I never thought to check it out. Would have to have the same setup and cam but different firing order. The double swap definitely smooths the idle out a little bit so I wonder if it pulls more vacuum?

The more you swap the more it pulls vacuum. Can cause issues with smaller cams. Cause it will pull to much vacuum and not open the power valves.

if nothing is changed on an engine except the firing order what would make it produce more vacuum?does each cylinder pull the same amount of air in on the intake stroke or am i missing something.

Wave front inside the intake manifold? Smoother idle / smoother and steadier vacuum?

i get that but bb said more vacuum.i can,t wrap my head around that and would like to learn how that could happen.

Probably has to do with intake reversion pulses canceling each other out. Smoother idle probably means less canceling of pulses.

kind of reaching for straws. lot of probably,s their.

This is the only thing that I could thing of. On the exhaust end of it it's no mystery. A motor with a double swap cam sounds like a lame duck compared to a standard firing order. We've seen 16" of vacuum over 3,000 rpm with some double swap motors. Which can be trouble especially when you have a 6.5 PV. Try to get the boat on plane and she starts snap crackle and popping because the carb will not flow any fuel. Put in a 10.5 PV (which are a pain to get) and she's good to go.

Who wave tunes an intake or exhaust at idle rpms?

Another thing to note on AFR adjustment is PVCR's. If off idle to cruise is good AFR but lean WOT or moderate accel, enlarge the PVCR's (Note: At a certain point high flow PV's are required). This will enrich the mix only when the PV's are open rather than across the board. My cruise was mid 12's to 13. WOT was 13.0-13.1. I enlarged both Pri and Sec PVCR's .005. Cruise still same (PV's are shut) and WOT is now 12.6. - 12.8