Merlin makes a 208 cc intake runner replacement head. I'm guessing that's what he used. The 990 heads they came with had 219cc I believe

Thats not even close to the specs on any Merlin head or stock 990
Stock 990 head is 119 cc chamber, 325cc intake runner , 2.19" intake valve, 1.88 exhaust valve

NO Merlin head was a 208cc intake runner
360 is more like it

Here is the best advice you will get....
Ask a local engine builder not a mechanic or Internet guy.
They are all just guessing !!!!!!!!

I told you on your other post single plane intake is wrong for 5200 rpm, you loose torque which is what is really needed to spin the prop.
If you have the big merlin heads with a single plane intake manifold with a stock 420 bottom end your LOOSING hp and Torque from a stock 420

That's a fact I've run 420s on the dyno many many many times

yes, checked em when new 3 years ago, I am sure they are still good, especially since both are identical..

I agree, just getting many opinions, last year I almost put stock 420 heads and intakes back on it, I still swear the heads were just a tad bigger when I ran numbers, but I am going back 5 years and cannot remember... I was new to BBC at the time and thought the combo was decent, but I am more educated now somewheat on BBC and wanted more opinions before I start changing things.

I figure with a smaller intake (DUAL PLANE) maybe will help me get some back until I decide what to do about the heads...

I am gonna try to find my numbers on heads I had logged somewhere to see what they were...

the merlin head i very similar to the gm head,so i think port volume is not the problem,that engine needs 34 or 35 deg of timing,and should be proped to spin 5200 rpm

I run 34 degrees timing now... with 21P props I was spinning 5000, so I would have to use 19-20P props to see 5200, no boat I ever heard of with these engines spin that small a prop, I have an issue to resolve..

just fyi,a carbed hp500 comes with a single plane intake,and max rpm is 5250 in stock form.

what gear ratio are your drives?

1.5

ya,something seems wrong,are u running the stock merc ignition,and does it have a rev limiter?