GT-40 bogs/lacks top end speed

I'm always interested. Whether or not I'm helpful, that's another question

I just realized in the bottom picture, that's the LP fuel pump.

The top picture is your pressure regulator. The other pictures are your LP fuel pump. The "bicycle" is the schrader valve to test the fuel pressure it should be 39 +/- 3.
A fuel pressure gauge is what you want. When you turn the key to the first position you should hear your pump pressurize the line, it should have the 39+/- 3 as stated above. Then check that it stays at 39+/- as you throttle up if it doesn't it may be that your pressure regulator is stuck in an open position.
Top end on that boat should be closer to 48mph @ 5200rpm thats what I get out of mine with a 1442 prop.
Also check your vaccuum pressure the fuel regulator works off vaccuum pressure.

I'm thinking you have a fuel pressure problem. The pressure should stay constant from idle to WOT.
I think it could be two things
1) HP pump going bad. (not maintaining pressure)
2) Fuel regulator stuck/gummed up, not fully closing. (Allows too much fuel to return to tank when accelerating. )
The fuel regulator opens with vacuum to allow the extra fuel not needed at idle and low rpm to return to the tank. As you throttle up the vacuum decreases and the regulator valve closes more, routing more fuel to the rails to maintain the 39+/- 3 psi

Since the fuel pressure starts at 40 then is fading with out the engine running I don't think its a vacuum problem.

Luchog wrote: what about leaking injectors?

Luch,
I would think that would make it run rich at idle and hard to start. He said the hole shot is good but stumbles at 15 - 20 (assuming mph not RPM)and he's only getting 38mph at WOT. He should be getting more like 48mph with that boat.
By the way scooter do you know what prop is on your boat. Not that it has anything to do with the engine problem just curious.

I checked with the PO about the prop.... he said as far as he knew, it's the original 3 blade prop from factory. Nothing in the manuel tells me what the standard prop size is. I was reading the manuel and it shed some light on why the pressure on the gauge was dropping with just the key in the on position. The FP will only run for 2 sec and then turn off. I will say.... it would go from 40 to 20 in less than 60 sec. I would have thought it would hold the pressure. May not indicate anything?

I haven't checked the plugs only because the PO said he replaced them summer before last. When I got her there were 311 hours, now there are 351. But, I will eyeball them asap.

When I drove to Alabama to see/get her, while on a ride, the PO mentioned it surging but it was so minimal I didn't notice it. Plus I tripping out on the idea of owning her, I was numb. So the problem has been there from day one..... but it's getting worse.

Thank you guys for attempting to help. So what's next?

Hello,
No one responded from my last reply, so what I realize are the plugs are good and it's not running rich. I bought a fuel pressure gauge and was checking the pressure last night at the schrader valve and this is my update:

I was not getting 39 psi while just turning the key on even though the FP was pressurizing. I would have to switch it on and off 2-3 times before it reached 39 psi. Cranked and in neutral I have 34, in gear up to 2000 rpm I have 34. I pulled the vacuum hose from the FPR and I got 40 psi and the rpm increased. Gun-Driver mentioned earlier that maybe it was the HPFP or the FPR.

Any suggestions? I'm finding out that EFI makes pinpointing an issue much harder.

Shawn hard to tell from those pictures of the plugs if it is running rich or lean.

It sounds as though the early GT40 engines did not have the FCC.

It is not often that we send people over to PN for troubleshooting.

GT40 Fuel Pressure Check

You apply a jumper to fuel pump relay to test.

Fuel Pump Relay

Detailed description of fuel pump relay.

The problem with these GT-40 EFI/MPI is that even though they're strong and efficient running motors they are hard to trouble shoot on descriptions. The computer monitors and controls everything from spark advance,fuel/air mixture,temperature,oil pressure to a self preserving knock sensor.
So lets say you get some bad gas its hard to pin point the problem because if the motor starts to detonate due to the poor quality fuel the motor protects itself with the knock sensor. So to diagnose a simple fuel problem it's harder with out putting the motor on the computer box to check codes and so forth.
This was a rough explanation but I hope you get the point I was trying to make.

Gun-Driver I know exactly what your saying. Down here in the south they would tell you.... "sho you right" (sure your right).

Bri892001.... that has been my gut feeling all along. That's the direction I am going. I have to order it, so maybe in a week I will have some good news.

That's good to hear. Glad you found it

So what should my speed be at WOT with a 95 GT 40 that has 350 hours and a 3 blade prop? How much does weight matter because I had 280+ pounds in the boat plus my 170?

Do you know what your RPMS are at WOT? Are they close to spec for that engine?

45mph doesn't sound bad to me. The Sport Nautique is a little bigger/heavier than a Ski Nautique, so I would think it wouldn't go quite as fast with the same motor.

That doesn't sound like a lot of weight but it is some weight and if they were in the front, that could matter too. Do you have the stock 3 blade or an ACME?

Oh, I think it was the angle of your avatar picture that made me think it was a sport. It kind of makes the front look longer.

Sounds like you're in the right range for RPMs. Yeah, I'd bed the difference has something to do with the prop then.

prop can be a determinant factor on wot speed...
anyway I was just kidding! lol...
Oh I´m getting 47-48mph(gps) wot @4700rpm (not sure if my tach is accurate) with the acme 1442.
I have a fresh carbed 351w with gt40p heads, perfomer intake, CR cam, 310 hp...