1984 75hp mercury outboard 4 cylinder running issue

[axiosteve]

My 1984 75hp mercury is giving me issuses. Im only getting 2000 rpm on the water 25mph wot. but she cranks without a load on it. running with a hose and muffs. could the wrong prop becausing my issue. It has a 13.25X17 on it thats how it was when i bought it. It ran like this when i first got it, i used it a bunch of times before i completly lost all top end wot. 5 mph tops. figured my high speed windings were bad replaced the stator and its back to 2000 rpm tops.

 

[racerone]

Re: 1984 75hp mercury outboard 4 cylinder running issue

Does primer bulb go hard when starting the engine ?------Post your compression numbers.----Then check to see if you have spark on all 4 leads.--------Have you looked at / replaced water pump impeller ?-------Sort this out before leaving the dock again.---If it is a fuel blockage in one carburetor you risk serious damage to the engine.

 

[GA_Boater]

Re: 1984 75hp mercury outboard 4 cylinder running issue

If it was running good and you lost top end (I really can't tell from your description), it's not the prop unless it fell off. But then it would over rev. Something else - It's not good to rev the snot out of it on muffs with no load.

Follow racer's ideas and get back with the results.

 

[axiosteve]

Re: 1984 75hp mercury outboard 4 cylinder running issue

It never ran above 2000 rpm since I've had it. Should the primer go hard when it runs? And would the water pump impeller cause this? I do have water comming out of the little tube . I'll check compression and spark.

 

[GA_Boater]

Re: 1984 75hp mercury outboard 4 cylinder running issue

Very doubtful it's the prop then. The motor just isn't running well. The primer should stay fairly hard when it's running and you should be able to squeeze it until it's hard before starting. The water pump impeller question is related to the motor overheating and it is considered normal to replace it when you don't know when it was replaced last and with seasonal use every other year afterwards or 100 hours.

 

[axiosteve]

Re: 1984 75hp mercury outboard 4 cylinder running issue

If the primer stays soft when its running would it be not getting enough fuel?

 

[Chris1956]

Re: 1984 75hp mercury outboard 4 cylinder running issue

25 mph at 2000 RPM with a 17P prop is not possible. Something else is wrong...speedo, tacho, pitch.

Do a compression and spark check and report back. What kind and weight of boat is that motor on?

 

[axiosteve]

Re: 1984 75hp mercury outboard 4 cylinder running issue

I'll check the spark and compression when I get a tester. Its on a 16 ft aluminum Grumman

 

[racerone]

Re: 1984 75hp mercury outboard 4 cylinder running issue

Boat should do about 35 to 40 mph.----Something is wrong and you need to determine what that is before leaving the dock again.

 

[axiosteve]

Re: 1984 75hp mercury outboard 4 cylinder running issue

I think the exhaust might be an issue possible blockage

 

[racerone]

Re: 1984 75hp mercury outboard 4 cylinder running issue

Sorry , but you have not answered any questions about compression and quality of the spark.--Spark must jump a gap of 7/16" or more does it do this -Yes or no ?----Did you replace water pump impeller when you bought the motor ?