1989 Mercury Black Max 150hp outboard Motor Problem

[sshotgun]

QUESTION ….. I have a 1989 Mercury Black Max 150hp outboard on my bass boat . It at ½ throttle or more will be running fine … THEN after running fine for 10 minutes or so , without throttling back … it will slow down to about ¼ speed for about 5 seconds , then speed back up for about 10 seconds , then slow down again , and then speed back up . Then after this starts happing , it will get real hard to start if I turn it off .

I have fresh gas in it … not sure where to start looking for the problem . Any ideas ??? What do you think ?

THANKS for the help .

 

[racerone]

Answer some questions.-----How did it run for you last season ?----Have you installed a new water pump impeller ?----Compression test results to be posted.-----Spark must jump a gap of 3/8" on all leads.-----If boat sat for any length of time the carburetors could be plugged.----Water in the fuel.----Plugged filters ?-----Fuel pump inspected ?----What happens when an assistant operates the manual fuel pump ?---Perhaps test run with another tank and fuel hose.

 

[Chris1956]

That sounds like a fuel delivery issue. Pump the primer. If that fixes it, rebuild the fuel pump.

Clean/replace all fuel filters, clean the antisiphon valve in the fuel line at the fuel tank. Some fuel tanks have a filter on the pickup that can get clogged.

 

[sshotgun]

Answer some questions.-----How did it run for you last season ?----Have you installed a new water pump impeller ?----Compression test results to be posted.-----Spark must jump a gap of 3/8" on all leads.-----If boat sat for any length of time the carburetors could be plugged.----Water in the fuel.----Plugged filters ?-----Fuel pump inspected ?----What happens when an assistant operates the manual fuel pump ?---Perhaps test run with another tank and fuel hose.

THANKS for the info ... I am not a mechanic ... just looking for some ideas on where to start looking for the problem I am having . Thanks for your imput .

 

[QBhoy]

Here is where my money would be, having the same engine as you…
Either bad fuel (despite you saying it’s fresh fuel..if you have some bad fuel left over in the bottom of the tank, fresh fuel doesn’t help much) or your stator or electrical/ battery connections are failing you. If you aren’t charging the battery, your engine will struggle on the ignition system.
Something like that will be your issue, I’d say.