jryan26

Hp500s converted to 540s, crane 741 cams, 800cfm carbs, stock gm heads, around 9.5 to 1 compression.
29 labbed hydros=about 80 on speedo @ 4300 rpms
27 hydros= 77 on speedo @ 4300 rpms
I cant get any more rpms! The power just seems to drop off at 4300 rpms.
I've been told that it may be anti-siphon valves in the fuel lines cutting off the fuel. Have any of you had this problem when going with bigger engines? What about ignition? I have stock merc ignition, but so do friends with over 725 hp motors with no problems.
Thanks
Merlin single plane intakes also.

paradigm shift

Sounds like a nice combination. I would check total timing should be around 32-34 degrees and get a fuel pressure gauge in it if you do not have one. Did you do a plug check and what do they look like? 800 carb shoudl support more RPM then you are seeing. Stock heads will limit you on a 540. Just my .02 Good luck.
Oh yea I am not a guru. I know enough to get myself in trouble.

HeavyChevSS

Is your timing set correct, last time I had work done on my motor I got it back and lost a lot of top end, bumped up the timing and picked up 400 rpm. Mechanic set it wrong from the get go.

jryan26

timing is at 36, fuel pressure is at 7lbs wot.

Turbojack

How did the HP500 perform in the boat? What prop did you use with HP500? RPM & Speed? Are the heads from the HP500? Is tach correct? New plugs & wires? New distributer cap?

PatriYacht

New valve springs?

mcollinstn

You've got it timed to 36.
If you have a knock detector, disable it to rule it out.

You've got 7psi at WOT.
Have you made sure that your floats are set on both bowls, and that your linkage is fully opening the secondaries?

Have you made sure you are getting unrestricted airflow to the motor at WOT?

Have you made sure your oil level in the new motor is correct? Sometimes the dipstick calibration can be off and you can turn it into a hydraulic blender inside.

DO you know for sure your cam timing? A 741 is supposed to be installed "as ground" which is plus 5.

Have you done a plug chop to check your fuel mix?
Have you removed (with plugwire pliers) each wire from the distributor at cruise speed to double check that all 8 are "playing well with each other"?

Done a compression check?

Without a comprehensive report on the above, I can't give much in the way of recommendations.

jryan26

Mcollinstn--the bowls are correct, and the linkage is fully opening. The pan is from the hp500s as are the dipsticks.
The cam was degreed 2 advanced above and beyond the plus 5. Could the fuel/water seperater be restrictive to fuel flow? New springs. It runs very smooth, and is very responsive up to 4300.

z.zuperboat

That combo will make 625hp with those heads.As far as fuel, if it was running lean the pistons would be melted by now.Your down about 1000 rpm thats alot.Any other changes to the boat gauges/drives/anything.The boat has good speed for 4300rpm. Need more info?How fast did it go with the 500?

jryan26

I never ran it with the 500s. The previous owner said 80mph in salt water with 26p bravos. They are in a straight bottom 32' fountain.