Mercury 150 xr4 will only go 10mph full throttle

[Jstrater]

Have a mercury 150 xr4. I believe it is a 1988. The motor starts really well and will rev all the way up in neutral. Will also work really well when hooked to does ears. When it is actions in the water and in gear, full Flored it only goes 7-8 mph. It appears that only when it's under load that this happens. Could it be a fuel issue? Maybe something to do with the carbs. Any thoughts?

 

[racerone]

Do a compression test.----------Then check for spark on all leads.------------Sounds like it is running on 3 cylinders.--------------And yes they all rev up in nuetral and on a hose.---That means nothing here.

 

[Cre73]

Do you have a tach? What RPM are you seeing?

 

[johnnygjr]

Are the throttle plates opening up all the way?? Throttle cable working correctly? Try pumping primer bulb while engine is running,this will supplement a weakened fuel pump.We need replies to our questions in order to help you. thnks.