Mercruiser 250 inline 6, power loss under load

[Cbastuan]

Hey guys,
Having issues with my mercrusier 250 inline 6, nothing but problems all last year and still continuing into this year.
Pretty much starts fine, idles a little rough but not too bad. On the muffs if i give it gas it is responsive and no hesitation while revving it up.
On the water however, when the motor is under a heavy load and trying to push the boat out and onto plane, the rpms will fluctuate and drop down then back up and eventually and finally once the boat comes into plane the motor runs fine, no rpm fluctuation or hesitation.
So where do i start? Here's a list of what I've done so far:
new fuel tank
all new fuel lines
new fuel/water separator filter
New plugs
New wires
New fuel pump
New distributor/rotor/cap
Rebuilt carburetor (twice)

I'm sure there's more I'm forgetting, but I'm running out of parts to replace, any ideas guys? I'm desperate
Thanks

 

[Rick Stephens]

Carburetor rebuild would be pretty high up my list. Double check timing. Compression check the motor, so you have a baseline on its suitability. If compression is low, do a leak down test to see if there are valve issues. Run the valve adjustment.

Those are all basic engine tasks on any motor when you have a problem. Start at likely stuff and proceed until you find the problem.

 

[Maclin]

Is it points type ignition? If so I didn't see "New points and condenser, dwell set then timing set" on your list.