my merc stalls at top speed

[northernboater]

I Have a 1979 Mercury 115 hp with the blue band. It takes off great but after 30 seconds starts to slow down. If I pull the throttle to 3/4, it runs fine. Just got in from cruising around the lake at 3/4 throttle. When I push it up to full it started to cut out then speed up and then cut out and so on. Pull it back to 3/4 and it would run good again. I did notice some spiking on the tack at 3/4 once in a while.
Anyone got any ideas as to what I can do to fix this problem?
Please help.
Jim

 

[Chris1956]

Re: my merc stalls at top speed

Normally that behavior is due to a waek fuel pump. The spiking tach is different than expected.

I would have someone pump the primer bulb as you cruise at full speed and see if it gets better. If so, rebuild the fuel pump. If not, report back on the tach.

 

[northernboater]

Re: my merc stalls at top speed

Hi Chris
Thanks it worked as you said. I opened it up and kept pumping the ball on the fuel line and it ran perfectly.
So.... there are 3 fuel pumps on my motor. How do I decide which is the weak pump? Do you have a good site that explains about these fuel pumps as my manual doesn't have anything on it.
Thanks Jim