Under what conditions would having a less restrictive flame arrestor reduce power output? In this scenario, would this indicate that the carb jetting is already rich and allowing the engine/carb to breathe better is in effect drawing in more fuel and compounding the issue? Or is it that the jetting is already lean and allowing the carb to breath better, that it is just introducing more air and making it worse in the other direction?

As a second part, would it be best to have the least restriction possible (best airflow) and then jetting up or down based off of this new airflow?

Thanks

It depends. How’s that for a great answer ? Lol.

Typically, if an engine is tuned well with no restriction from the flame arrestor, installing a restricted flame arrestor will cause the carb to run richer.

So, you guys with dirty flame arrestors - yes there are lots if you - take it off and clean it. Once a year is typically fine.

Okay, retread your post.

Yes, if you are close to losing power due to too lean a/f and you install a less restrictive FA, this can causevyoubto lose some power.

Carburetors also depend on certain direction if airflow for the bleeds and boosters. It’s possible some FA’s mess this up. Many have seen this on dyno and racetrack with cars and air filters . Some scoops can screw this up too.

Worked on a boat (disaster) once, 509’s with 8-71 blowers. Had those tiny pie dish flame arresters barely the size of the carb flange, filled with belt dust etc, worn out distributors, timing all over the place.
put 4 new flame arresters on of proper size, 2 new msd distributors, plugs, wires and made the mistake of giving it back to the customer without putting an air/fuel meter on and running it myself. Boat ran awesome for 30 minutes

Now what if the carb is jetted base on this particular restricted flame arrestor? They have been paired together on two different builds but never dyno tuned. Is there anything to be gained by opening the restriction (removing the majority of the mesh) and adding jet, or does that just put the balance back to where it started and no power to be gained?

What flame arrestors are we comparing? A little more info might help. If you tell me you put a K&N on and lost power I'd say get rid of the K&N...

How are the Spark plugs reading?

This is very true. I’ve seen some engines lose power with no filter on top. A little bit of air tension helps straighten out the flow into the carb.

it was a short story,the aluminum was abundant.

Many FA's have sharp bends close to the air horn of the carb causing a lot of turbulence. I put a vacuum gauge on my engine and noticed that I was pulling vacuum running above 5500rpm with my factory FA. I found a velocity stack unit that was designed around optimal flow into the carb that uses a K&N 14"diameter x 4.625" tall marine approved filter. Yes, I had to re-jet the carb and no I don't pull a vacuum now. Fortunately the taller FA worked better for my boat. I have NACA ducts pulling in air and I opened up the hole in the hatch so the filter sits right inside where the fresh air is coming in.