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    Posted: September-23-2020 at 8:29pm
Hi Everybody, I just now joined this site because it looks like there is a lot of GT40 engine knowledge here.  I don't own a CC, I just bought a 95 Hydrodyne with the GT40 engine.  I just sold an 89 Supra TS6M.   Wasn't really looking for a boat.
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?
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Post Options Post Options   Thanks (0) Thanks(0)   Quote slmskrs Quote  Post ReplyReply Direct Link To This Post Posted: September-23-2020 at 9:25pm
Welcome.  Lots of GT40 folks here.

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....  
Gordon '97 Ski Nautique, GT40
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Post Options Post Options   Thanks (0) Thanks(0)   Quote agetech Quote  Post ReplyReply Direct Link To This Post Posted: September-23-2020 at 10:20pm
#7 Injector-
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.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote slmskrs Quote  Post ReplyReply Direct Link To This Post Posted: September-23-2020 at 11:00pm
Originally posted by agetech agetech wrote:

#
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.
Gordon '97 Ski Nautique, GT40
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Post Options Post Options   Thanks (0) Thanks(0)   Quote MrMcD Quote  Post ReplyReply Direct Link To This Post Posted: September-24-2020 at 5:32pm
Add 1 quart of ATF to your next tank of fuel and see if it clears your injector issues.   ATF automatic trans fluid is good for cleaning and lubricating injectors.  Won’t hurt anything and may help.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote agetech Quote  Post ReplyReply Direct Link To This Post Posted: September-24-2020 at 6:21pm
Originally posted by MrMcD MrMcD wrote:

Add 1 quart of ATF to your next tank of fuel and see if it clears your injector issues.   ATF automatic trans fluid is good for cleaning and lubricating injectors.  Won’t hurt anything and may help.

This is the kind of info I knew I could find here
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Post Options Post Options   Thanks (0) Thanks(0)   Quote DVskier Quote  Post ReplyReply Direct Link To This Post Posted: September-24-2020 at 6:26pm
Sea foam will also take care of cleaning injectors. 
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Bri892001 Quote  Post ReplyReply Direct Link To This Post Posted: September-24-2020 at 6:33pm
Increased heat does cause increased electrical resistance.  Maybe a traditional tuneup including new plug wires?
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Post Options Post Options   Thanks (0) Thanks(0)   Quote 8122pbrainard Quote  Post ReplyReply Direct Link To This Post Posted: September-24-2020 at 6:44pm
Originally posted by DVskier DVskier wrote:

Sea foam will also take care of cleaning injectors. 

That's what it states on the can! LOL


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Post Options Post Options   Thanks (0) Thanks(0)   Quote KENO Quote  Post ReplyReply Direct Link To This Post Posted: September-24-2020 at 7:18pm
Now that you got all kinds of info on cleaning injectors, you should probably define engine cold and engine warmed up if you want any kind of maybe useful answers to your original question Wink

Is engine cold something like you start it, idle 10 feet out from the dock and punch it?
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Post Options Post Options   Thanks (1) Thanks(1)   Quote agetech Quote  Post ReplyReply Direct Link To This Post Posted: September-24-2020 at 8:04pm
Originally posted by KENO KENO wrote:

Now that you got all kinds of info on cleaning injectors, you should probably define engine cold and engine warmed up if you want any kind of maybe useful answers to your original question Wink

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.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote slmskrs Quote  Post ReplyReply Direct Link To This Post Posted: September-24-2020 at 9:12pm
Originally posted by KENO KENO wrote:

Now that you got all kinds of info on cleaning injectors, you should probably define engine cold and engine warmed up if you want any kind of maybe useful answers to your original question


While I was waiting for the big guns to load, I figured we'd pass the time talking about my issue.

So, it literally just slows down while at WOT. Real fast, or gradually such that it's not really noticeable without looking at the tach?
Gordon '97 Ski Nautique, GT40
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Post Options Post Options   Thanks (0) Thanks(0)   Quote agetech Quote  Post ReplyReply Direct Link To This Post Posted: September-24-2020 at 10:13pm
It's like somebody just pulled the throttle back a bit, sudden drop off, really noticeable.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote MrMcD Quote  Post ReplyReply Direct Link To This Post Posted: September-25-2020 at 3:29am
Excess Timing can give you the results you speak of. Cold the engine runs fantastic, better than ever. As combustion heat builds it can start detonating which robs a ton of power.
Not simple Pinging, but full detonating. Detonation can be quiet, happens when a cylinder fires the fuel charge before the piston is at TDC, this early fire tries to push the piston back down while 7 other cylinders and the flywheel are trying to push the piston up into the early explosion.
In detonation the engine runs great and then suddenly feels like you let partially off the throttle. The exhaust tone will change slightly also. If you wish to rule this out back your timing off 3-4 degrees and go see if the problem goes away. If the timing is over advanced at high RPM it can create a hot spot in one or more cylinders which will start glowing red while running hard, this red hot spot starts working like a spark plug and ignites the fuel charge before the spark plug sparks.   Any rough spot in a head chamber can be the source of a hot spot. Too hot of a spark plug can be a source of a hot spot. Serious racers would polish the head chambers and the valve and piston heads to eliminate any sharp corner or rough spot that could start detonation. Newer engines with Aluminum heads pretty much eliminated the need for this polishing as they come out of production pretty darn smooth today with the CNC machines doing the work.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote KENO Quote  Post ReplyReply Direct Link To This Post Posted: September-25-2020 at 5:15am
Originally posted by agetech agetech wrote:

Originally posted by KENO KENO wrote:

Now that you got all kinds of info on cleaning injectors, you should probably define engine cold and engine warmed up if you want any kind of maybe useful answers to your original question Wink

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  Wink
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Post Options Post Options   Thanks (0) Thanks(0)   Quote MrMcD Quote  Post ReplyReply Direct Link To This Post Posted: September-25-2020 at 12:43pm
Good point Ken, his description of the engine laying down while under full power sounded like typical detonation, you are correct if it is Fuel injected the computer controls timing.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Gary S Quote  Post ReplyReply Direct Link To This Post Posted: September-25-2020 at 1:29pm
And even then the computer can only do so much-







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Post Options Post Options   Thanks (0) Thanks(0)   Quote MrMcD Quote  Post ReplyReply Direct Link To This Post Posted: September-25-2020 at 5:17pm
Gary what caused your engine failure?
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Post Options Post Options   Thanks (0) Thanks(0)   Quote KENO Quote  Post ReplyReply Direct Link To This Post Posted: September-25-2020 at 5:36pm
Originally posted by MrMcD MrMcD wrote:

Gary what caused your engine failure?

The guy he bought it from did

He bought it like that for a project  Wink
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Post Options Post Options   Thanks (0) Thanks(0)   Quote agetech Quote  Post ReplyReply Direct Link To This Post Posted: September-25-2020 at 6:47pm
Going to check timing next time out.  Still need a fuel pressure tester.  Will report back!  Want to get it 100% before winterizing.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Gary S Quote  Post ReplyReply Direct Link To This Post Posted: September-25-2020 at 7:39pm
What he said Wink.  Heads were already rebuilt so they were clean,valve covers rockers,push rods, block sludge wise were clean. But the oil left in the pan was nasty. Bearings worn but not scored or damaged. Crank only required a polish. Injectors were gone but new in the box Motorsport ones were included.LP pump making noise. I have new pumps and several ECM's  When I get it running I'll keep an eye on everything and test the knock sensor. Something made #8 go bad and it's failure spread debris through several other cylinders
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Post Options Post Options   Thanks (0) Thanks(0)   Quote KENO Quote  Post ReplyReply Direct Link To This Post Posted: September-25-2020 at 8:25pm
Originally posted by agetech agetech wrote:

Going to check timing next time out.  Still need a fuel pressure tester.  Will report back!  Want to get it 100% before winterizing.

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?
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Post Options Post Options   Thanks (0) Thanks(0)   Quote agetech Quote  Post ReplyReply Direct Link To This Post Posted: September-25-2020 at 9:49pm
I can check the timing.  Was a powered equipment mechanic in the Air Force.

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Post Options Post Options   Thanks (0) Thanks(0)   Quote MrMcD Quote  Post ReplyReply Direct Link To This Post Posted: September-25-2020 at 10:08pm
I can't comment on the boat applications but the same Injection system was used in many Ford Cars and Trucks, OK, a similar automotive system that looks like the Marine version. In those applications we saw many melted pistons in #8 cylinders. The plug wire routing allowed #7 and #8 to cross fire in automotive applications. Maybe it is happening in the GT40 Marine engine also? If cross fire is happening you will find Halo's where the spark is blowing out of the wire. The Halo looks like it's name, a ghost like 3/8" halo on the wire. If you see one the wire is cross sparking. Good Luck.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Bri892001 Quote  Post ReplyReply Direct Link To This Post Posted: September-26-2020 at 11:15am
When checking timing on a GT-40, don't forget about that little puck, thing, I can't remember what it's called, but you have to unplug it to get back to base timing.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote 8122pbrainard Quote  Post ReplyReply Direct Link To This Post Posted: September-26-2020 at 11:20am
Originally posted by Bri892001 Bri892001 wrote:

When checking timing on a GT-40, don't forget about that little puck, thing, I can't remember what it's called, but you have to unplug it to get back to base timing.

Snout??


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Post Options Post Options   Thanks (0) Thanks(0)   Quote Gary S Quote  Post ReplyReply Direct Link To This Post Posted: September-26-2020 at 11:34am
Spout
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Originally posted by Gary S Gary S wrote:

Spout

Thanks Gary. At least I was close! 


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Post Options Post Options   Thanks (0) Thanks(0)   Quote agetech Quote  Post ReplyReply Direct Link To This Post Posted: October-08-2020 at 9:11pm
Okay-
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.

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First, pressure does not evaporate when the key is shut off.  There is a spec, it will say something like pressure should hold for five minutes and slowly decline over the next 30 minutes.  If you are losing pressure instantly you have a large leak.  Either the pressure regulator is stuck open or your injectors are not closing.  Black smoke on a restart points to injectors.

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.
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