merc only going up to 3000 rpms under load?

[broken_line]

i have a clasic 50 45 hp mercury motor and when i use the warm up lever it will rev up just fine but as soon as i put it in gear it only goes up to 3000. lift the prop out of the water and it will go up to 5500 rmps just fine but as soon as it goes back in the water it drops to 3000. it idles and runs fine just doesnt go fast.. carb plugged up?

[boatfixer]

Spark

Compression

Fuel

Have you checked compression?

Do you spark on all 4?

[broken_line]

spark and compression were good! spark was good enough to zap the snot out of me 2" away.

i took the carbs off, what a pain in the REAR!! but i noticed that the bottom main jet had a little junk in it. if its not the carbs what else could it possibly be? theres great fuel pressure.

[BoxMN]

Classic 50... I have that motor as well. Main thing I have found, and helped with others before, was that the tank needs to be vented, meaning the screw on the top of the tank needs to be open. I am sure you know that, but it can rattle down tight, and then you get funky acting symptoms, not sure if it could be what you are seeing, but worth a check. Good luck.

[JimBuck]

I've got a 89 Mariner 45 which is nearly identical to the Classic 50. I had similar symptoms you were describing and I believe BoxMN had the idea of checking the vent cap. They will rattle shut over time. Hopefully it's only that. I'd try switching out gas tanks to see if anything changes as well.

[marine_man]

I presume this motor isn't new to you right? It used to work fine?

marine_man

[broken_line]

i baught the motor used and it ran great.

i cleaned out the carbs (what a pain!) and entire fuel system, cleaned spark plugs checked timing and made it so the throttle plate opened all the way not 90% and put some sea foam in the tank.

it used to only go about 26 mph now it does a good 31 (:

thanks for the tips guys! its an on board tank so the vent cant close on its own