easy job to drop down a couple of jet sizes on both primary and secondary sides till the plugs look right.
Order either one and don’t look back. You’ll love the difference moving from vacuum to mechanical secondaries makes.