The boat has been sitting for a while because the previous owner has cancer.  It had a misfire when I bought it, replaced #7 injector and took care of that.  It also had engine surge at full throttle, so in went a fuel filter, plugs, swapped gray to black TFI, and blew out the fuel lines.  Engine runs good now.

Now the big question- Engine cold it will tach to 4800rpm and is strong out of the hole.  After it warms up it will only tach to 4600rpm but hole shot is still good.  Why is it dropping 200rpm?

I honestly don't know what mine redlines at; I never go over 36 in the course anyway.  I'd be nervous going WOT with a cold engine; I never do it for more that a few seconds when hot anyway.  I assume you are barefooting.

Just throwing a few things out there that I would normally not think of checking first and then end up doing a lot more work when it could have been an easy fix....:

* I assume there is a difference in your top speed between cold and warm WOT, correct? 

* Have you verified the RPMs independently (PerfectPass or another RPM display)?

* Any difference in your fuel pressure at the rails (cold and warm)?

* Does the temp gauge work correctly (making sure there's no issue with the thermostat)?

I defer to the GT40 masters on this one and whether there's a theory.  Sorry, but I won't test mine cold and then warm to see if there is a generic difference....  ;)

BTW, I'm curious how you determined it was the #7 injector.  I have what appears to be a miss sometimes at idle (less so when warm) but it is gone as soon as I give it some throttle.  Wondering if it might be an injector but don't want to throw eight injectors at it to see if it's the problem....  

Took all the injectors out of the left bank (where I could definitely hear the misfire) and checked them with a 9 volt radio battery.  The dead one would not click.  Also blew carb cleaner through the good ones.  Some good youtubes are out on this.

Yes, top speed drops off from 46 to 43 mph.

Temp gauge appears to be working  fine, shows 140.

Next purchase is a fuel pressure gauge.

Normal thermostat for the GT40 is 160 (at least for the ones in Nautiques).  Don't think that would affect it though. 

From a fuel and ignition standpoint, I can't think of anything off hand other than lower pressure (not enough to make it stall, but enough to not let it get to highest RPM--pressure regulator).  But you'll need to get that gauge.  Our GT40 experts will need to jump in on that.

Another thing that comes to mind is your tranny.  Fluid changed recently?  I would expect less parasitic drag from the tranny when hot, but if there are no fuel/ignition issues, I'd look for other sources of drag.  My logical minds says your RPMs would increase when hot, not the other way.  But if something is failing, it could cause more drag when warm, such as a dying raw water pump that tends to seize up a little as it gets warm.  Alternator with the same issue, or even the water pump.

On the tranny, you could drop it in the water cold, and then turn the prop shaft coupling at the tranny by hand and see how loose/tight it is.  Then run it till it's hot, shut it down, and then try turning it again by hand and see if there's any difference.

Just a few things to consider as well....

Tks for the tip on the injectors.  I'll get my engine stethoscope out and see if I can hear any issue on each of the injectors at idle.

This is the kind of info I knew I could find here

That's what it states on the can! 

Is engine cold something like you start it, idle 10 feet out from the dock and punch it?

Launch boat, idle to dock, idle and wait for tow driver, idle out to river channel, then punch it.  About 5 minutes of warm-up time.  Will run up to 48-4900rpm and 46 mph.  About 45 seconds later tach drops 200rpm along with top speed.  No misfires or stumbles just looses top speed.  I'm thinking low pressure fuel pump is getting weak.  Boat has 2 separate pumps, no fuel module.

So the engine isn't really cold, it's been warmed up before punching it 

You could take some timing out of the engine like MrMcD said but, it's a gt40 complete with all those electronic controls including a knock sensor and it'll take the timing out automatically if the computer decides it needs to which may be what's happening.

You could start by making sure the timing is set where it should be.

What octane gas are you using?

What happens when you slow down to idle speed and punch it again?

Do you have the 301 page gt40 service manual like what used to be in the reference section?

Questions, questions  

The guy he bought it from did

He bought it like that for a project  

Since you didn't ask and didn't say whether you have the gt40 manual, is it safe to assume you know how to check the timing on the gt40 since it's new to you and a little different than what you might be used to?

Snout??

Thanks Gary. At least I was close! 

Put a fuel pressure gauge on the engine, turn key on pressure goes to 40psi then drops to 0 when pump cycles off.  Maybe 1 out of 8 cycles pressure will stay at 40psi.  Weak high pressure pump?  Took vacuum line off of the pressure regulator and no signs of gas, sucked on the vacuum line and it holds vacuum.  

Went to the lake to do some testing but overheated at dock after launch, new impeller on order, was missing 5 blades.  Caught it before any damage.  Was going to replace before next season anyway.  I have a pump on another boat I can borrow for a few days until I get the parts.

On the impeller failure you need to find all the parts the tore off.  Chunks of the old impeller will plug your cooling passages and cause overheating.  Takes time but dig for them till you account for a complete impeller.

Mark

I ran the pumps for over a minute and the engine started fine with no black smoke.  I think the injectors are OK.  But the pressure is definitely leaking off somewhere.  I don't know why the pressure doesn't drop to 0 every time.

I have seen guys just clamp off the rubber return hose with a simple clamp.  This can damage the hose so don't clamp too tight.

Sounds like you never got to high speed because of the impeller problem.

Replaced the Intake Air Temp Sensor after I read in the manual that it would cause a high idle if that sensor was bad.  Tested with an Ohm meter and the sensor was erratic, readings were jumping all over the place.  

Took it to the lake yesterday and I think it is cured, engine sounds really strong and pulls hard.  Got a top speed of 45 on the Airguides and Latnex.  

Now working on the exhaust manifold riser gaskets leaks.  25 year old bolts don't come out very easy.

Just wanted to let everyone who posted with help what the final outcome was!

If you apply heat and then more of the mix it can draw the lube in deeper.  If you rush they will break normally if they are rusted firm.  Glad you got it running better, I am also curious, what about the lost 200 RPM.

me2

Got both risers off, 2 broken bolts, took the manifold to a friend  who is a machinist and he welded a nut to the broken bolts and got them out (right side).  Left side has been off before and was packed with RTV silicone.  Also, the water jacket on the exhaust side of the riser is almost completely blocked.  I don't know how the engine cooled at all.  I'll post a picture later.  

Good to know!