1980's 2 stroke mecury power problem

[Turtleman87]

So I just bought this older bass boat. Has a 50hp mercury outboard. Carbs were just redone. Starts fine and goes into gear fine. When I push h the throttle to full power the motor doesnt increase RPMs and slugs along at 5-10 mph. If I have it run like that after a couple minutes it will randomly increase rpms and speed. It will get to about 35-40 mph. It will stay that way for a couple minutes die back down to 5mph the rpms also fall. This cycle will repeat over and over every couple of minutes....
Any ideas where to start?

 

[flyingscott]

Try pumping the primer ball when this is happening. If the primer ball makes the motor run better look at the fuel pump. If it makes no difference test compression and spark . Report back with those results.

 

[Chris1956]

If pumping the primer bulb doesn't fix it, you may have two problems. Lack of acceleration is likely an idle mixture setting which is too lean. Open the idle mixture screws 1/8 turn and see if that helps.

The high speed/low speed issue is likely fuel delivery. Pumping the primer should diagnose that, but the fix is to make sure there are no fuel restrictions (dirty filter, clogged antisiphon valve..), and the fuel pump is working properly. A rebuild of the fuel pump is a pretty cheap fix.