GT40 Engine Questions

Welcome.  Lots of GT40 folks here.

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.

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.

Increased heat does cause increased electrical resistance.  Maybe a traditional tuneup including new plug wires?

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 

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?

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.

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.

Gary what caused your engine failure?

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

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.

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.

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.

Gary S wrote: Spout

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.