Engine idles fine, but doesn't accelerate much

[BCajun]

I recently bought a 2003 Response LXI with Indmar Monsoon 335.  I recently changed the fuel pump to get it running.  It idles fine, revs up (only tried to about 1100 rpm), and sounds great with hose attached!  I put it in the water to test it out and it still idles great both in and out of gear.  When I tried to get going, it barely accelerated and would not get up and go.  I don't think the RPM's increased past 1500 or so (just a guess).  It started vibrating and shaking when trying to go, but it never killed.  I pulled the throttle back to idle and ran fine again so I turned around and put it back on the trailer in fear of messing something up.  Has anyone had a similar problem?  I checked the prop and shaft and they do not look damaged or bent.  

[Eagleboy99]

That sounds like what happened when I've had bad gas/water in the tank.

[BCajun]

I know the boat sat for a while before I bought it (probably a year or two).  I siphoned the old gas out and put fresh non-ethanol gas when I changed the fuel pump.  Do you think it could still have some old gas working it's way through the system or possibly have some water that stayed in the tank after siphoning (if there was any to start with)?

[Eagleboy99]

Run a can of Seafoam through it.  Or it might be your injectors.

[drh]

Speaking from experience double check that any plug wires didn't get knocked loose. May not be what's going on here but it's easy to check...you'd be surprised how smooth it will idle on 7 cylinders.

[justgary]

2 hours ago, drh said:

Speaking from experience double check that any plug wires didn't get knocked loose. May not be what's going on here but it's easy to check...you'd be surprised how smooth it will idle on 7 cylinders.

Yup.  For that matter, go ahead and change the cap and rotor.  Or at least peek under the cap and take a look at the oxide buildup and carbon traces. 

[BCajun]

I just wanted to update the thread in case any others have similar issues.  I checked spark and compression, that was all good but I found two cylinders were dead.  I pulled injectors and a couple of them were clogged with rust from the fuel line fittings.  I brought them to get cleaned, flow tested, etc.  One injector was completely dead and one was still not keeping up, so I'm getting quotes on fuel injectors now.