1985 90hp Merc - need help diagnosing

[90merc]

I have had this motor for 4 years and it has served me very well. At the end of last year it started to act up... starts and idles fine but when you hit the throttle it bogs...I had a bad power pack and replaced it, fuel cell was removed and drained, diaphram on fuel pump replaced, carbs are clean n set.
I use both a particulate fuel filter and water fuel separator ...so fuel and fuel supply are no longer issues...spark plugs are new...I am stumped!!!! please help...

 

[James R]

Re: 1985 90hp Merc - need help diagnosing

May still be a problem with the fuel pump or carbs. However a compression check would be in order. You could have one or more cylinders blown. Check this first before spending more money and time.

 

[90merc]

Re: 1985 90hp Merc - need help diagnosing

Thanks ... my buddy is comin over tomorrow with a compression tester...my fingers are crossed. I forgot to mention that there was a repair at the end of last season to the gasket at the exhaust port in one of the cylinders that was leaking water into the cylinder...could still be leaking???

 

[cannonford57]

Re: 1985 90hp Merc - need help diagnosing

while you are accelerating,push the choke in when it starts bogging,if it picks up and EVERYTHING is in as a pristine state as you say it is,the labereth(sorry about the spelling) seals on the reed blocks are shot..

 

[90merc]

Re: 1985 90hp Merc - need help diagnosing

here's the latest update....
compression test completed and all is perfect.
took it out again and ran off a cruise a day....
still problem persisted so I attempted to use the choke while bogging...
it would loose RPM and start to stall out when choke was used.
I was able to get the boat up to about 3600rpm by slowly applying throttle and depressing the choke from time to time.
Still it sounded like it really wanted to go every 30 sec would be a surge, slight rev and then back to bog...like cutting out...
but that was way more than I have gotten out of it in a couple of weeks!
I'm stumped!!!!
Please help!
90merc

 

[narvlebenoit]

Re: 1985 90hp Merc - need help diagnosing

Ihave a 81 hp 90 put new switchboxes a few months ago still did the same boging down put new stator today sounds a lot better I will try it out tommorow and see what it does .

 

[James R]

Re: 1985 90hp Merc - need help diagnosing

Before going back to the fuel side lets check the spark side. Still sounds like one or more cylinders are not performing. With the motor running pull each of the spark plug wires in turn and see if there is any difference in rhythm. I use a clean leather work glove and take care not to hold onto any part of the motor casing. A heavy rubber glove would be better of course. If when pulling there is no change in rhythm then that one is not firing. However some Mercs have a system that cuts out two cylinders at idle. Not sure if yours is one but I don't think so. A simple spark tester from auto supply house could also prove useful. This is just an adjustable gap device,Very cheap.
This simply takes place of a plug and helps you visually determine the strength a week spark at turn over speed and may indicate a bad plug wire, coil or switch box.
These tests don't cost anything and require a minimum of dismantling.

 

[Chris1956]

Re: 1985 90hp Merc - need help diagnosing

James, The 2+2 Mercs do not have idle circuts in two of the cylinders. The spark is constant.

This motor should be an inline six. If you do have spark to all cylinders, try richening the carbs a bit, and test by accelerating after each adjustment. Oprn the top carb 1/8 turn (CCW) and test acceleration. Next try mid carb and then bottom carb. Repeat sequence if necessary.

 

[James R]

Re: 1985 90hp Merc - need help diagnosing

Thanks for the info Chri1956. I should have pulled the drawings and I would have seen.
Just lazy.