No more power past 3/4 Throttle
 

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

Re: No more power past 3/4 Throttle
 

Did you change valve springs??

Don't ask why I know about this :D

Re: No more power past 3/4 Throttle
 

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

Re: No more power past 3/4 Throttle
 

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.

Re: No more power past 3/4 Throttle
 

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.

Re: No more power past 3/4 Throttle
 

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.

Re: No more power past 3/4 Throttle
 

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.

Re: No more power past 3/4 Throttle
 

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

Re: No more power past 3/4 Throttle
 

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

Re: No more power past 3/4 Throttle
 

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.

Re: No more power past 3/4 Throttle
 

I did a plug reading at 4k which puts me just into the secondaries and the plugs were very lean. that was 80 front 82 back. So I upped to 82 front 84 back that worked good at 4k. I have not taken a reading at WOT yet I'm going to check that this weekend. Transom is clean though so I don't think I would be that rich.

Re: No more power past 3/4 Throttle
 

I'm running a -8 from the pump to the carb. Gauge is at the pump. I am running in line fuel filters by earls. They are after the gauge. I could try to move the senders up by the carbs to make sure I'm getting it up there. or take the filers out for a run.

A plug reading this weekend should tell me if I have a fuel problem. It does make sense. Especially the jet changes with no rpm or mph changes.

Re: No more power past 3/4 Throttle
 

Not sure if this will help but I had a 31 with 502's, hyd roller, dart int, stock heads with bowl blend, 800 carb, gill exh with a 4 1/2 tail pipe and ran 79 on GPS with 25 Mirage wheels at 5450. I figured the motors made 450hp or so. 27's ran 82 at 5200.

Re: No more power past 3/4 Throttle
 

Bob that does help. It helps me realize something is definatly wrong... What kind of 31. How much did it weigh.. I know this boat of mine should go deep in the 70's if not the low 80's.

Re: No more power past 3/4 Throttle
 

Are the carbs in box stock configuration? An 850 on a 502 should be real close the way it comes from Holley. If it isn't in stock configuration I would put it back and start over from ground zero. Have you tried to run the motors without the flame arrestors just to see if it makes a difference? I also wonder what a single plane manifold would do compared to the air gap on this motor, particularly over 5000 RPM. The headers are still your biggest problem.

Re: No more power past 3/4 Throttle
 

Vette I started from box stock. They ran good but it was lean.. So I started upping jets 2 sizes at a time 82's up front are good. I have not made a plug reading at WOT but I have stepped up from 78's in the rear all the way to 86's and there has been no change in rpm or speed. That and some of the responses on this thread are leaning me toward poor fuel delivery. I am going to yank the fuel filters this weekend. That will make my gauges more accurate. Currently my gauges are before some inline fuel filters I added. I am thinking they could be restricting flow. I also will be taking a WOT plug reading.

I agree manifolds are killing me but I think I still should see at least md 70's even with those manifolds.

As soon as I scrape up 5 grand I will replace them... Problem is they have to be custom made. so cost is crazy...

Re: No more power past 3/4 Throttle
 

Leathl I had a 31 Sutphen around 7500-8000#.

Re: No more power past 3/4 Throttle
 

I had mine on the scale with me in it and half tank of fuel it was 7200...