I have a 71 glastron with a JC pump, unknown cut on the impeller. Ive done a few things to add power to the 350 that's currently in it, but can't seem to get the thing to turn more rpm. 4200 max currently and from when I originally purchased it.
Original setup was just a goodwrench 350 (like 8.2:1 compression)with a edelbrock vacuum 600, osco manifolds, performer intake.. basic, reliable.
New setup currently:
Ported fresh double humps set up for.500 lift 64cc chambers
Thin .015 head gaskets
9.2:1 best I could math.
Performer intake
Holley 670 vac seconds
Mechanical and vacuum advance is locked out at 33⁰
Clay smith .450 lift .280 advertised duration  on a 108⁰ center (degreed in)
 
Still no more rpm than it originally had. It comes out of the hole pretty good, doesn't really drink much fuel compared to the edelbrock carbed setup. Definitely more crisp response and smooth running.
I'm guessing the boat had a big block in it originally, as I don't think glastron offered the Sbc. They did have the 351w that year, but with the bell housing being BPO I'm conflicted. Would it make sense this thing has a AA cut or?
I  plan to eventually get a place diverter, just curious what's going on with it before I add that. 
I'm all ears!
Long time lurker and diesel/horsepower junkie.

highly doubt it has an AA impeller in it but doesn't matter anyways as the motor refresh netted no increased RPM.  You need to look at what stayed the same on the boat between the 2 engines.

Same distributor?  rev limiter?

Same fuel lines?  possible too liitle fuel flow to increase HP?

Same spark arrestor?  too small causing a restriction at the 250hp range?

Not enough air entering the engine compartment?  have you tried running with no cover?

Have you checked fuel pressure or vacuum at WOT?


Brother on law has an older commander 16' jet with stock 350 and A impeller and i think he turns 4300 or 4400 rpm which matched the 265HP rating. 


Something is up, sorry for so many questions but hopefully you find it.....

The distributor is a factory gm hei, I locked out vacuum and mechanical advance on it during the swap. So it kind of changed.
Never have measured vacuum on the engine, still has the 6.5 power valve and idles clean though.
Fuel pressure is good, the gauge on my log bounces between 7 and 12 psi, new filters etc.
I've tried without the arrestor and without the cover when dialing In vacuum secondary springs, no noticeable difference. Same fuel lines 3/8".
I'm probably going to tear into the pump Sunday to see what impeller I have, no i.d. tag on the pump. I was told today i should be running a b+ impeller.  It's just weird, my buddy has exactly the same boat with a jg pump and a A cut and the 351w. The boat will turn like 5300 but he has a place diverter.
Thanks for your reply.

It does sound like you have a A2 impeller.
My SBC, about 325 hp, turns an A impeller to 4600/4650 rpm.

Between the parts in your SBC, I'd think you'd be at a similar HP level.

As far as a 351 and 5300 rpm, either it's one stout 351, at about 475 HP, or it's a loose pump, (most likely)

Dan'l

Will the gain in rpm with a b cut net more mph? The cam Power band is good to 6300 rpm, guessing the heads probably are maxed on flow right about there also. I plan to build a nice roller engine this winter, be nice to just cut it once and be done.

It has a A2, and a junk shaft. 🙂

Figured I'd add along here... vacation this week so a little extended relaxation.
Flashback a few weeks, I bit the bullet tearing into my pump to discover my a2 impeller.  I read nothing but good about Tom and Jet boat performance  so I decided to call one afternoon,  the guy totally hooked me up with a B/C CUT  impeller, new shaft. He got me the parts fast too (knowing I wanted to boat, also probably a little annoyed some Farmer in Illinois keeps asking tons of questions.)
Fast forward a little, take the boat out with the new pump( huge difference with hole shot and top end, boats enjoyable, runs 52mph.) Turning those new enjoyably rpms runs my pan dry and it tosses a rod!. I already had in the works a .060 over 350 (360):Forged flat tops trw''s(the old ones),  balanced steel rotating assembly, sourced some 200cc runner 64cc chamber ALUMINUM heads, 7 quart pan ,HV oil pump, Went through and fixed a bunch of previous owners mistakes (the motor mounts were in different holes the engine was not lever or straight with stirgers or pump, thoroughly cleaned the hull, yeah the boathull loll. Bright white epoxy to show them Leaks.)

anyhow, plopped everything into the motor, full roller goodies, .474 lift custom clay smith cam degreed at 106⁰ center , swedged pushrod. Etc etc

The fucking boat rips! So fun, went and did a break in Friday, back home 40 miles, reset valves, see where I ended up with timing and add some fuel (lean can be too mean sometimes). Took it out today and ran it a little harder... 56 mph on the GPS down river full of fuel just me!
 
I

What's next? Place diverter? A little better bowl, mine is a JC and I feel it may be cracked somewhere? Everytime  I'm floating an go to the back of the boat and see a slight snail trail if you know what I'm saying. 😃

Dirty boathull