GT-40 bogs/lacks top end speed

Hey Brian - I did replace the plug wires back on 7-19, and have been out 5 times since then and the boat has run perfect. I had to go at least 3 times before I felt for sure it was fixed. I appreciate your help.

Thank you for the advice. Wires have been ordered and I will let you know if that is the fix.

The Distributor Cap looked ok. If I were to replace the wires, do they have to be marine grade or can I pick some up at the local part store? One thing we noticed is how the wires are bundled together and touching the block. There just wasn't a lot of slack there and I could see where if the wires were bad they might ground out on the motor. Is that normal for the wires to be tightly bundled and laying againt the block?

Hey CCF,

I am still dealing with the same surging issue that I've had for the past 3 summers with no resolve. Thought I would seek your advice again. Fuel pressure @ idle: ~37psi
Fuel pressure w/regulator disconnected @ idle: ~41psi
Fuel pressure @ WOT: ~39psi
Fuel Pressure w/regulator disconnected @ WOT: ~41psi

Motor would surge when under a load (typically between 35-40mph)
Fuel pressure remained consistent during surges

Motor continued to surge irregardless of whether or not the regulator was connected

Schrader valve on the fuel line gave us some issues. It was loose so we tightened it. However, once it was tightened the valve stayed partially open and leaked fuel. We loosened it and this stopped the leak.

Checked resistance in the TPS w/multi-meter. Needle made a clean sweep from idle to WOT.

Note that one of the plug wires arced on my hand. Wires are supposedly only a couple of years old according to PO. They are marine grade 8mm wires.

Not sure where to go from here. When we're at a cruising speed between 35 and 40 (it does surge at slower speeds but it's more noticeable here) the boat feels like it has lost most of its power for maybe half a second and then regains power. It is a very sharp, noticeable contrast. The motor doesn't sputter or sound like it's missing, it just sounds like the key has been switched off and the motor naturally reduces rpms but then instantly regains all power. Sounds like it would either be fuel or ignition related. Is it possible to have fuel problems but still maintain high pressures during the surging?

Any suggestions would be appreciated.

Hey Guys,
It has been a while since I have updated the thread. Since replacing my HPFP the boat has ran so much better! I wish I could say there have been 0 surging issues but I can't. It has been very minimal though.... only a couple of times. I have been skiing trying to make up for lost time,so all is well.

Fly100, this is what I'm getting out my 1995 SN with the GT40 EFI: 45 mph @ 4800 rpm. Im running a broke mans 3 blade prop LOL.

I'm all smiles because we able to play in slick water.

Walter,
Seems my problem was my high pressure fuel pump. I say that carefully because I haven't been out for a good 2 hour ski yet. The initial ride was smooth but I wont know for sure till maybe Thursday when I go looking for slick water.

What exactly is your boat doing?

I didn't suspect it would. So its actually better than candy, cause after you lick the red off its not as good. Ha

No.

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...

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.

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?

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?

That's good to hear. Glad you found it

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.

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.