2x4 1850 holleys Jetting?

[Excaliber]

I'm running two fours on my 427 FE side oiler also. 600 cfm vacuum secondaries. I forget the exact jet size now but I know it's low 60's or under and TO lean! I was running 660 Holley center squirters (drag race only, no power valve and mechanical secondaries). THOSE had 73's all around (8 jets total). I dropped them to 68's and it was MUCH better. I think I'll pull the 68's out and put them in my 600's.

BUT, I been playing with the power valve and what a difference THAT makes!!!! My old PV's (on the newer 600's) were 3.5" when I bought them, which was fine when my radical roller cam (which pulled almost NO vacuum). With my new "streetable" flat tappet cam and lower compression (from 12.5 down to 9.8) those 3.5" PV's NEVER opened! Talk about running lean and missing big time at cruise rpm.

Now I know YOUR running rich, but don't rule out the POWER VAVLES as a cause. All though, 73's still sound to big to me. If your PV's are rated at a vacuum higher than your cruise or idle vacuum signal they will be OPEN and thus will richen the mixture big time! I rigged up a vacuum gauge with a long hose and set the gauge on the windshield held in place with the wiper arm. Then I could read IDLE vacuum AND cruise vacuum. Idle and cruise were (surprise) about the same, 11 inches or so. SO, I installed 10" PV's. MAN what a difference!!!! Cruise was smooth, throttle response was WAY better. Transition from idle to "rev up" was instant and no more "back fire" and hestitation. WORLD of difference.

Now cruising at 75 or 80 I feel that kind of "jerky" uneven hit and miss thing, subtle, but it's there. Just a "touch" more gas and the PV opens and everything smooths right out. BUT I'm thinking I shouldn't have to "open the PV" at cruise rpm to get "smooth", thus an increase in jet size is now warranted. One thing at a time you see, PV or jets till you nail the problem.

In your case I would do two things.
1. Check my PV rating against my idle/cruise vacuum signal.
2. Decrease the jet size into the 60's.

Every cam, compression ratio, engine specs are different. YOUR carb has to match YOUR engine. There is no easy answer here. Set your PV size (vacuum rating) JUST under your idle/cruise vacuum by a point or two. Idle at 11, go with 10 or 9.5 PV's. Any vacuum signal ABOVE 10 or 9.5 will keep the PV "closed". A touch more throttle, vac drops below 10 and the PV opens and introduces more fuel, think of it as a "third main jet". You DON'T want it to open unless you NEED it to. Open at cruise rpm/vacuum will cause big time "rich" to occur, passing or going up a hill then it SHOULD open due to slightly lower vacuum signal!

Hope this helps.