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GT40 still not reaching top speed |
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87MCProstar 
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Joined: June-08-2010 Location: Northwest CT Status: Offline Points: 25 |
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 Topic: GT40 still not reaching top speedPosted: August-06-2020 at 6:18pm |
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Hey everyone, a buddy just picked up a 97 with the Pro Boss. We replaced the fuel filter and have seen a little increase in top speed with barefooter 36 to 40, we are replacing plugs/wires/cap/rotor. If we don’t see true top speed where else should we look?
Thanks in advance Tim |
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MrMcD
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Joined: January-28-2014 Location: Folsom, CA Status: Offline Points: 3749 |
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Posted: August-06-2020 at 6:22pm |
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You just got this boat, how old is the gas in your tank?
What prop are you running and at what elevation? |
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87MCProstar
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Posted: August-06-2020 at 9:02pm |
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He has run a couple tanks of gas through. The evelbation of the lake is 1,300’. He’s not sure of size/pitch of prop just knows it’s a 4 blade.
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MrMcD
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Joined: January-28-2014 Location: Folsom, CA Status: Offline Points: 3749 |
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Posted: August-06-2020 at 11:47pm |
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Mine is a Pro Boss, 95 Ski Nautique closed bow. Runs 46+ with an Acme 422 at 1,500 ft elevation so you should be close if you have the similar hull and prop.
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slmskrs
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Joined: January-03-2012 Location: SF bay area, CA Status: Offline Points: 522 |
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Posted: August-07-2020 at 2:23am |
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What are your RPMs at WOT? And just to confirm, you have the ProBoss carb rather than the GT40EFT, correct? MrMcD, what is your WOT RPMs @46MPH+? I don't recall what mine is; been a long time since I've gone more than 36MHP... ;)
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Gordon '97 Ski Nautique, GT40
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slmskrs
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Joined: January-03-2012 Location: SF bay area, CA Status: Offline Points: 522 |
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Posted: August-07-2020 at 2:25am |
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Oh, sorry, didn't notice GT40 in the title. I might hook up a pressure gauge to the fuel rail and see if there is any drop in PSI at WOT.
If you don't see any issue there, you might want to do a compression test to rule out bad rings on a piston or two.
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Gordon '97 Ski Nautique, GT40
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MrMcD
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Posted: August-07-2020 at 3:22am |
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Mine is EFI, GT40, 1995 was the first year for this engine, it will RPM at wide open between 4,800 and 5,100. I am pretty sure with the 422 it only turns 4,800 but formerly I ran a 224 Acme and it turned just over 5,000, on a good day 5,100. I am speaking from memory and I have not opened it all the way up this year. I am sure it is recorded in old posts on here from when I bought this prop. When the 224 was new the boat was 2-3 MPH faster top speed. I liked that prop but it was damaged and never the same after repairs.
I found I get a touch more RPM on premium fuel. 91 octane. Sounds like yours is off or has too much prop to turn.
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87MCProstar
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Posted: August-07-2020 at 8:47pm |
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His is EFI, we changed fuel filter, plugs, wires, cap and rotor looked decent. Went for a spin after and was only hitting 42.0 at 4800rpm off the PP gauge. Doesn’t seem to be missing, sputtering, running rough. Maybe it is a fuel pressure issue idk, but Went to change the coil wire to distributor but the new kit from skidim was different than wore that was on there. The connection at the coil wasn’t the same as the distributor. Don’t know how to import pics while posting from phone. So maybe it’s a lack of spark strength?? I’m at a loss
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87MCProstar
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Posted: August-07-2020 at 8:51pm |
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Sounds like we need to pull not to find out what prop is on it
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slmskrs
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Posted: August-09-2020 at 12:59am |
I'm pretty sure your PP data is dead-on, but does PP more or less match what your tach is showing? Sometimes the 'inverted' setting in PP is better; don't know if that would change how PP counts RPM, but just want to make sure you are really turning 4800 at 42 mph. 42 is a bit slow, but generally you're not going to get a whole lot more than around 45 or so regardless because of the hull wetted surface. I would definitely pull the prop. It could be that the prop was improperly repaired in the past. However, since it is the low end of RPMs AND low end top speed, I have my doubts on it being the prop (although worth getting it checked/repaired). I also don't think it is fuel pressure because I once had an issue with pressure dropping at higher RPMs, and it was a very noticeable drop in power. So, I would still probably do a compression check if you think it is the engine.
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Gordon '97 Ski Nautique, GT40
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Fast351
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Joined: September-18-2006 Location: Twin Cities, MN Status: Offline Points: 35 |
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Posted: August-16-2020 at 3:30pm |
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4800 RPM is about the max you want to run. RPM is directly related to prop speed, so if the engine is turning 4800, but you're only doing 42 MPH, the prop may be the issue.
Also, use a GPS speedo app on your phone to verify speed. That will be more accurate than a paddle wheel. FWIW, my 01 with a GT40 does around 47 MPH with an Acme 422. It spins to about 5K RPM. |
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2001 Ski Nautique 196
2007 Air Nautique 211 TE |
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87MCProstar
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Posted: August-16-2020 at 9:15pm |
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Okay I can check compression and will download a GPS Speedo app to double check what PP is showing. I’ll also have hime check the prop size and condition.
Thanks for the help! |
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MrMcD
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Posted: August-17-2020 at 3:29am |
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Had our 95 out the past few days. 422 Acme, it ran 46 mph at 4,800.
This was at 1,500 ft but temps were hitting 109* today when I ran it one person in the boat and half tank of gas. This 422 is pretty new and never hit till tonight. Found an underwater log while idling in the 5mph area. Prop still ran good after the hit but I will need to look for a bent blade.
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Jonny Quest
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Joined: August-20-2013 Location: Utah--via Texas Status: Offline Points: 2974 |
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Posted: August-17-2020 at 12:16pm |
McD: too bad about the log strike. These CNC props are amazing when “virgin” but repairs can be hit-and-miss (pun intended). Hopefully your repair works out well. I almost look at these props as disposable.... JQ
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Current
2003 Ski Nautique 206 Limited Previous 2001 Ski Nautique Open Bow 1994 Ski Nautique Open Bow Aqua skiing, ergo sum |
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Fast351
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Posted: August-17-2020 at 10:04pm |
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I banged my prop up and had it repaired. It is nice and smooth (although it whistles at 1200-1300 RPM). I did have to bump my perfect pass up about 30 RPM at 32 MPH, so it lost "something" although it appears to be very minor.
Someday I might buy a shiny new 422. Or maybe not.
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2001 Ski Nautique 196
2007 Air Nautique 211 TE |
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MrMcD
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Posted: August-17-2020 at 10:10pm |
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I ran it after the hit, could not pick up on any vibration and everything seemed normal. Odd since I was driving and felt the hit, I spun around and saw about a 5" diameter log rolling under the surface, our lake is pretty clear, 10' plus visibility at least.
There were people waiting for me at the dock so I went and got them and when we returned I could not find the log again. Maybe I was lucky. A quick visual did not pick up any sign of contact but I am telling you it was a thud when it hit and the boat lurched a little bit.
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87MCProstar
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Posted: August-19-2020 at 8:33am |
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I have seen posts about a database for non-marine GT40 parts. Does anyone know that location?
And would a faulty ignition coil with a poor spark be causing this speed issue? |
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KENO
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Joined: June-06-2004 Location: United States Status: Offline Points: 11093 |
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Posted: August-19-2020 at 9:46am |
The link below is probably what you're looking for with info scattered all over the thread Bad coils usually have things like misfires, stalling etc associated with them, not a smooth running engine that doesn't get enough speed in the boat. Like somebody already mentioned 4800 RPM and 42 MPH just don't go together on that boat Don't you have a neighbor who spends some time on this site and has about 600 or so props hanging around that might let you try one?
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rosconole
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Posted: August-19-2020 at 5:02pm |
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Wait Keno doesnt KNOW a GT 40 answer!!!!! he should be sent to the penalty knowledge box of mastercrafts forum for atleast 3 months!
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1989 ski nautique ,1991 barefoot nautique, 1993 Mustang Cobra 1998 5.0 Magazine shootout invitee
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87MCProstar
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Posted: August-19-2020 at 5:28pm |
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Okay just trying to narrow things down. Connection anode didn’t look normal.
I don’t know if he has 600, I’ve reached out through text and had been helping. He just isn’t comfortable with GT40 stuff |
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KENO
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Posted: August-19-2020 at 6:43pm |
That's enough out of you "Shaker"
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KENO
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Posted: August-20-2020 at 7:09am |
OK...................So I might have rounded up a little on the number of props he has It seems like another prop to try would help you determine if it's an engine issue or a prop issue though. Did you figure out what it has for a prop, or if the tach reads right yet?
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8122pbrainard
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Joined: September-14-2006 Location: Three Lakes Wi. Status: Offline Points: 41045 |
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Posted: August-20-2020 at 7:35am |
Tim, Fill me in on this anode. Maybe I missed something or maybe I can learn something?
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87MCProstar wrote: