I just built 2 502's over the winter. They are stock compression. Heads have been pocket ported. Cam is a 238 248 Comp Cams Extreme Marine, 850 Holley DP's, Air Gap intakes. The exhaust is crap stock manifolds with gil dry risers.

The boat runs like a raped ape to 5k. I still have a 1/4 throttle left. But the last 1/4 throttle does nothing. No rpm increase. No speen increase. Not even a change in sound of the motor.

The boat is a 30 Sutphen, Props are 25 pitch Mirage's. They are in good shape no nicks. at 5k I'm 70mph gps. It's like I hit a wall boat wont go no faster rpm no higher.

Carbs are new 4 corner idle, 6.5 pv front and back, 82 fronts 86 rears. I have gone form 82 rears to 86 rears no change in performance. I have yet to pull a plug after a hard run I am going to do that this weekend. At midrange plugs look good.

Just wondering if this is normal is it possiable I'm making peak hp there. I would think I should get some increase somewhere with 1/4 stick left. Throttles are opening all they way at carb.

Rob

Did you change valve springs??

Don't ask why I know about this

Good thought but yes I did. Springs are also new and were individually checked before installation. Motors were professionally built. Not that no one is capable of maikng mistakes. But I would bet money the motors are right. Leak downs are good. Compression check is good as well. Cams installed straight up.

a few more things:

Timing is at 11 Base with a V8-24 module. Which should put full advance at 35.

Cap, wires, plugs, rotors all new.

Fuel pressure at 8 on one motor 7.5 on the other at 5k.

Rob

Pull your flame arrestor off and look down the carb to see how much your throttle butterflies actually move with the last 1/4 throttle movement. Depending on the geometry of your throttle linkage they may not me moving much at all. If the carb is slightly too large the motor could act like it is at WOT even though there is still some left on the stick. Just a thought.

good thought I'll check that..

I don't think the carb is too big, per every formula I have seen 850 is right on the money for a 502 spinning 5400. I know now the cam seems to be too big.

I agree with wet vette, make sure you are getting WOT at the carb, but also may be a fuel delivery problem. I replaced my fuel tanks and had the same problem, normal operation untill the last 1/4 throttle...and nothing. Turned out the pick-ups in the fuel tanks were too small. I had prochargers, but maybe something in your system is not allowing enough fuel.

Good call guys I looked at it during my lunch and the last 3 clicks on the throttles do very little. At 3/4 throttle I am almost as wide as they will open.

That actually is bad news though. Now the question comes up as to why I can't spin a 25 pitch prop past 5k. The boat only weighs 7k loaded. Motors should be close to 500hp each even with the ****ty exhaust. I would guess 550 plus with some good exhaust.

Per BAM's caculator with 1000 hp and 7500 as the weight the boat should go 82. Even at 800hp it says the boat should go 72

The prop slip calc is right on the money. 1.50 drive 25 pitch 5000 rpm 12% slip 70mph.

Again fuel delivery.... double check fuel pressure at wide open...UNDER LOAD...!!!!

Are the secondary jets to large??????????????

I have 7.5 psi on one motor and 8 on the other at 5k under load. The guages never move from idle to 5k thats what they read. i have switched senders and the read the same.