I know there are several SBC boat owners hanging around here and wanted to give an update on something I tried this past weekend. I have a 394 (4.060 bore x 3.800 stroke) aluminum head 10:1 engine. I'm running the popular edelbrock rpm airgap intake. When I built this I went with a Howards hyd. roller cam with 225/231 @ .050 with 112 LSA. I added a PCV valve this past week, but to do so added a 1" open spacer to the top of the intake to clear the elbow for intake vacuum for the PCV. I was shocked when I gained 460 rpm up top over running without the spacer. I jetted up (3) sizes to keep on the rich side in case the PCV caused some lean conditions. It does now have a hesitation coming off idle that wasn't there before the spacer, but I'm hopeful I can tune that out. I'm curious if anyone else has tested spacers with a similar combination? I'm thinking of adding the 1/2" HVH super sucker that reaches below the bottom of the spacer to smooth the transition into the 1" open for a total of 1 1/2" of spacer. Two seasons ago this was a 52 mph boat, that I was convinced the hull wouldn't allow going any faster. I'm now at 68. All through minor incremental changes.

That’s great.
Your big small block must have really wanted more air. I forget, what size is your carb ?

Its an old 3310 vac secondary 780 Holley. I've updated it with the quick change secondary spring kit and power valve protection.

Hesitation might be cured by opening idle mixture screws a tad more. Give it a try. Yeh, idle mixture screws add fuel every throttle position and rpm.

First step- set your float levels in the carb. Lots of times guys start "tuning" while float levels are crazy off.

With that cam, you probably need more initial timing.

Try this - hook up a vacuum gage to the manifold. Set your idle speed at 850 rpm. Start adding ignition timing, watching vacuum gage. As timing increases, vacuum increases, rom increases. Adjust timing a bit. Slow idle back down to 850. Keep doing this until vacuum no longer increases. That's where your motor wants the initial timing set. But if you end up much higher than stock (you will) then you have to change your advance module so your full advance still ends up 34-36 degrees.
Anyhow, once you optimize your initial and max timing, adjust your idle mix screws (you'll need to keep readjusting your idle stop screw also, to maintain 750-800 rpm).
Then - after you've optimized timing, adjusted idle mix screws for best idle, reset idle speed stop for proper idle speed - NOW it's time to play with your off-idle hesitation. SLOWLY INCREASE throttle off idle in gear underway to try to find the hesitation. If there is a hesitation, you can either richen the idle mix or you can slightly peen/redrill the idle air bleeds.

If you can move the throttle SLOWLY with no hesitation, but you get a stumble when you move it normal speed, you can make sure your pump squirter linkage doesn't have slop or a dead spot. There are also different accel pump cams that have different shot profiles.

Holley tuning is straightforward.

The increase in performance with the spacer is expected in a motor like yours, but you lose some airflow signal strength at low speeds and you have to accommodate that in your tuning.

Did you run the spacer before jetting it up? If not it could very well be that you were lean to begin with. As others said bring the idle screws out another 1/8 to 1/4 turn to get over the off idle stumble. Since the engine likes the increased plenum area with the spacer on a dual plane you may very well find a good bit more with a single plane.