gt-40 lacking top end

Hello everyone,

I recently purchased a 97 Ski Nautique with the GT-40 engine. We've put about 50 hours on it so far this year, and I'm a little disappointed with it's lackluster performance. I'm looking for ideas on what I need to do to get that last 2-3 MPH that this boat should be capable of.

Here's the story -
For about 4 years now, we've had a 1994 Ski nautique with the 351HO Pro-Tec engine, carb. Purchased with 450 hours, now has 1020 hours. This has always been an extremely fast boat - it does 47-48 on the GPS, with a 4 blade OJ, @5050-5100 rpm. I've always been very happy with the performance side of this boat, but upgraded to the TSC this year for the much better wakes/tracking in the course.

My new boat is a 97 with the GT-40 engine. It is the newer version with the FCC. I thought nothing of purchasing a boat with 950 hours, given that my old one didn't lose an ounce of performance from 450-1000 hours (and i've seen boats at ski schools go 3000+ hours with no engine issues). I skied/drove the boat before buying, but never took it up to the top speed.
The boat tops out at 43-44 on the stargazer, @4650 rpm. Hole shot is OK, but not as good as I would like). Has not changed with any of the tuneup parts I put on it over the last 50 hours of engine time. Runs smooth, Idles perfect. It's perfectly fine for most skiing, but for footing and short setup slalom it leaves something to be desired. On one hand, the published numbers put the GT-40 at 310 ponies, while the Pro-Tec cranks out 285, but the TSC hull probably has more drag than the older 'no wake zone' hull. I was kind of expecting the gt-40 to push 45-46 mph and turn 5000 rpm with the 422 prop.

Here's what I've done so far:
Plugs/Wires
Cap/Rotor
Fuel Filter
fuel pressure measures 38@wot (not sure how accurate my gauge is)
strut bush
t-stat

Where should I turn next to get the last bit of power out of this engine? or do I just have unrealistic expectations from this hull/engine combo? Maybe I'm just spoiled from skiing behind 6 liters at tournaments...

My 98 will get to 46 with no problem with a 422. Because I don't know what else to ask, have you cleaned your flame arrestor lately?

Some carb cleaner spray from the inside out, let it dry thoroughly, and reinstall. Motor likes to breathe. Even if this doesn't solve your top end problem, it's cheap maintenance that needs to be done at least once per year from the looks of what comes out of my flame arrestor in the fall.

east tx skier wrote: have you cleaned your flame arrestor lately? Some carb cleaner spray from the inside out, let it dry thoroughly, and reinstall. Motor likes to breathe. Even if this doesn't solve your top end problem, it's cheap maintenance that needs to be done at least once per year from the looks of what comes out of my flame arrestor in the fall.

Doug,
What are you getting off the arrestor? Black? I'm wondering if you have some misaligned sheaves chewing up the belts. I simply mention this since I have had no issues on my boats and I haven't cleaned the arrestors in years! Unless there's a problem under the dog house, I don't consider the cleaning to be an annual requirement.

Can you use an advance light and just check it no -loaded? Just curious. I dont know squat about about GT 40s.

Sounds like prop to me though, another 200 RPM might get you where you want to be.

I really don't think it's the prop. I've tried 2 props - the 422 which I know is THE correct prop for this setup, and a 4 blade oj. My older, less powerful boat can spin both of them up over 5k, and the gt-40 struggles to get them up to 4600. with a footer in tow it will not get over 4300, and is really slow to climb from 3900 up. last year we had 2 long line footers behind the '94, and we were throttled back to run 42mph.

just to clarify - here's how i checked the timing:

in neutral, 2000 rpm, spout disconnected. timing light indicates 5 degrees advance (static base timing)

put the spout back in - timing advances and engine has more power.

I'll try kicking it up to 8 degrees before we ski again. The 'jumpy' mechanical tach has got me thinking stator?

by stator i was referring to one of these:
http://www.skidim.com/prodinfo.asp?number=MP716SB

It goes inside the distributor. I'm not really sure what/where a pip is.

I think I'll try and borrow a friend's code reader and check the advance.

if that checks out, i think it's time for a compression test. I'd hate to think that there could be internal problems with only 1100 hours, but you never know. Possibly cam wear? this would manifest itself more at high rpm operation...

derekskier wrote: by stator i was referring to one of these: http://www.skidim.com/prodinfo.asp?number=MP716SB It goes inside the distributor. I'm not really sure what/where a pip is. I think I'll try and borrow a friend's code reader and check the advance. if that checks out, i think it's time for a compression test. I'd hate to think that there could be internal problems with only 1100 hours, but you never know. Possibly cam wear? this would manifest itself more at high rpm operation...

direct link

You mentioned you have Perfect Pass.

Is it doing it's start-up routine properly. That is, is it taking up all the slack in the throttle cable as it should. Howabout when you turn that knob, see any slack?

well, off to do the compression test. Maybe jody will chime in while I'm gone...? He seems to know everything about these old hog's.

Hey guys,

Didn't have time to run the compression test today. I did run a fuel pressure test again with a gauge that I know to be accurate - about 29 psi at idle, 39 at WOT. I think it's safe to say it's not a fueling problem.

I'll try and do the compression test tomorrow.

Good find
Consider a 351W is going to like 36 degrees total, shouldn't matter how the fuel is delivered.
Its a function of stroke and rod length, compression ratio and combustion chamber shape.

got the new distributor in and timed to 8 degrees. went for a ski/test drive today and it made no difference. Well, I take that back, the idle might have been a little smoother and the tach no longer 'jumps' around. My top speed was still 4630 rpm.

I did do a few wide open take offs to watch the revs. When I do a wide open hole shot, rpms jump to about 2700 immediately, then climb fairly quickly to 3700. After that, It's pretty anemic pulling up to 4300.

Maybe someone who has experience with high hour engines here can chime in - do my compression numbers look bad enough to explain this power loss?

I did do the old trick of pouring a bit of motor oil in the cylinder when taking the compression readings, and the numbers jumped by about 5 - 10 psi across the board. I guess this means the bottom end of the motor is probably fine, If there is an issue it would be valve/head related.

Your thoughts?

I did not remove all plugs when performing the compression test. I guess that means I did it wrong.

I'm getting really sick of this power issue. I'd hate to spend $1000 on a valve job and a new cam only to find performance still less than stunning. Has anyone ever had a valve job done on one of these that could comment on the effectiveness?

Also, I don't really consider 1100 hours all that high, given that my 94 is a speed demon with a carburetor engine with over 1000 hours...

I guess my only other option is to find a low hour Econtrols 6 liter. Anyone have a source for one of those? (yes i know i can get a new one for 10k)

If your 96 only went 43, then you would expect the newer hull of the 97 to be a bit slower.   If this is the case, then maybe I was just expecting more top end than this boat can deliver. I think its time to look for a bigger engine and zo, although it may be cheaper to sell and buy a newer boat that came with zo from the factory

I ended up tearing the engine down to figure out what was going on, and found two problems:

#2 fuel injector was completely non-functional, #6 injector 20% deficient - flowed 19 lb/hr vs the specified 24 lb/hr. Neither of these injectors responded to ultrasonic cleaning, so they were replaced. The entire injector set now tests between 24-25.25 lb/hr.

driver side head was cracked. I originally intended to have a valve job done, but the machine shop discovered a crack between the valve seats on cylinder #1. apparently it cannot be repaired. replaced heads with two rebuilt gt-40p heads from clearwater.

engine runs great now- much smoother at speed, and has the power that I was expecting all along. with a 422 it turns ~5150 rpm.

Anyways, I have one gt-40 head casting F3ZE for sale.
Fully rebuilt- cleaned, magnaflux, new valve seats, etc. let me know if you need it.