More Merc 350/260 HP Challenges HELP

[mphatc]

We took our other boat out yesterday as the Corsican has more work required, long sad story.

And now I have two boats that need work!

Our other boat is a 1988 Chapparal 2350 with a Merc 350 / 260 hp Alpha set up. Has ~ 300 hours use, and has been as reliable as any bone stock Chevy Nova . . .
till yesterday . . . .

running wide open it used to run 50 - 53 mph at 4900 rpm.
Yesterday it did that after warm up for a short run, I slowed , slower enough to just be out of the 4 barrels. In a little while it wouldn't maintain speed, and it only got worse, soon it was at full throttle and the speed gradually dropped to as low as 4200 rpm and 25 mph (4000 rpm ussually is closer to 40 mph). A this point I returned to the dock rather than risk the need for a tow . . . BTDT enough!!!

here's what I noticed . . .

at idle a small rattle from behind the engine
a vibration at speed, though minor
smooth idle, normal oil pressure, operating temp., normal start up, and normal response to throttle up in neutral . .
lower prop shaft spins freely in neutral

I've ruled out both the drive hub and the prop hub as I would expect the engine to over rev if either failed.

I've spent my entire summer friggen with Boats, and have yet to have a whole day on the water . . . WTF . . why me???

TIA for any assistance!
Mario L.

[Rootsy]

i say it is time to go through the Q-jet, fuel screen and water seperator...

the small knock is most likely the flappers in the exhaust tubes.. the rubber burns off edges and they clack.

[mphatc]

James,


I can't be convinced that is it . . . here's why

if I had any fuel starvation the engine would run lean at full throttle and sputter in some fashion. I've never experienced an engine that funs out fuel which will continue to run smoothly at full throttle . . . ;)

this boat has thru the drive exhaust, so there's no flappers, this sounds like it's coming from the drive coupling, drive plate are on the engine.

Mario :banghead: :banghead:

[DONZI]

Ign. or coil breakdown ?

[Rootsy]

mario,

the THRU PROP exhaust has flappers just below the turn downs...

and i dont think she is running lean i would be checking the vacuum pot on the Q-jet... that controls the secondary opening...

if it was ignition she'd be popping one way or another... it would not be smooth...

either she isn't getting the air she needs and it is acting as a restrictor plate or your are OVERFUELING the air and it is a flat kinda bog...