by those cam specs..looks like that may be the limiting factor...Rob

The cam is plenty big enough but it really is old school and any hyd. flat tappet has trouble controlling the valves above 5000 rpm. I'd find a good cyl. head guy to do a valve job and set up a new set of springs with a little more pressure than Crane reccomends. Or just change to Crane 741 hyd. roller cams. They are tried and true and will get you 700 hp in a set-up like this. Btw, what ignition timing are you using? in a set-up like this I would use 30-32 degrees.

It would be good to try a set of 25 pitch props. No boost until 4000 rpm's suggests that you are lightly loading the engines right now. They may pull the 25's to a higher speed and show more boost at a lower rpm.

Ding ding ding on the timing. how much diferent timing did you use on the dyno? Did you try it to 32,33,34,29,28 etc?

You could have to much or to little.

Carb should be fine. A lot of thes set ups historically used the twin 750's actually.

Roller cam is a good idea, but there is no reason he shouldn't be spinning past 5k IF he has the right spring.

Try adjusting the timing a little.

Check and make sure the belt is not slipping. Do you have the notched pulley set up or the grooved pulley set up. That could be why you are not making the proper boost.

What exhaust do you have on the boat. That could be slowing you down.

Just other factors.

If you had the arrestors off on the dyno, that is not the problem now.

Thanks for the continued advice. Was on a cruise to the Carib. back now in cold Roch. NY. I have to think hard on the timing and total advance. I am running a Crane Hi -6M ignition with a 650Sc Ign. Curve. I think I am running 10-12 initial with 32 total advance. I only have a set of 3 balde 19P props to test with so not sure if that would do much for my research.

Let me know if you have any more thoughts.

thanks

Not sure of this info I will have to check back with my engine builder. i'll have more info this week.

Back from a cruise to the Carib. Now back in cold roch. NY
I am Not real familiar with cam's and specs. does this crane 741 Hyd. roller have the same specs as what I have or would I order a Crane 741 to match my specs??. I am sure I am running 32 total advance. I am using a Crane Hi-6M running the SC650 Ing. curve.

Right now I am only running 4 Blade Hydro. Eng 23P props. I am also running 1.36 Bravo 1's. I am changing over to XR's for next year 1.35's. I have to reprop. i am trying to make all the right decisions before I buy a new set of wheels.

Thanks for the input. All opinions are valued.

Jeff thanks for the Input i was off on a cruise in the Carib. back now.

I didn't do alot of timing adjustment on the dyno. i was letting the Dyno guy take the lead when testing. I am running a 16 Rib grooved belt. Not a Cog belt so I am sure I have some slipping How much ?? I have stainless Marine exhaust
I have one of the engines out right now. Should I have My engine guy Test/look at the springs. I think I need to find a new engine builder in my area. I'm not confident with my guy as far as a blower motor he does not have alot of experience with blowers. If I switch to a Hydr. Roller cam what can I expext from that.

cocktails,i put a 509 with 8.0 compression in a boat i had.i used a bds 8 71 with a pair of holley 750 carbs,msd 6m ignition,gm heads with ports cleaned up,crane 741 hyd roller&spring kit.this setup made 720 hp at 5600 rpm at 8 lbs boost,also had cmi headers,not manifolds.your 420 can easily make 8 lbs of boost.i think 32 or 33 total timing is right,i always lock the distributor advance on all blower motors so you have total timing all the time.one more thing to consider,are you getting enough air with the engine hatch closed,if i opened mine up 2 inches i picked up 2 mph.[more and cooler air]my thinking is your valve springs are to weak and won,t let the engine rev.i have seen it happen before.oh,i forgot to mention,i used[and still have]a 27 pitch 3 blade,slight cavatation until on plane,then hooked up real good.