Mercury 75hp misfiring?

[lrice01]

Greetings board members,

I'm spending a week in the California delta chasing stripers. Today was my first day on the water. Boat started out running fine and later in the day, I experienced a lack of power on the hole shot and got the feeling the outboard was misfiring. Ran perfectly until the problem started. It's a 1994 Mercury 75hp 3 cylinder 2 stroke. Any thoughts?? Thanks...........Lee

 

[Maxz695]

Re: Mercury 75hp misfiring?

Fuel obstruction bad plugs bad ground bad wires water intrusion stuck float clogged jet or filter faulty fuel pump cracked lines leaking lines bad coils If fuel injected clogged injectors leakijng fuel rail can you at least narrow it down so I don,t have to go on and on? Sorry just sat down to dinner 10 52 pm with alot of people up the east cost. First thing check compression on all cylinders second check spark on all cylinders. lastly check fuel system to all cylinders. Thats a great way to start

 

[lrice01]

Re: Mercury 75hp misfiring?

Thanks for your response. Sorry, I didn't mean to be so vague. I changed the spark plugs last evening, will go out today to see if there is any improvement. It seems to run fine at idle but, starts to sputter when I increase to high idle. When I'm on the water, and increase to full throttle, there seems to be less power and of course, it takes longer to get on plane. Once on plane, it seems to run fine, no sputtering or coughing, it just runs slower. I guess I'm thinking fuel related. Thanks very much for your time and response........Lee

 

[Maxz695]

Re: Mercury 75hp misfiring?

Please let us know where your standing after testing the new plugs.

 

[CharlieB]

Re: Mercury 75hp misfiring?

Continued operation at large throttle opening can cause engine damage if your current problem is a fuel supply problem. The motor can be running low on fuel in the upper carb, leaning out the fuel mixture causing a lack of power and raising combustion temp high enough to melt the piston.

This is why is is CRITICAL to diagnose and repair immediately to prevent extensive engine damage.

Grab the primer bulb and hold a steady pressure on it to see if that improves the condition, if so this proves a fuel supply problem, such as a weak fuel pump, poor fuel hose, dirty filter, etc.

How did those old spark plugs look? Did replacing them make an improvement?

Have you done the spark test to see if you have a hot blue arc across a 7/16 inch gap?

 

[lrice01]

Re: Mercury 75hp misfiring?

Will do! Forgot to mention this is a carbureted engine. Thanks for your help with this.............Lee

 

[lrice01]

Re: Mercury 75hp misfiring?

Plugs looked ok, meaning there wasn't any conclusive evidence of a problem. Haven't done the plug test. Thanks for the warning. I wasn't aware of this. I'll proceed with caution while running. I'm trying come to a logical reason why this problem came on so suddenly. Thanks for your response..............Lee

 

[lrice01]

Re: Mercury 75hp misfiring?

Here's an update. This morning before heading out to check if the spark plug change made a difference, I started up the engine and as it idled, I turned the fuel mixture screw on each carburetor counterclockwise 1/2 turn and then returned it to it's original position. I did this to each carburetor in turn. When I went out on the water and accelerated, the problem had vanished. Now, from my automotives days, which is just too many years ago to mention, I know you want to make one change at a time to determine what the problem was. I guess it seemed to me to be a fuel starvation issue so my logic was, maybe one of the jets were obstructed. So, I want to thank you all for chiming in on this and I truly appreciated all the responses. I would very much like to hear any thoughts you might have on this. Does what I did and does the corresponding result make sense to you?? Thank you again, I get to go fishing tomorrow!

 

[CharlieB]

Re: Mercury 75hp misfiring?

So you changed the plugs AND messed with the idle mixture, but, you returned the idle mixture to their original position so really the only change was the plugs.

Enjoy the fishing!

 

[lrice01]

Re: Mercury 75hp misfiring?

That's correct. I just thought maybe I dislodged an obstruction when I adjusted the mixture screws. Do sparkplugs typically fail that suddenly?

 

[Maxz695]

Re: Mercury 75hp misfiring?

Yes they can just quit in an instant. Sometimes they work up until a certain pressure then fail. I used to use a spark plug cleaner in high school and it had a tester that blew air pressure unto the plug to test if it would with stand acceptable air pressure. If it didn,t fire at that pressure it was trash.

 

[lrice01]

Re: Mercury 75hp misfiring?

Maxz695,
Thank you very much for that information. This has been a very educational experience. I'm glad I keep extra plugs onboard!. Thanks again for your help.

 

[Maxz695]

Re: Mercury 75hp misfiring?

A grandma tool kit and extra plugs are as important as my saftey kit when I go out

 

[lrice01]

Re: Mercury 75hp misfiring?

I couldn't agree more!