Pismo10

All runs fine for 1/2mile then runs great up to 3000rpm then sputters and loses power. Then runs fine below plane at 2000rpm all day, try 3000 again, loses power. 1988 Mercruiser 454 with TBolt IV. Gumout down carb while sputtering did nothing so thinking electrical. Removed gas cap, no help. New cap and rotor(rather green) did not help. Jumping + batt to +coil next to rule out key switch, lanyard etc.

Any thoughts appreciated!

GPM

Just a guess, fuel pump

Pismo10

That was my first thought. I am hoping the Gumout spray test eliminated fuel supply issues. While it was sputtering at 3000rpm I sprayed some then lots down the carb bores and it changed nothing at all. I was hoping it would roar back to life for at least a moment or change behavior somehow but it did not.

Wally

Is gumout actually flammable or is just a solvent? Might try starting fluid instead....
You may still have a carb problem....possible the secondary circuit is clogged...
Fuel pump could still be an issue as well....not enough pressure to keep the carb filled trying to get over the 3k rpm
Could also be broken springs on the intake/exh vavles and the valves floating at higher rpms...
Have you done a compete tune up? new plugs wires cap rotors etc? all kinds of things can happen with bad components...
Start with the simplest or easiest/cheapest and go from there

Pismo10

Gumout is gasoline in a spray can with an octane rating of about 80-90. Ether/starting fluid has an octane rating about 30-40 so I try not to use that. An engine will run all day on gumout, done it.
Boat runs perfectly at all speeds for first 5-10min then the issue begins so I do not think broken springs as that would be an issue all the time.
Cap and rotor done, no help. Plugs, wires next but they all look good.
Issue is very on/off like turning key on/off quickly. Coil? Module? Vacuum in tank (took cap off, no help), bowls running out sounds logical. Boat has an electrical external fuel pump.
Thanks for the ideas.

105Fountain

Choke sticking closed.

GPM

Good Luck !

SB

How old is the dist sensor and is it a Merc one ? Same question for coil

What is voltage to coil when issue appears ? Do you have to jump battery to start the boat ?

Pismo10

No idea how old coil and Merc dist sensor are. I jumped a wire direct from + Batt to + Coil with no help, same problem, loses power after 5-10min, wont plane anymore. Volt meter reads 12.7-13.2 area. Have not checked at coil but jump wire should have given it full 13 area volts. The delay puzzles me, fine at all rpms for first 5-10min then issue begins. Sounds coil possibly. Jump wire should rule out key switch, lanyard, etc. The tach really drops off when it sputters, tach jumps right down to 1500 then right back to 3000, back a forth quickly.

Pismo10