Double pump-- mech. seconds--Holly Gurus

[Cornpatch MO]

I am just about ready to tune my 750 cfm double pump, mechanical secondarys Holley
68 primaries--76 secondary jets. . 440 mild street engine . Runs good as is, but would like to see if it could do better. No dragstrip for times , locally.
1--hard to read plugs on todays gas, but tail pipes show rich on the inside
2-- is about 8 steps between primary and secondary jets about right ? to much or to little?
3--About how many steps , up or down , should I try on the jets?
4-- I would guess the first direction would be to try a leaner mixture?
I would appreciate any EXPERIENCED input................Thanks..............................MO

 

[66Satellite47]

I was never able to get much from "plug color" especially on the street motor. As far as jetting it does depend on the specific List Number carb you have, need to look up what stock is. The number of steps between primary & secondary jet sizes depends on if there is a power valve in both primary & secondary. My experience with many Holley's tells me you need to change at least 2 number sizes to see anything, maybe 4 numbers. Drag strip performance or a good air/fuel ratio meter is the way to know. If you think it's rich check the primary power valve, replace as needed. IMO a 750 DP can work well even on a mild street 440, but it still could take a fair amount of tinkering. Accelerator pump squirters, pump cams & the idle position screws on both primary & secondary may need adjustment.

 

[Cornpatch MO]

I was never able to get much from "plug color" especially on the street motor. As far as jetting it does depend on the specific List Number carb you have, need to look up what stock is. The number of steps between primary & secondary jet sizes depends on if there is a power valve in both primary & secondary. My experience with many Holley's tells me you need to change at least 2 number sizes to see anything, maybe 4 numbers. Drag strip performance or a good air/fuel ratio meter is the way to know. If you think it's rich check the primary power valve, replace as needed. IMO a 750 DP can work well even on a mild street 440, but it still could take a fair amount of tinkering. Accelerator pump squirters, pump cams & the idle position screws on both primary & secondary may need adjustment.

I don't think mine has power valves . I will check that out. There are so many things that can be done with a Holley, I just want to start with the jets............................MO